The proteolysis adaptor, NblA, is essential for degradation of the core pigment of the cyanobacterial light-harvesting complex.

Abstract

The cyanobacterial light-harvesting complex, the phycobilisome, is degraded under nutrient limitation, allowing the cell to adjust light absorbance to its metabolic capacity. This large light-harvesting antenna comprises a core complex of the pigment allophycocyanin, and rod-shaped pigment assemblies emanating from the core. NblA, a low-molecular-weight protein, is essential for degradation of the phycobilisome. NblA mutants exhibit high absorbance of rod pigments under conditions that generally elicit phycobilisome degradation, implicating NblA in degradation of these pigments. However, the vast abundance of rod pigments and the substantial overlap between the absorbance spectra of rod and core pigments has made it difficult to directly associate NblA with proteolysis of the phycobilisome core. Furthermore, lack of allophycocyanin degradation in an NblA mutant may reflect a requirement for rod degradation preceding core degradation, and does not prove direct involvement of NblA in proteolysis of the core pigment. Therefore, in this study, we used a mutant lacking phycocyanin, the rod pigment of Synechococcus elongatusPCC7942, to examine whether NblA is required for allophycocyanin degradation. We demonstrate that NblA is essential for degradation of the core complex of the phycobilisome. Furthermore, fluorescence lifetime imaging microscopy provided insitu evidence for the interaction of NblA with allophycocyanin, and indicated that NblA interacts with allophycocyanin complexes that are associated with the photosynthetic membranes. Based on these data, as well as previous observations indicating interaction of NblA with phycobilisomes attached to the photosynthetic membranes, we suggest a model for sequential phycobilisome disassembly by NblA.