The quaternary structure of a unique phycobiliprotein: B-phycoerythrin from Porphyridium cruentum

B-phycoerythrin, from the unicellular red alga Porphyridium cruentum, was crystallized in the rhombohedral space group R3 with a=111.0Å and α=116.8° or A=B=189.1Å and C=60.1Å and γ=120°. Density measurements on the crystals indicate that the hexagonal unit cell can acconmodate three cylindrical molecules, 109Å in diameter and 60Å in height, each of approximately 275,000 daltons. The crystallographic symmetry of the unit cell requires at least 3-fold symmetry for the particle. However, the particle stoichiometry has been reported as (αβ)₆γ and this composition is also supported by SDS gel electrophoresis on the crystalline protein. These results are discussed in light of preliminary model calculations on the quaternary structure of B-phycoerythrin.