Assembly Apparatus of Light-Harvesting Complexes; Identification of Alb3.1-cpSRP-LHCP Complexes in the Green Alga Chlamydomonas reinhardtii.

Abstract

The unicellular green alga, Chlamydomonas reinhardtii, contains many light-harvesting complexes (LHCs) associating chlorophylls a/b and carotenoids; the major light-harvesting complexes, LHCIIs (types I, II, III, and IV), and minor light-harvesting complexes, CP26 and CP29, for photosystem II, as well as nine light-harvesting complexes, LHCIs (LHCA1-9), for photosystem I. A pale green mutant BF4 exhibited impaired accumulation of LHCs due to deficiency in Alb3.1 gene which encodes the insertase involved in insertion, folding and assembly of LHC proteins in the thylakoid membranes. To elucidate the molecular mechanism by which ALB3.1 assists LHC assembly, we complemented BF4 to express ALB3.1 fused with no, single, or triple HA tag at its C-terminus (cAlb3.1, cAlb3.1-HA, or cAlb3.1-3HA). The resulting complemented strains accumulated most LHC proteins comparable to wild-type levels. The affinity purification of Alb3.1-HA and Alb3.1-3HA preparations showed that ALB3.1 interacts with cpSRP43 and cpSRP54 proteins of chloroplast signal recognition particle cpSRP and several LHC proteins; two major LHCII proteins (types I and III), two minor LHCII proteins (CP26 and CP29), and eight LHCI proteins (LHCA1, 2, 3, 4, 5, 6, 8, and 9). Pulse-chase labeling experiments revealed that the newly synthesized major LHCII proteins were transiently bound to the Alb3.1 complex. We propose that Alb3.1 interacts with cpSRP43 and cpSRP54 to form an assembly apparatus for most LHCs in the thylakoid membranes. Interestingly, PSI proteins were also detected in the Alb3.1 preparations, suggesting that the integration of LHCIs to a PSI core complex to form a PSI-LHCI subcomplex occurs before assembled LHCIs dissociate from the Alb3.1-cpSRP complex.