When cyanobacteria are starved for
nitrogen, expression of the NblA
protein increases and thereby induces proteolytic degradation of
phycobilisomes, light-harvesting complexes of pigmented
proteins.
Phycobilisome degradation leads to a color change of the cells from blue-green to yellow-green, referred to as bleaching or
chlorosis. As reported previously, NblA binds via a conserved region at its C terminus to the alpha-subunits of
phycobiliproteins, the main components of
phycobilisomes. We demonstrate here that a highly conserved stretch of
amino acids in the N-terminal helix of NblA is essential for
protein function in vivo. Affinity purification of
glutathione S-transferase-tagged NblA, expressed in a Nostoc sp. PCC7120 mutant lacking wild-type NblA, resulted in co-precipitation of ClpC, encoded by open reading frame alr2999 of the Nostoc chromosome. ClpC is a HSP100 chaperone partner of the
Clp protease.
ATP-dependent binding of NblA to ClpC was corroborated by in vitro pull-down assays. Introducing
amino acid exchanges, we verified that the conserved N-terminal motif of NblA mediates the interaction with ClpC. Further results indicate that NblA binds
phycobiliprotein subunits and ClpC simultaneously, thus bringing the
proteins into close proximity. Altogether these results suggest that NblA may act as an adaptor
protein that guides a ClpC.ClpP complex to the
phycobiliprotein disks in the rods of
phycobilisomes, thereby initiating the degradation process.