Identification of mammalian TOM22 as a subunit of the preprotein translocase of the mitochondrial outer membrane.
Saeki K,
Suzuki H,
Tsuneoka M,
Maeda M,
Iwamoto R,
Hasuwa H,
Shida S,
Takahashi T,
Sakaguchi M,
Endo T,
Miura Y,
Mekada E,
Mihara K.
Institute of Life Science, Kurume University, 2432-3, Aikawa-machi, Kurume 839-0861, Japan.
A mitochondrial outer membrane protein of approximately 22 kDa (1C9-2) was purified from Vero cells assessing immunoreactivity with a monoclonal antibody, and the cDNA was cloned based on the partial amino acid sequence of the trypsin-digested fragments. 1C9-2 had 19-20% sequence identity to fungal Tom22, a component of the preprotein translocase of the outer membrane (the TOM complex) with receptor and organizer functions. Despite such a low sequence identity, both shared a remarkable structural similarity in the hydrophobicity profile, membrane topology in the Ncyt-Cin orientation through a transmembrane domain in the middle of the molecule, and the abundant acidic amino acid residues in the N-terminal domain. The antibodies against 1C9-2 inhibited the import of a matrix-targeted preprotein into isolated mitochondria. Blue native polyacrylamide gel electrophoresis of digitonin-solubilized outer membranes revealed that 1C9-2 is firmly associated with TOM40 in the approximately 400-kDa complex, with a size and composition similar to those of the fungal TOM core complex. Furthermore, 1C9-2 complemented the defects of growth and mitochondrial protein import in Deltatom22 yeast cells. Taken together, these results demonstrate that 1C9-2 is a functional homologue of fungal Tom22 and functions as a component of the TOM complex.
PMID: 10900208 [PubMed - indexed for MEDLINE]