Multiple forms of a type I phycoerythrin from a Chroomonas sp. (Cryptophyceae) varying in subunit composition

Four forms of phycoerythrin have been isolated by chromatofocusing, with isoelectric points of 7.16, 6.24, 5.46 and 4.98. Each isoprotein has a type I absorbance maximum at 545 nm and a shoulder at 564 nm with a small shoulder at 644 nm. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate and urea revealed that the four isoprotein fractions contain a β-subunit of 20.5 kDa but are different with respect to the α subunits. The pI 7.16 isoprotein lacked the α polypeptide at 12.0 kDa, whereas the pI 4.98 and pI 5.46 forms contained predominantly the 12.0 kDa α polypeptide. The predominant pI 6.24 isoprotein contained α subunits of both 11 kDa and 12 kDa. Ion exchange chromatography of the α subunits showed that both existed as two differently charged species. Isoelectric focusing confirmed the existence of multiple forms of phycoerythrin. Eight or nine species of phycoerythrin were separated, consistent with the previously postulated α₂β₂ structure (Martin, C.D. and Hiller, R.G. (1987) Biochim. Biophys. Acta 923, 88-97). None of the multiple forms isolated appears to be a specific intermediate in energy transfer between bulk phycoerythrin and chlorophyll a.