Transport of proteins into and across the thylakoid membrane

The biogenesis of thylakoid proteins is a complex issue that requires the o peration of at least four pathways within the chloroplast. Two of the pathw ays are used for soluble lumenal proteins, where the proteins bear cleavabl e targeting signals that are recognized by one of two distinct translocases . These pathways differ in fundamental respects. A subset of lumenal protei ns are transported in an unfolded state by a typical Sec system, whereas ot hers are transported by a novel class of translocase that appears to functi on primarily in the transport of fully-folded proteins. Related protein tra nslocases have now been shown to operate in a wide variety of bacterial spe cies, suggesting a wide spread requirement for the translocation of folded proteins across biological membranes, Numerous integral membrane proteins a re also targeted into the thylakoid membrane, and these too follow at least two distinct routes. Some proteins use a signal recognition some proteins would be very toxic, even lethal, if active Some proteins use a signal reco gnition particle-dependent pathway that requires GTP and unidentified appar atus in the thylakoid membrane. Others, however, require none of the known targeting factors and may insert spontaneously into the membrane. In this a rticle, the rationale behind this pathway complexity is discussed in relati on to the properties of the substrate proteins and the evolutionary origins of the chloroplast.