 |  |
| ||||
Copyright © 2001, The American Society for Cell Biology Molecular Cloning and Characterization of Phocein, a Protein
Found from the Golgi Complex to Dendritic Spines Monty Krieger, Monitoring Editor ‖Corresponding author. E-mail address:
monneron/at/lncf.cnrs-mrs.fr. Received July 27, 2000; Revised December 7, 2000; Accepted January 9, 2001.  This article has been cited by other articles in PMC. | |||||||||||||||||
Abstract Phocein is a widely expressed, highly conserved intracellular
protein of 225 amino acids, the sequence of which has limited homology
to the ς subunits from clathrin adaptor complexes and contains an
additional stretch bearing a putative SH3-binding domain. This sequence
is evolutionarily very conserved (80% identity between
Drosophila melanogaster and human). Phocein was
discovered by a yeast two-hybrid screen using striatin as a bait.
Striatin, SG2NA, and zinedin, the three mammalian members of the
striatin family, are multimodular, WD-repeat, and calmodulin-binding
proteins. The interaction of phocein with striatin, SG2NA, and
zinedin was validated in vitro by coimmunoprecipitation and pull-down
experiments. Fractionation of brain and HeLa cells showed that phocein
is associated with membranes, as well as present in the cytosol where
it behaves as a protein complex. The molecular interaction between
SG2NA and phocein was confirmed by their in vivo colocalization, as
observed in HeLa cells where antibodies directed against either phocein
or SG2NA immunostained the Golgi complex. A 2-min brefeldin A treatment
of HeLa cells induced the redistribution of both proteins.
Immunocytochemical studies of adult rat brain sections showed that
phocein reactivity, present in many types of neurons, is strictly
somato-dendritic and extends down to spines, just as do striatin and
SG2NA. | |||||||||||||||||
INTRODUCTION Neurons have unique structural and functional polarity: they
extend a single, usually long and thin axon and numerous shorter,
thicker dendrites (Dotti et al., 1988 To validate such a hypothesis, we searched for potential interactors of
striatin, and by means of the two-hybrid strategy, we identified a
novel, intracellular, 26-kDa protein, phocein (named after the Greek
founders of the port of Marseille). Because we recently showed that two
proteins, SG2NA and zinedin, share with striatin identical
protein-protein association modules (Castets et al., 2000 The sequence of phocein was found to contain stretches of homology with
the ς subunits of clathrin adaptor complexes. Clathrin, a coat
protein, is linked to different types of vesicles by various sets of
adaptor protein complexes (AP), heterotetramers comprising two heavy
chains or adaptins and two light chains, a μ and a ς chain (Schmid,
1997 The data presented in this study show that phocein, although mostly present on the Golgi complex in unpolarized HeLa cells, is seen within adult rat brain neurons from the perinuclear area down to the smallest dendritic branches. | |||||||||||||||||
MATERIALS AND METHODS Two-Hybrid Assay A fusion protein comprising the LexA DNA-binding domain and
striatin was used as a bait to search for fusion proteins expressed by
a construct containing a rat brain cDNA library (Matchmaker;
Clontech, Palo Alto, CA) and the activation domain of Gal4
(Dagher and Filhol-Cochet, 1997  ). A full-length striatin insert
(Castets et al., 1996 ) was ligated into the pLex 11 vector
(a gift from M.C. Dagher, Commissariat à l'Energie Atomique,
Grenoble, France) in-frame with the LexA DNA-binding domain, yielding
plasmid pLex-stri. L40 yeast strain cells grown in minimal medium were
transformed with pLex-stri, using the lithium acetate method (Gietz
et al., 1992 ). The Lex-stri fusion protein was stably
expressed in L40 cells, as verified by immunoblotting
using anti-striatin antibodies ( Castets et al., 1996 ). L40
cells expressing Lex-stri were transformed with the plasmid library.
From 5 × 106 colonies obtained 5 d
after cotransfection, 63 colonies were His+. They
were tested for β-galactosidase (β-gal) activity by a color filter
assay using the substrate
5-bromo-4-chloro-3-indolyl-d-galactoside.
