Comparative Immunology of the Phycobilisome Linker Polypeptides

Abstract

Since the discovery of linker polypeptides by Tandeau de Marsac and Cohen-Bazire (1), these proteins have been found in all cyanobacterial and red algal phycobi 1 isome (PBsomes) examined. They are critical to the ordered assembly of this very efficient light harvesting antenna. They can be grouped into those polypeptides essential for the assembly of the PBsome rods and rod to core attachment and those needed for the association of allophycocyanin (APC) into the core and its attachment to the thylakoid (2,3). We have identified the function of these linker polypeptides in Nostoc sp. These PBsomes, not atypical of other cyanobacteria, are composed of an APC core, phycoerythrin- (PE) and phycocyanin- (PC) containing rods and five additional polypeptides of 95, 34.5, 34, 32 and 29 kD. The 95 kD polypeptide anchors the PBsome to the thylakoid membrane and is the direct mediator of excitation energy transfer from the PBsome to chl a (4); the 29 kD polypeptide attaches the rods to the core (5). in cells grown in cool white fluorescent light, the 32 kD polypeptide links two PE hexamers, and the 34 kD polypeptide mediates association of PE and with PC hexamers. in red light adapted cells, levels of PE and the associated 32 and 34 kD polypeptides are much reduced; in their stead are rods composed of PC and a 34.5 kD polypeptide which serves to link PC hexamers (6).