Localization of the small CAB-like proteins in photosystem II.

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.