Organization and Stability of Polypeptides Associated with the Chlorophyll <italic>a-b</italic> Light-Harvesting Complex of Photosystem-II

In the oxygen-evolving photosystem-II (PSII) of higher plant chioroplasts and green algae, most of the light-harvesting function is performed by the chlorophyll (Chl) a-b-protein complex (LHC-II). On the average, the LHC-II contains about 210 Chl (a+b) molecules per PSII reaction center. The polypeptide composition, copy number and organization of assembly in the LHC-II complex are not fully understood at present. This work utilized the chlorina f2 mutant of barley (lacking Chl b and having a LHC-II antenna of only 13 Chl a molecules) to determine the organization and stability of assembly of proteins in the LHC-II. High-resolution SDS-PAGE and immunoblot analysis showed the presence of four main constitutive polypeptides in the wild-type LHC-II (termed here subunits a, b, c and d) with molecular masses in the range 30–25 kDa. Of those, only subunit d (a 25 kDa polypeptide) was found to occur at an equal copy number per PSII reaction center in both wild-type and in the Chl b-less chlorina f2 mutant. All other subunits were either absent or existed in much lower amounts in the mutant. Subunit d is a polypeptide constituent of the major Chl-protein subcomplex (CPII) of the LHC-II. It is stably incorporated in the thylakoid membrane in the absence of Chl b and probably binds the 13 Chl a molecules in the residual LHC-II antenna of the chlorina f2 mutant. We propose that, of all LHC-II polypeptides, subunit d is most proximal to the PSII core and may serve as a linker in the process of excitation energy transfer from the bulk LHC-II to the PSII reaction center in chloroplasts.