Interactions of linker proteins with the phycobiliproteins in the phycobilisome substructures of Gloeobacter violaceus.

Gloeobacter violaceus PCC 7421 is a unicellular oxygenic photosynthetic organism, which precedes the diversification of cyanobacteria in the phylogenetic tree. It is the only cyanobacterium that does not contain internal membranes. The unique structure of the rods of the phycobilisome (PBS), grouped as one bundle of six parallel rods, distinguishes G. violaceus from the other PBS-containing cyanobacteria. It has been proposed that unique multidomain rod-linkers are responsible for this peculiarly organized shape. However, the localization of the multidomain linkers Glr1262 and Glr2806 in the PBS-rods remains controversial (Koyama et al. 2006, FEBS Lett 580:3457-3461; Krogmann et al. 2007, Photosynth Res 93:27-43). To further increase our understanding of the structure of the G. violaceus PBS, the identification of the proteins present in fractions obtained from sucrose gradient centrifugation and from native electrophoresis of partially dissociated PBS was conducted. The identification of the proteins, after electrophoresis, was done by spectrophotometry and mass spectrometry. The results support the localization of the multidomain linkers as previously proposed by us. The Glr1262 (92 kDa) linker protein was found to be the rod-core linker L(RC) (92), and Glr2806 (81 kDa), a special rod linker L(R) (81) that joins six disks of hexameric PC. Consequently, we propose to designate glr1262 as gene cpcGm (encoding L(RC) (92)) and glr2806 as gene cpcJm (encoding L(R) (81)). We also propose that the cpeC (glr1263) gene encoding L(R) (31.8) forms the interface that binds PC to PE.