Suppression of CP29 Causes Instability of the PSI-LHCI/II Supercomplex in Chlamydomonas reinhardtii Under State 2 Conditions

Abstract

The light harvesting-complexes II (LHCIIs) reversibly migrate between photosystem I (PSI) and photosystem II (PSII) during state transitions. Recently, it was reported that three minor monomeric LHCIIs (CP29, CP26 and LhcbM5) were associated with PSI and formed PSI-LHCI/II supercomplex during state 2 in the green alga Chlamydomonas reinhardtii. In this study, we generated knock-down mutants of CP29 to collect further insights on this monomeric LHCII during state transitions. In a transformant b4i-8, whose expression of CP29 was nearly completely suppressed, a PSI-LHCI/II supercomplex was not detected under state 2-promoting conditions from detergent-solubilized thylakoid membranes. However, the in vivo spectroscopic analysis showed that the PSII antenna size was reduced upon a transition from state 1 to 2 in b4i-8 cells as in wild type. These results suggest that CP29 is required for the formation of PSI-LHCI/II supercomplex, but not for the dissociation of the antenna from PSII.