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, α^18.2, β^20.6, γ^31.6 (γ''), γ^34.6 (γ), and no colorless polypeptide; its subunit composition was 6α^18.2:6β^20.6:1 γ^31.6:2γ^34.6. The α and β subunits were distributed within a acidic pH range from 5.0 to 6.0 in denaturing IEF and the γ 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 γ (αβ)₃ γ (αβ)₃γ'' and γ (αβ)₃ γ''(αβ)₃ γ and that the R-PE participate in the rod domain assembly of P. urceolata phycobilisomes by stacking each of its trimer (αβ)₃ face-to-face with the aid of one γ subunit (γ or γ'').