Research
Mechanisms of Protein Import into Chloroplasts
Most proteins in chloroplasts are encoded by the nuclear genome and post-translationally imported from the cytosol. Nuclear-encoded chloroplast proteins can be divided into two groups: proteins targeted to the interior of chloroplasts and proteins inserted into the outer membrane. Most interior proteins are synthesized as higher molecular weight precursors with N-terminal extensions called transit peptides. The import process requires ATP and a proteinaceous receptor complex (the translocon) in the chloroplast envelope. In contrast, most outer membrane proteins are synthesized at their mature size without cleavable transit peptides. Less is know about how proteins are inserted into the outer membrane.
Our lab is interested in understanding the mechanisms of protein import into chloroplasts. For protein targeting to the outer membrane, we have identified the outer-membrane targeting signal from two outer membrane proteins and analyzed their insertion requirements. We have found that outer-membrane and interior protein import pathways converge at the protein-conducting channel Toc75 (8). We are also studying protein import into the interior of chloroplasts using a genetic approach (2). From the mutants we characterized, we found that a chloroplast-specific galactolipid is important for protein import into chloroplasts, and the translocon complex is under specific developmental and cell-type-specific regulations (3). We have also identified the first transcription regulator of the translocon genes (2), a co-chaperone in the translocon complex that coordinates translocation steps on the stromal side of the translocon (5, 12), and a possible channel component for protein translocation across the inner membrane (10). Through analyses of the interactions and assembly (11) of translocon components, a model for sequential steps of protein import into chloroplasts is being built.