Plasmids from the 58 His+ Lac
Z+ colonies were prepared according to the method
of Kimmel and Berger (1987) . After electroporation in Escherichia
coli HB101 cells of Leu− phenotype, the
selected library plasmids were rescued by complementing the
Leu− phenotype on minimal medium. The 58
selected colonies were accounted for by only two plasmids, encoding
inserts of 1.7 and 2.5 kilobases (kb), respectively, named pGAD
10-phocein-1.7 and pGAD 10-phocein-2.5. Inserts were sequenced using
the specific primers of pGAD10 and primers designed by a “gene
walking” strategy (ESGS, Evry, France). The 1.7-kb sequence was fully
included in the 2.5-kb sequence, which contained an open reading frame
(ORF) of 678 bp, encoding phocein, preceded by 7 bp and followed by a
3′-noncoding sequence ending by a poly-A stretch (phocein sequence, −7
to +2506).Northern Blots Total RNA from various rat tissues were purified using TRIZOL
(Life Technologies, Grand Island, NY). Each RNA (10 μg) was
electrophoresed on 1% agarose-6% formaldehyde gels and transferred on
Nytran-plus membranes (Schleicher and Schuell, Keene, NH). A 659-bp
phocein probe (nucleotides −7 to +652) was obtained by digesting
pGAD10-phocein-2.5 with EcoRI. Phocein and actin probes were
labeled by random priming with [α32P]dCTP.
The blots were hybridized overnight at 42°C in the presence of
formamide. After hybridization, the membranes were washed several times
at 50°C in 0.1× SSC and 0.1% SDS. Membranes were exposed at
−70°C with amplifying screens, using Fuji (Tokyo, Japan) films.Production of a GST-Phocein Fusion Protein and Obtaining
Antibodies A 2-kb BamHI/BglII fragment of the
selected library plasmid was subcloned in a pGEX4T-3 vector (Amersham
Pharmacia Biotech, Arlington Heights, IL), yielding pGST-phocein
encoding the full-length phocein sequence in-frame with that of
glutathione S-transferase (GST). E. coli JM109
cells were transformed and, upon induction by 0.1 mM isopropyl
β-d-thiogalactoside, expressed high levels of
GST-phocein (52 kDa). The cells were lysed, and the fusion protein
contained in the soluble fraction was purified on
glutathione-Sepharose. Two rabbits were immunized with the purified
fusion protein according to published procedures (60–120 μg per
injection). Antisera were tested on Western blots of purified
GST-phocein and rat brain subfractions. Anti-GST-phocein antibodies
were affinity purified either on strips of blots of GST-phocein or on a
GST-phocein affinity resin (obtained by coupling 3 mg of GST-phocein to
1 ml of CNBr-activated Sepharose 4B (Amersham Pharmacia Biotech). The
blots or resin were incubated for a few hours with the anti-phocein
serum and washed, and the antibodies were eluted with 0.1 M glycine-HCl
buffer, pH 2.5. The antibody solution was adjusted to pH 7.5. It was
mixed with 50% glycerol and 0.1% bovine serum albumin (BSA) and kept
at −20°C.Coimmunoprecipitation and Pull-Down Assays Rat brain homogenates were fractionated, using buffers
containing either 0.1 mM Ca2+ or 1 mM EDTA, into
cytosol and a 100,000 × g pellet, containing membranes
(the detergent-soluble fraction) and cytoskeleton (the
detergent-insoluble fraction) (Bartoli et al., 1998 ).
Protein was determined by the microBCA method (Pierce, Rockford, IL).
