Phycobilisome Structure of Porphyridium cruentum

Abstract

Purified phycobilisomes of Porphyridiwm cruentum were solubilized in sodium dodecyl sulfate and resolved by sodium dodecyl sulfate-acrylamide gel electropboresis into nine colored and nine colorless polypeptides. The colored polypeptides accounted for about 84% of the total stainable protein, and the colorless polypeptides accounted for the remaining 16%. Five of the colored polypeptides raning in molecular weight from 13,300 to 19,500 were identified as the a and ,8 subunits of aflophycocyanin, R-phycocyanin, and phycoerythria Three others (29,000-30,500) were orange and are probably related to the y subunit of phycoerythrin. Another colored poly- peptide had a molecular weight of 95,000 and the characteristics of long wavelength-emitting aBophycocyanin. Sequential dissociation of phycobili- somes, and analysis of the polypeptides in each fraction, revealed the association of a 32,500 molecular weight colorless polypeptide with a phycoerythrin fraction. Tne remaining eight colorless polypeptides were in the core fraction of the phycobilisome, which also was enriched in allophy- cocyanin. In addition, the core fraction was enriched in a colored 95,000 dalton polypeptide. Inasmuch as a pobpeptide with the same molecular weight is found in thylakoid membranes (free of phycobilisomes), it is suggested that this polypeptide is involved in anchoring phycobilisomes to thylakoid membranes.