Old and new pathways of protein export in chloroplasts and bacteria

Targeting of chloroplast proteins to the thylakoid membrane is analogous to bacterial secretion, and much of what we know has been learned from secret ory mechanisms in Escherichia coli. However, chloroplasts also use a Delta pH-dependent pathway to target thylakoid proteins, at least some of which a re folded before transport Previously, this pathway seemed to have no cogna te in bacteria, but recent results have shown that the HCF106 gene in maize encodes a component of this pathway and has bacterial homologues. This Del ta pH-dependent pathway might be an ancient conserved mechanism for protein translocation that evolved before the endosymbiotic origin of plastids and mitochondria.