Following translation, the PBPs assemble into the subunits that form the phycobilisome. During this process, they undergo a number of structural and chemical modifications required for proper functionality. One of the non-covalent changes that occurs is the stabilization of the almost invariant ThrP77 residue (Figure 11.1a, white square), in a configuration of dihedral angles that belong to a typically non-permitted region of the Ramachandran plot ) 51]) This is due to constraints that occur during the trimerization process that brings this residue into close contact with the a84 bilin, thereby shielding it from the solvent and shifting its absorption to the red. This configuration could also, potentially, shield the a84 bilin from the effects of linker binding, unlike the P84 bilin which is accessible to the linker in the trimer cavity.
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