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Translocation of chloroplast precursor proteins into the 

thylakoid space. (A) The precursor protein contains an N-terminal chloroplast signal sequence (red), followed immediately by a thylakoid signal sequence (brown). The chloroplast signal sequence initiates translocation into the stroma by a mechanism similar to that used for the translocation of mitochondrial precursor proteins into the matrix space, although the translocator complexes, named TOC and TIC (for translocator in the outer and inner chloroplast membrane, respectively), are different. The signal sequence is then cleaved off, unmasking the thylakoid signal sequence, which initiates translocation across the thylakoid membrane. (B) Translocation into the thylakoid space or thylakoid membrane can occur by any one of at least three routes: (1) a Sec pathway, so called because it uses components that are homologs of Sec proteins, which mediate protein translocation across the ER and bacterial plasma membrane; (2) an OXA-like pathway, so called because it uses a chloroplast homolog of the OXA translocase; (3) a TAT (twin arginine translocation) pathway, so called because two arginines are critical in the signal sequences that direct proteins into this pathway, which depends on the H+ gradient across the thylakoid membrane. The OXA-like pathway makes use of a chloroplast SRP that lacks an RNA subunit. This specialized SRP located in the stroma recognizes a thylakoid-directed signal sequence and functions exclusively post-translationally because it is found in a separate compartment from the ribosome that made the thylakoid precursor protein