Abstract
State transition is a short-term balancing mechanism of energy distribution between PSI and PSII. When PSII is preferentially excited (State 2), a pool of mobile LHCII proteins is thought to migrate from PSII to PSI, but biochemical evidence for a physical association between LHCII proteins and PSI in State 2 is weak. We report the isolation of PSI-LHCI supercomplexes from Chlamydomonas cells locked into State 1 and State 2 by sucrose density gradient centrifugation and gel filtration chromatography. Three LHCII polypeptides associated with a PSI-LHCI supercomplex were identified as two minor monomeric LHCII proteins and one major LHCII protein Type II. The corresponding phylogenetic tree indicates that among the LHCII proteins associated with PSII, these are most similar to the LHC proteins for PSI. These LHCII proteins, which have been viewed as belonging solely to the PSII complex, are now postulated to shuttle between PSI and PSII during state transitions.
