Degradation of the cyanobacterial protein pigment complexes, the phycobilisomes, is a central acclimation response that controls light energy capture. The small protein, NblA, is essential for proteolysis of these large complexes, which may reach molecular mass of up to 4 MDa. Interactions of NblA in vitro supported the suggestion that NblA is a proteolysis adaptor that labels the pigment proteins for degradation. The mode of operation of NblA in situ, however, remained unresolved. Particularly, it was unclear whether NblA interacts with phycobilisome proteins while yet part of the large complex or alternatively, interaction with NblA necessitates dissociation of pigment subunits from the assembly. Fluorescence intensity profiles demonstrated preferential presence of NblA::GFP at the photosynthetic membranes, indicating co-localization with phycobilisomes. Furthermore, fluorescence lifetime imaging microscopy provided in situ evidence for interaction of NblA with phycobilisome protein pigments. Additionally, we demonstrate in vivo the role of NblA as a proteolysis tag based on the rapid degradation of the fusion protein NblA::GFP compared to free GFP. Taken together, these observations demonstrate in vivo the role of NblA as a proteolysis adaptor. Additionally, interaction of NblA with phycobilisomes indicates that dissociation of protein pigment subunits from the large complex is not a prerequisite for interaction with this adaptor and furthermore, implicates NblA in disassembly of the protein pigment complex. Thus, we suggest that in the case of proteolysis of the phycobilisome the adaptor serves a dual function: undermining the complex stability and designating the dissociated pigments for degradation.
