Spectroscopic study of the light-harvesting protein C-phycocyanin associated with colorless linker peptides

C-Phycocyanin (PC) trimers associated with linker polypeptides were isolate d from the phycobilisome (PBS) rods of Synechococcus sp, PCC 7002. L-X(Y) r efers to a linker polypeptide (L) having an apparent mass of Y kDa, located at position X in the phycobilisome where X can be R (rod), C (core) or RC (rod-core junction). Measurements of the absorption, fluorescence and excit ation anisotropy of PC trimer, pC(.)L(R)(32.3) and (PCLRC28.5)-L-. complexe s document the spectroscopic modulation of each linker polypeptide on the P C chromophores, The difference spectra between the PC trimer and the PC-lin ker complexes show that although the effect induced by the linker polypepti des is qualitatively similar in behavior, the extent of the modulation is g reater in (PCLRC28.5)-L-.. Measurements taken at 77 K show that a red-wavel ength component of the PC trimer absorption-fluorescence spectra is the tar get of the linker's influence and that this component is altered to a great er extent by L-RC(28.5). In addition the 77 K absorbance of the PC trimer r esolves band features that are consistent with an excitonic coupling intera ction between neighboring alpha 84 and beta 84 chromophores, These band fea tures are also evident in the absorbance of (PCLR32.3)-L-. but are absent i n (PCLRC28.5)-L-. indicating that L-RC(28.5) may be perturbing the coupling interaction established in the PC trimer alpha 84-beta 84 chromophore pair s. Structurally, the linker polypeptide should disrupt the C-3 symmetry in the central cavity of the associated phycobiliprotein and this asymmetric i nteraction should serve to guide the transfer of excitation energy along PB S rods toward the core elements.