For immunoprecipitation assays, brain fractions were precleared by
incubation at 4°C for at least 1 h with preimmune serum-coated
Pansorbin cells (Calbiochem, San Diego, Ca; 1 ml of cell suspension for
10 mg of brain protein).For coimmunoprecipitation assays, batches of 200 μl of a 10%
suspension of washed Pansorbin cells preincubated in 1% BSA-containing
Tris saline buffer (TBS buffer; 50 mM Tris-HCl, pH 7.4, and 150 mM
NaCl) were incubated with 140 μg of rabbit preimmune immunoglobulins
or affinity-purified anti-phocein, anti-striatin, anti-SG2NA, and
anti-zinedin antibodies, in the presence of 0.1% BSA (Castets et
al., 2000 For pull-down assays, 2.5 μg GST or purified GST-phocein were incubated with 40 μl of 50% glutathione-sepharose for 2 h at 4° in TBs containing 0.1% BSA. After three washes in TBS, 200 μl (about 400 μg protein) of rat brain cytosol or lubrol-soluble fraction were added to the resin and incubated overnight at 4° with gentle agitation. After extensive washes with TBS, the resin pellets were treated as above. Immunofluorescence Studies and Fractionation of HeLa cells:
Brefeldin A (BFA) Treatment HeLa and Hep-2 cells (American Type Culture Collection,
Manassas, VA) were grown in Eagle's medium modified by Dulbecco
(DMEM), supplemented with 10% fetal calf serum, 2 mM
l-glutamine, penicillin, and streptomycin (GIBCO-BRL, Grand
Island, NY). For treatment with BFA (Sigma, St. Louis, MO), HeLa cells
grown on coverslips were incubated for 1, 2, and 10 min at 37°C with
5 μg/ml BFA in DMEM or with a 1:2000 dilution of ethanol in DMEM as a
control. Cells were then fixed and processed for immunofluorescence.For immunofluorescence studies, cells grown on coverslips were washed in phosphate saline buffer (PBS), fixed in a solution containing 3.7% paraformaldehyde and 30 mM sucrose, for 30 min at 4°C. The cells were washed once in PBS and, after quenching for 10 min, were washed in PBS containing 50 mM NH4Cl and washed again in PBS supplemented with 1 mg/ml BSA. The cells were incubated with primary antibodies in permeabilization buffer A (PBS containing 1 mg/ml BSA and 0.05% saponin or 0.1% Triton X-100) for 45 min at room temperature. After two washes in buffer A, the cells were incubated for 45 min at room temperature in buffer A containing the labeled secondary antibody. After two washes in buffer A and one in PBS, the cells were mounted on microscope slides in 100 mM Tris-HCl buffer, pH 8.5, containing 100 mg/ml Mowiol (Calbiochem) and 25% (vol/vol) glycerol. The antibodies used were a mouse monoclonal antibody CTR433 (a gift of M. Bornens, Institut Curie, Paris, France); a mouse monoclonal antibody raised against clathrin light chains (American Type Culture Collection, CON-1); a mouse monoclonal antibody raised against γ-adaptin (Sigma, A 4200); preimmune rabbit immunoglobulins (Sigma); Texas Red-conjugated goat anti-mouse immunoglobulins and Alexa 488-conjugated goat anti-rabbit immunoglobulins (Molecular Probes, Eugene, OR); Cy5-conjugated goat anti-mouse immunoglobulins (Amersham-Pharmacia-Biotech). Subcellular fractionation was performed according to the method of
Monneron and d'Alayer (1978) Immunohistochemical Study of Rat Brain Sections Adult Wistar rats were deeply anesthetized using a mixture of
0.5 ml of ketamine (50 mg/ml, Rhône-Mérieux, Lyon,
France) and 0.37 ml of xylazine (2 mg/kg, Bayer, Elkhart, IN).
They were transcardially perfused with 400 ml of 0.1 M phosphate
buffer, pH 7.4, containing 4% paraformaldehyde. The brain and adrenal
glands were removed and postfixed in the same solution. Vibratome
sections, 30–40 μm, were cut and processed for immunocytochemistry
at the optical level, using the immunoperoxidase method as described
previously (Bernard et al., 1997 ). Briefly, sections were
preincubated in PBS containing 10% normal goat serum and incubated for
15 h at room temperature in a solution containing the primary
antibodies. Primary antibodies were affinity-purified anti-phocein
antibodies (0.1 to 0.5 μg/ml in PBS containing 1% normal goat
serum). Control antibodies were unimmunized rabbit antibodies (Sigma),
and solutions of affinity-purified anti-phocein antibodies preadsorbed
on the blotted phocein fusion-protein. The sections were washed,
incubated for 1.5 h in biotinylated goat anti-rabbit antibodies
(1:200; Vector Laboratories, Burlingame, CA), washed, incubated for
1 h in an avidin-biotin-peroxidase solution, and washed.
Peroxidase was revealed by using 3,3′-diaminobenzidine in the presence
of 0.01% H202, with nickel
salt enhancement. Low-magnification light microscopy images were
acquired with a light microscope (Nikon, France S.A.,
Champigny-Sur-Marne, France) coupled to a 3-chip charge-coupled
device camera linked to a computer (Power Mac; Macintosh). The
acquired image dimensions, 768 × 576 pixels, were reduced to
obtain a final resolution of 300 dots per inch.
