16.17: Protein Transport to the Outer Chloroplast Membrane
Chloroplast outer membrane proteins encoded by the nucleus are synthesized in the cytosol. Soon after synthesis, they bind cytosolic factors such as 14-3-3 protein and the Hsp70 chaperones that keep these precursors in an unfolded state until their translocation.
Two models describe the mechanism of precursor recognition and entry across the outer membrane through the TOC complex. Model 1 suggests the newly synthesized precursor binds to the TOC receptor 159 and forms a complex. TOC159-precursor complex docks onto the outer chloroplast membrane via another TOC receptor, TOC34, stimulating GTP hydrolysis of TOC34 and TOC159. This allows TOC159 to associate with TOC central channel and facilitate precursor translocation across the outer membrane.
Model 2 suggests that the transit signal of the precursor protein is phosphorylated at its C-terminal by an unknown kinase. TOC34 receptor functions as the initial receptor and binds to the phosphorylated transit signal, undergoing GTP hydrolysis. TOC34 dephosphorylates the transit peptide itself before transferring it to the TOC159 GTPase. GTP hydrolysis by TOC159 further promotes peptide translocation through the TOC complex into the intermembrane space.
As the precursors reach the TIC complex on the inner membrane, a stretch of polyglycine residues close to the transit signal, blocks further translocation. As a result, the precursor gets stalled across the TIC complex and is not translocated into the stroma. Plastid type 1 peptidases present in the intermembrane space cleave the polyglycine signal. The processed precursor is transferred to an insertion protein called outer envelope protein (OEP) or outer membrane protein (OMP) that folds and inserts the processed proteins into the outer membrane of the chloroplast.