Characteristics of an R-Phycoerythrin with Two γ Subunits Prepared from Red Macroalga Polysiphonia urceolata

An R-phycoerythrin (R-PE) was isolated by gel filtrations on Sepharose CL-4B and Sephadex G-150 from the phycobiliprotein extract of the marine red macroalga Polysiphonia urceolata Grev and further purified by ion exchange chromatography on DEAE-Sepharose Fast Flow. The purified R-PE showed three absorption peaks at 498 nm, 538 nm, 566 nm and one fluorescent emission maximum at 577 nm. Although the R-PE showed a single band on the examination by native PAGE, it exhibited two very close bands at pH about 4.7 in native isoelectric focusing (IEF). Polypeptide analysis of the R-PE demonstrated that it contained four chromophore-carrying subunits, alpha(18.2), beta(20.6),gamma(31.6) (gamma'),gamma(34.6) (gamma), and no color-less polypeptide; its subunit composition was 6 alpha(18.2):6 beta(20.6):1 gamma(31.6):2 gamma(34.6). The alpha and beta subunits were distributed within a acidic pH range from 5.0 to 6.0 in denaturing IEF and the. subunits were in a basic pH range from 7.6 to 8.1. These results reveal that the prepared R-PE may exist in two hexamers of. (alpha beta)(3) gamma(alpha beta)(3) gamma' and gamma(alpha beta)(3) gamma'(alpha beta)(3)gamma and that the R-PE participate in the rod domain assembly of P. urceolata phycobilisomes by stacking each of its trimer (alpha beta)(3) face-to-face with the aid of one gamma subunit (gamma or gamma').