High-magnification images were obtained from scanned photomicrographs
(1200 × 1200 pixels). An image-editing software (Adobe Photoshop;
Adobe Systems, Mountain View, CA) was used to create montages, which
were printed on a dye sublimation printer. Only contrast and brightness
of images were adjusted digitally.Miscellaneous In Vitro Transcription Translation Assay. A phocein-encoding
plasmid pcDNA 3-phocein was obtained by inserting the phocein insert
contained in pGAD 10-phocein-2.5 into the NotI site of
plasmid pcDNA 3 (Invitrogen, San Diego, CA). Transcription-translation
in vitro assays were performed using the TNT T7 system (Promega,
Madison, WI) and pcDNA3-phocein, in the presence of
[S35]methionine (1000 Ci/mmol; ICN, Costa Mesa,
CA).Gel Filtration. Brain cytosol (3.7 mg of protein in 3 ml of
TBS containing 1 mM EDTA) was performed on a Biogel A 5m column
(1.9 × 85 cm; calibrated as indicated by d'Alayer et
al., 1983 ). Calibrating proteins were porcine thyroglobulin (TG;
r = 9.8 nm), β-gal (r = 8.2 nm),
catalase (cat; r = 5.2 nm), and alcohol dehydrogenase
(ADH; r = 4.6 nm).Sucrose Gradients. Brain cytosol (400 μl, 0.6 mg of
protein) was layered on 11-ml sucrose gradients, 15–45%, and
centrifuged for 16 h at 105.000 × g at 4°C.
After centrifugation, 21 fractions (450 μl each) were collected,
starting from the bottom of the gradient. Calibrating proteins were ADH
(7.4 S), cat (11.3 S), apoferritin (17.2 S), and TG (19 S). | |||||||||||||||||
RESULTS Identification of Phocein and Domain Prediction A yeast two-hybrid screen of a rat brain library conducted with a
LexA-striatin fusion protein yielded a clone containing an insert of
2.5 kb encoding a 225-amino acid (aa) ORF. The corresponding protein,
of 26 kDa theoretical molecular weight, has been named phocein
(Figure 1A). The ATG codon lies within a
classical eukaryotic translation start sequence. An in vitro
transcription-translation–coupled assay showed that the plasmid
encoding phocein directs the synthesis of a protein of 26 kDa, the
predicted molecular mass (Figure
2, lane A; in lane B, brain cytosol
present on the same blot was revealed by anti-phocein affinity-purified
antibodies). Two polyadenylation signals are present in the
3′-untranslated sequence at nucleotides 923 and 2456 (Baillat and
Castets, unpublished results). A BLASTN 2 search resulted in
several matches (Altschul et al., 1997 ). One, a mouse cDNA
named 2C4D (970 bp, accession number U01138) is 98% identical with
phocein cDNA (Temeles et al., 1994 ). However, a 1-bp
frameshift with respect to the phocein sequence, due to a sequencing
error, results in a shorter ORF with a different C-terminal deduced
protein sequence. Several other matches occurred in human cDNAs
and genomic sequences, some of which contain the entire phocein gene,
encoding protein sequences 100% identical with the rat sequence. Some
of these sequences localize to chromosome 11. Search in expressed
sequence tag databases allowed us to predict the correct mouse
cDNA sequence, encoding a putative protein of 26 kDa, 100% identical
with the rat sequence, and we were able to establish that there are
several stop codons upstream of the translation start ATG. The
Drosophila melanogaster gene encoding phocein is now known
(chromosome II; accession number AAF57380) and encodes a protein 223 aa
long that is 80% identical to the human sequence. In the worm
Caenorhabditis elegans chromosome III, a sequence
(AAB59173) encodes a predicted protein sequence of 223 aa, which
is 67% identical to the human sequence.
In addition to finding orthologues of phocein in different species by a
Blast search, a Proscan search (Bairoch et al., 1997 Between the two blocks containing homologous stretches, phocein
displays a 100-aa–long stretch (aa 61–160) that has no counterpart in
ς subunits and contains a putative SH3-binding domain (aa 120–124,
underlined in Figure 1A) of the type PxxDY recently described by
Mongiovi et al. (1999) Phocein Is Expressed in Many Tissues Multitissue rat Northern blots (Figure
3 A) were analyzed with a phocein probe
(Figure 3B). Phocein transcripts were prominent in cerebellum, brain,
spinal cord, and especially the adrenal gland. Two transcripts of 2.5
and 1.2 kb were present, the 2.5-kb transcript being more abundant. The
presence of two transcripts could be explained by the fact that in rat
there are two polyadenylation sites. Blots exposed for a longer time
also revealed phocein transcripts in muscle, heart, and several other
tissues.
