Protein transport into and across the thylakoid membrane

Abstract

Protein transport into or across the thylakoid membrane of higher plant chloroplasts proceeds via at least four different, protein-specific pathways which are mechanistically distinct. The Sec-dependent pathway requires nucleoside triphosphates and a SecA protein in the stroma, the ΔpH-dependent route is apparently exclusively dependent on the transthylakoid proton gradient, the SRP-dependent pathway requires a transthylakoid ΔpH, GTP and a chloroplast homologue to the 54 kDa subunit of the signal recognition particle and, finally, there is a spontaneous protein transport mechanism for which no physiological prerequisite has so far been found. The Sec-dependent and ΔpH-dependent pathways are utilized predominantly by hydrophilic proteins of the thylakoid lumen, whereas the other two pathways have been described for integral proteins of the thylakoid membrane. With me exception of the SRP-dependent pathway, which is used by signals that are uncleaved in the mature part of the protein, all other transport routes involve cleavable, signal peptide-like thylakoid transfer domains that are removed after transport on the lumenal side of the thylakoid membrane. The distinction between the Sec-dependent and ΔpH-dependent pathways is dictated by the transit peptides, whereas the passenger proteins do not influence the transport route, but demand, in some instances, additional physiological conditions. Apparently, only three pathways are of prokaryotic origin, whereas available evidence suggests that the ΔpH-dependent pathway was newly developed in chloroplasts. So far, only SecA and SecY from the Sec-dependent pathway and the SRP54 homologue from the SRP-dependent pathway have been characterized at the molecular level; nothing is known so far about the composition of the translocation machinery from the ΔpH-dependent pathway.