Protein Import and Routing Systems of Chloroplasts

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Abstract
Plastids are ubiquitous plant cell organelles that perform many essential functions. Chloroplasts are the most com-plex type of plastid, both structurally and functionally, con-taining six distinct compartments and performing essential processes, such as fatty acid and amino acid biosynthesis in addition to their well-known role in photosynthesis. The many plastid proteins needed for these and other essential metabolic pathways derive from two genetic systems. The plastid genome encodes,100 proteins (Sugiura, 1989), but the vast majority of plastid proteins are encoded by nuclear genes and synthesized as precursors in the cytoplasm. Consequently, specific and reliable protein transport mech-anisms are needed to direct these precursor proteins to their proper location within chloroplasts. Most studies of protein transport to and within chloroplasts have been performed with isolated organelles, whereby the general process of protein transport, as schematically pre-sented in Figure 1, can be broken down into three distinct phases: specific targeting of precursor proteins to plastids, transport across the two envelope membranes, and in many cases, sorting to the proper plastidic compartment. One major conclusion derived from these studies is that proper localization of cytoplasmically synthesized chloroplastic proteins is accomplished by specific interactions between targeting sequences contained within the precursor proteins and the transport machinery contained within chloroplasts. This review focuses on the transport process and the ma-chinery that mediates it; it also includes a brief consideration of targeting sequences. Many excellent reviews have cov-ered earlier progress in understanding chloroplastic protein transport (Keegstra et al., 1989; de Boer and Weisbeek, 1991; Schnell, 1995, 1998; Cline and Henry, 1996; Kouranov and Schnell, 1996; Fuks and Schnell, 1997; Lübeck et al., 1997a; Heins et al., 1998); these reviews can be consulted for additional details.