Rabbit antibodies were raised against a GST–whole-length phocein fusion protein. They were affinity purified and used to follow phocein expression at the protein level. As shown in Figure 3C, cytosolic phocein was abundant in brain and cerebellum, in agreement with the Northern blot data, and in the adrenal gland. However, the amount of cytosolic phocein in this gland was not as high as would be expected from the abundance of the transcripts; the adrenal gland also contained a sizable amount of particulate phocein, but other tissues did also. In spleen, on the contrary, the amount of protein was larger than expected from the Northern blots. Intestinal phocein migrated with a slightly different apparent molecular mass than in other tissues; furthermore, one additional abundant protein of slightly lower apparent molecular mass was immunolabeled in liver. Brain Phocein Is Both Cytosolic and Associated with Membranes Brain fractionation showed that phocein is distributed both in the
cytosol (40%) and in the particulate, detergent-soluble fraction
(60%). Phocein was wholly solubilized from a 100,000 ×
g pellet using nonionic detergents such as Lubrol-PX or
zwitterionic detergents such as
3-([3-cholamidopropyl]dimethylammonio)-2-hydroxy-1-propanesulfonate.
Striatin and the other proteins of its family, zinedin and SG2NA,
behave in the same way (Castets et al., 1996 , 2000 ; Bartoli
et al., 1998 ).Data from gel filtration experiments and sucrose gradient
centrifugations showed that brain cytosolic phocein was present in
fractions corresponding to proteins or protein complexes of larger size
than expected for each monomer. Gel filtration showed that, although
some cytosolic phocein eluted in fractions compatible with the expected
Stokes radius (~4 nm if globular), most of it eluted in fractions of
much larger Stoke's radii, 7–10 nm (Figure
4 A), similar, for instance, to values
obtained for clathrin adaptor complexes (7.0 for AP-1 and AP-2, 8.5 for
AP-3, 6.5 nm for AP-4; Dell'Angelica et al., 1999
Phocein Interacts with All Members of the Striatin Family The two-hybrid strategy in yeast demonstrated that phocein
directly interacts with striatin. Coimmunoprecipitation and pull-down
assays using brain fractions were performed to confirm this interaction
in vitro and to see whether phocein also interacts with zinedin and
SG2NA, which share the same protein-protein association modules.
Striatin (Figure 5, lane 1, top), SG2NA
(Figure 5, lane 2, top), and zinedin (Figure 5, lane 3, top) contained
in rat brain cytosol were coimmunoprecipitated along with phocein by
anti-phocein antibodies (Figure 5, lanes 1–3, bottom) but not by
rabbit unimmunized antibodies (Figure 5, lane 4). Coimmunoprecipitation
of these proteins also occurred when detergent-solubilized membranes
were used (Baillat and Monneron, unpublished results).
Conversely, phocein from rat brain cytosol (Figure 5, lanes 5–7,
bottom) or from solubilized brain membranes (Baillat and
Monneron, unpublished results) was coimmunoprecipitated along
with striatin (Figure 5, lane 5, top), SG2NA (Figure 5, lane 6, top),
and zinedin (Figure 5, lane 7 top), using the respective
affinity-purified antibodies, whereas striatin, SG2NA, and zinedin were
not detected when Pansorbin cells coated with control antibodies were
used (as in Figure 5, lane 4).
In pull-down experiments, in which glutathione-Sepharose beads saturated with GST-phocein or with GST were incubated with rat brain cytosol or solubilized membranes, endogenous striatin was retained on GST-phocein–coated beads (Figure 5, lane 9), as well as SG2NA (Figure 5, lane 10) and zinedin (Figure 5, lane 11), but not on GST–coated beads (Figure 5, lane 8). Phocein and SG2NA Colocalize over the Golgi Area of HeLa Cells Because subcellular localization of proteins can be
conveniently studied in HeLa and Hep-2 cells, which are amenable to
treatment with various drugs, we studied the localization of phocein
and SG2NA in the two cell lines. SG2NA was expressed in both, whereas
striatin was not, a fact consistent with the restricted expression of
the latter protein to a few species of neurons (Castets et
al., 1996 ). Because the results obtained with the two cell lines
were very similar, only the results obtained with HeLa cells are shown.
