CORE SUBSTRUCTURE OF THE HEMIELLIPSOIDAL PHYCOBILISOME FROM THE RED ALGA PORPHYRIDIUM-CRUENTUM

The allophycocyanin core of hemiellipsoidal phycobilisomes from the re d alga, Porphyridium cruentum, was isolated by chromatography on hydro xylapatite and subsequent density gradient centrifugation. Electron mi croscopy of negatively stained core complexes revealed a tricylindrica l structure with a width of 21 to 23 nm in face view and a depth of 12 to 14 nm in side view. Fluorescence emission spectra of these complex es were similar to those of whole phycobilisomes confirming the presen ce of the two ''terminal energy acceptors allophycocyanin B (APB) and the high molecular linker polypeptide L(CM). The polypeptide compositi on analyzed by SDS-PAGE showed the ''anchor'' polypeptide L(CM), alpha (AP), alpha(APB), beta(AP) subunits, a low molecular weight linker wit h M, 13500 and a blue-colored polypeptide of M(r) 19800. A complex con taining APB could be isolated from a ''trimeric'' allophycocyanin frac tion of the density gradient by agarose gel electrophoresis in the pre sence of ampholytes. This complex shows a polypeptide composition of ( alpha(APB) alpha(2)(AP) beta(3)(AP)).L(C)(13.5) and contributes to the core with 30 to 35% of total ''trimers''. In comparison to the alloph ycocyanin cores from hemidiscoidal phycobilisomes of cyanobacteria and red algae, all data concerning the core of the hemiellipsoidal phycob ilisomes from Porphyridium cruentum strongly suggest that there is no increase in size but an increase in its APB content. A model comprisin g the allophycocyanin core and the membrane integral photosystem II pa rticles presents the structural and possible functional consequences o f the results.