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Mechanisms and early evolution of protocellular transport
PI: Pohorille, Andrew
This proposal is devoted to investigations of the emergence and early evolution of nutrient transport across cell walls, which was an essential step in the origins of cellular life. We assume that the earliest mechanism of transport was permeation through membranes, and it must have sufficed to deliver the required monomers and charged species to the interior of protocells. We further argue that, once simple proteins became available, some of them were recruited into membranes and self-organized into channels. Despite their simplicity, these channels were capable of mediating transport of ions and small molecules across protocellular walls at greatly increased rates, thus facilitating faster evolution. To test key elements of our hypotheses we will examine simple, molecular models of unassisted and protein-mediated protocellular transport and generalize our results in terms of their implications for the origins of life on Earth and elsewhere in the universe. Our studies will be pursued through computer simulations and validated in laboratory measurements. By combining recent theoretical improvements with advances in massively parallel computing we expect to achieve efficiencies that have not been reached in protobiological simulations.
We propose two specific aims. One is to determine the mechanism and rates of unassisted transport of ribonucleotides, their molecular components and their di- and triphosphate derivatives across membranes formed by fatty acids or phospholipids. Our focus on ribonucleotides is motivated by their role as the building blocks of RNA, which is thought to be the first self-replicating and catalytic polymer. The results will not only help to develop more fully the RNA World hypothesis but will also guide experiments aimed at creating laboratory models of protocells.
Our second aim is to explain how transmembrane channels formed through self-association of small, helical proteins can efficiently mediate transport across protocellular walls, and how the channel size and amino acid sequence influences efficiency and selectivity of transport. As a model, we will use a simple channel formed by small, trichotoxin proteins. We expect that the results of this study combined with our pervious work on related systems will allow us to establish the minimal requirements for proteins to mediate selective ion transport, and to develop a comprehensive theory of the earliest evolution of ion channels.
The proposed studies support Goal 3, Subgoal 3C of the NASA Strategic Plan and directly addresses Goal 3 of the Astrobiology Roadmap. By focusing on transport of nutrients, which both facilitated and constrained evolution of protocells, this work is relevant to objectives 3.2 and 3.4 devoted to the origins and evolution of functional biomolecules and origins of cellularity and protobiological systems, respectively.
