The cyanobacterial small CAB-like proteins (SCPs) consist of one-helix proteins that resemble transmembrane regions of the light-harvesting proteins of plants. To determine whether these proteins are associated with protein complexes in the thylakoid membrane, an abundant member of the SCP family, ScpD, was marked with a His tag, and proteins co-isolating with His-tagged ScpD were identified. These proteins included the major Photosystem (PS) II components as well as FtsH, which is involved in degradation of the PSII complex. To ascertain specific interaction between ScpD and the PSII complex, the His-tagged protein fraction was subjected to two-dimensional blue native/ SDS-PAGE. Again, PSII components were co-isolated with ScpD-His, and ScpD-His was found to interact most strongly with CP47. ScpD association was most prominent with the monomeric form of PSII, suggesting ScpD association with PSII that is repaired. Using antibodies that recognize both ScpC and ScpD, we found the ScpC protein, which is very similar in primary structure to ScpD, to also co-isolate with the PSII complex. In contrast, ScpE did not co-isolate with a major protein complex in thylakoids. A fourth member of the SCP family, ScpB, could not be immunodetected, but was found by mass spectrometry in samples co-isolating with ScpD-His. Therefore, ScpB may be associated with ScpD as well. No association between SCPs and PSI could be demonstrated. On the basis of these and other data presented, we suggest that members of the SCP family can associate with damaged PSII and can serve as a temporary pigment reservoir while PSII components are being replaced.In organisms performing oxygenic photosynthesis, sunlight is absorbed by chlorophylls and other pigments, and absorbed excitation energy is transferred to the reaction centers, where the photochemical process of converting excitation energy to chemical (redox) energy takes place. These pigments are bound to proteins to keep them in their proper location and orientation so that the energy transfer is efficient and rapid and so that toxic triplet states can be quenched effectively. In plants, the vast majority of pigments, including chlorophylls a and b and various carotenoids, are bound to a family of integral membrane proteins called the light-harvesting complex (LHC).

Edited by Richard CogdellKeywords: Photosystem I Photosystem II Small Cab-like protein Protein turnover Half-life time a b s t r a c tThe half-life times of photosystem I and II proteins were determined using 15 N-labeling and mass spectrometry. The half-life times (30-75 h for photosystem I components and <1-11 h for the large photosystem II proteins) were similar when proteins were isolated from monomeric vs. oligomeric complexes on Blue-Native gels, suggesting that the two forms of both photosystems can interchange on a timescale of <1 h or that only one form of each photosystem exists in thylakoids in vivo. The half-life times of proteins associated with either photosystem generally were unaffected by the absence of Small Cab-like proteins.

. However, the half-life time of chlorophyll was 5-fold shorter in the photosystem I-less/ scpABCDE ؊ strain than in the photosystem I-less strain even when grown at low light intensity (ϳ3 mol photons m ؊2 s ؊1 ) (32 ؎ 5 and 161 ؎ 25 h, respectively). In other photosystem I-less mutants that lacked one to four of the scp genes the chlorophyll lifetime was in between these two values, with the chlorophyll lifetime generally decreasing with an increasing number of inactivated scps. In contrast, the chlorophyll biosynthesis rate was only marginally affected by inactivation of scps except when all five scp genes were deleted. Small Cab-like protein deficiency did not significantly affect photoinhibition or turnover of photosystem II-associated ␤-carotene. It is concluded that SCPs do not alter the stability of functional photosystem II complexes but retard the degradation of photosystem II-associated chlorophyll, consistent with the proposed involvement of SCPs in photosystem II re-assembly or/and repair processes by temporarily binding chlorophyll while photosystem II protein components are being replaced.