THE ROLE OF LIPIDS IN PLASTID PROTEIN-TRANSPORT

The elaborate compartmentalization of plant cells requires multiple me chanisms of protein targeting and trafficking. In addition to the orga nelles found in all eukaryotes, the plant cell contains a semi-autonom ous organelle, the plastid. The plastid is not only the most active si te of protein transport in the cell, but with its three membranes and three aqueous compartments, it also represents the most topologically complex organelle in the cell. The chloroplast contains both a protein import system in the envelope and multiple protein export systems in the thylakoid. Although significant advances have identified several p roteinaceous components of the protein import and export apparatuses, the lipids found within plastid membranes are also emerging as importa nt players in the targeting, insertion, and assembly of proteins in pl astid membranes. The apparent affinity of chloroplast transit peptides for chloroplast lipids and the tendency for unsaturated MGDG to adopt a hexagonal II phase organization are discussed as possible mechanism s for initiating the binding and/or translocation of precursors to pla stid membranes. Other important roles for lipids in plastid biogenesis are addressed, including the spontaneous insertion of proteins into t he outer envelope and thylakoid, the role of cubic lipid structures in targeting and assembly of proteins to the prolamellar body, and the r epair process of D1 after photoinhibition. The current progress in the identification of the genes and their associated mutations in galacto lipid biosynthesis is discussed. Finally, the potential role of plasti d-derived tubules in facilitating macromolecular transport between pla stids and other cellular organelles is discussed.