As shown in Figure 6, antibodies against
phocein (A) and SG2NA (D) strongly revealed a seemingly tubular,
reticulated, juxtanuclear network that appeared to be the Golgi
apparatus. The staining observed with either antibody was considered to
be specific because it was not observed with preimmune immunoglobulins
or with antibodies raised against striatin or antibodies preincubated
with the respective antigens (Baude and Monneron, unpublished
results). The presumptive Golgi labeling was confirmed by the use of
the CTR433 antibody, which recognizes a medial Golgi resident protein
(Figure 6, B and E) (Jasmin et al., 1989 ). The
colocalization of phocein and SG2NA with the CTR433-immunolabeled
protein was indicated by classical immunofluorescence (Benmerah,
unpublished results) and confirmed by combined confocal images,
showing yellow staining of the Golgi apparatus in the combined images
(Figure 6, C and F). Phocein and SG2NA only partially colocalized with
clathrin and AP-1 (γ-adaptin) at the level of the trans-Golgi network
(Benmerah, unpublished results). The use of confocal microscopy
showed that, close to some areas of the plasma membrane, bright signals
from SG2NA were observed (Figure 6, D, arrows). Such labeling was not
seen in the case of phocein.
Phocein Association with the Golgi Complex Is Sensitive to BFA Golgi-associated clathrin coats containing the adaptor complex
AP-1 are dispersed away from Golgi membranes by BFA, whereas plasma
membrane-associated clathrin coats containing the adaptor complex AP-2
are not modified by BFA (Klausner et al., 1992 ; Robinson and
Kreis, 1992 ). When HeLa cells were treated for 2 min with BFA, AP-1
became dispersed throughout the cell (Figure
7, compare A and B). A shorter time (1
min) lead to little effect on AP-1 localization (Benmerah,
unpublished results). Phocein was dispersed in exactly the same
way as was AP-1 (Figure 7, 2-min treatment, compare C and D; Benmerah,
unpublished results). With respect to the BFA treatment, SG2NA
behaved just as did phocein and AP-1 (Benmerah, unpublished
results). The effect of BFA was specific of Golgi-associated coats
because a 10-min treatment had no effect on plasma membrane-associated
clathrin coats, as shown by the fact that the peripheral punctate
clathrin staining seen at the plasma membrane was not affected (Figure
7, compare insets in G and H). Phocein was not detected where coated
pits were observed (Figure 7, compare insets in F and H).
Next, we investigated whether phocein actually binds membranes in HeLa
cells. Subfractionation of lysed HeLa cells on sucrose gradients
yielded, in addition to nuclei, three different fractions: 1) floated,
light membranes, originating from plasma membrane, smooth endoplasmic
reticulum (ER), and Golgi; 2) cytosol; and 3) heavy membranes
(mitochondria, some nuclear membranes, lysosomes, and cytoskeleton) (%
protein content of the fractions: 5, 30, 65) (Monneron and d'Alayer,
1978 In the CNS, Phocein Immunoreactivity Occurs in Neurons and
Is Somato-dendritic Immunocytochemistry at the optical level was applied to rat brain,
cerebellum, brain stem, and adrenal gland sections using
affinity-purified antibodies. Immunoreactivity for phocein was present
throughout the rat brain (all cortical layers, including both pyramidal
cells, Figure 8A, and nonpyramidal cells;
amygdaloid, septal, habenular, and thalamic complexes; hippocampus, all
layers; caudate-putamen), cerebellum (essentially molecular layer and
Purkinje cell bodies, glomeruli, and some Golgi cells, Figure 8 B; deep
cerebellar nuclei), and brainstem. The strongest labeling occurred
in the motor nuclei of cranial nerves in the pons and the bulb
(Figure 8, C–F).
