The precursor protein contains an N-terminal chloroplast signal sequence (red), followed immediately by a thylakoid signal sequence (orange). The chloroplast signal sequence initiates translocation into the stroma through a membrane contact site by a mechanism similar to that used for translocation into the mitochondrial matrix. 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 four routes: (1) a Sec pathway, so called because it uses components that are homologs of Sec proteins, which mediate protein translocation across the bacterial plasma membrane (discussed later), (2) an SRP-like pathway, so called because it uses a chloroplast homolog of the signal recognition particle, or SRP (discussed later), (3) a ΔpH pathway, so called because it is driven by the H+ gradient across the thylakoid membrane, and (4) a spontaneous insertion pathway that seems to require no protein translocator for membrane integration.