In all the examined brain structures, immunoreactivity for phocein was present only in neurons, not in glial cells. The labeling was intracytoplasmic, restricted to cell bodies and dendrites (Figure 8, A-F). Importantly, phocein was excluded from axons, just as were striatin and SG2NA. Nuclei were unlabeled (Figure 8, A–F). In the soma (Figure 8, A–D and F), the labeling was particulate and often reticulated, suggesting staining of the ER. Perinuclear staining was usual, probably due to the staining of the Golgi apparatus (Figure 8, A, C, and D). Labeling was vermiculated all along the dendrites (Figure 9, A and E: proximal dendrites). Preliminary ultrastructural studies of the cerebellum molecular layer indicate that phocein is present within spines, associated with membrane profiles (Y. Bailly, unpublished data). In addition to brain structures, the adrenal gland was studied. The medulla strongly reacted with anti-phocein antibodies, whereas the cortex was much less stained (Monneron, unpublished results). Phocein and SG2NA distributions in this gland are thus comparable. Control experiments showed that 1) the omission of anti-phocein antibody resulted in the removal of all staining; 2) no staining was obtained when either preimmune sera or rabbit control immunoglobulins were used as primary antibodies; and 3) preabsorption of the anti-phocein antibodies with blotted GST-phocein resulted in the absence of staining. | |||||||||||||||||
DISCUSSION The rationale to identify the partners of striatin was that major
cellular and physiological effects had been observed following striatin
down-regulation (Bartoli et al., 1999a Fractionation of brain homogenate shows that phocein is distributed, in
approximately equal amounts, in the cytosol, where it behaves as a
protein complex, and in the membrane fraction. The proteins belonging
to the striatin family are identically distributed (Castets et
al., 2000 An interesting finding was the sequence homology between the N- and
C-terminal domains of phocein and the ς subunits of adaptor proteins,
the two homologous phocein domains being separated by an additional,
intervening stretch containing a putative SH3-binding motif. The
sequence homology between phocein and ς subunits led us to
hypothesize that phocein might be a component of a novel coat, quite
different from known adaptors. Immunolocalization studies of phocein in
cultured cells (HeLa and Hep-2 cells) indeed showed that phocein is
conspicuous over the Golgi area, where it colocalizes with SG2NA. To be
recruited onto the Golgi complex, AP-1 and AP-3 need the intervention
of ADP-ribosylation factors (Arfs), at variance with the AP-2 complex
at the plasma membrane. Treatment of cells with BFA, which inhibits the
activity of Arf exchange factors, results in the dispersion of
Arf-dependent coats and, within minutes, in the fragmentation of the
Golgi complex, which is cycled back to the ER (Donaldson et
al., 1992 Although the localization of phocein to the Golgi complex in HeLa cells
and its sensitivity to BFA are compatible with the hypothesis of a role
for phocein in vesicular traffic, at the moment we are lacking
functional data to support that idea. The distribution of phocein
within polarized cells such as neurons neither confirms nor denies this
hypothesis. Within adult brain neurons, phocein is not confined to the
Golgi complex. As seen in rat brain sections, phocein immunolabeling,
although it filling the soma, is found within dendrites down to the
most distal and tenuous branches and spines. Noticeably, the axons are
unstained. Striatin, SG2NA, and phocein thus share exactly the same
subcellular distribution. Such polar distributions are quite important
to stress, because, if the hypothesis of a relationship between phocein
and vesicular traffic holds true, no neuronal coats or proteins
involved in vesicular traffic have been, to our knowledge, demonstrated
to be restricted to either axons or dendrites, with the exception of
EEA1 (Wilson et al., 2000 It is noteworthy to recall that phocein is highly conserved throughout the animal kingdom: the phocein orthologues found in D. melanogaster and C. elegans are unusually conserved (80% identity between fly and human, 67% between worm and human). Studies of phocein in such species should therefore help determine its function, which appears to be a very conserved cellular process. | |||||||||||||||||
ACKNOWLEDGMENTS The pertinent suggestions and comments from Pietro De Camilli have been very helpful. We also thank Domenica Borgese, Bruno Goud, André LeBivic, and John Bergeron for critical reading of the manuscript. We thank Michel Bornens for supplying the CTR433 antibody, Yann Goureau for expert assistance in confocal microscopy, and Yves Colette for supplying HeLa cells. This work was supported by Centre National de la Recherche Scientifique and by grants from the Association pour la Recherche sur le Cancer (ARC 9318 to A.M. and F.C. and ARC 9679 to A. Benmerah) and Association Française contre les Myopathies (AFM FRN 210/6481). Abdelaziz Moqrich was supported by the Association pour la Recherche sur le Cancer (1999) and by the Lilly Foundation (2000). | |||||||||||||||||
Abbreviations used: | |||||||||||||||||
REFERENCES
| |||||||||||||||||
