Abstract
The marine dinoflagellate Amphidinium carterae (Plymouth 450) releases several water-soluble peridinin-chlorophyll a proteins after freezethawing. These chromoproteins have a molecular weight of 39.2 x 10(3) and are comprised of noncovalently bound peridinin and chlorophyll a and a nonoligomeric protein. They have distinct isoelectric points and may be resolved into six components by either isoelectric focusing on polyacrylamide gel or ion exchange chromatography. The predominant chromoprotein, which has a pI of 7.5, constitutes about 90% of the extractable peridinin-chlorophyll a protein. It consists of an alanine-rich apoprotein of molecular weight 31.8 x 10(3) stoichiometrically associated with 9 peridinin and 2 chlorophyll a molecules. Additionally, the peridinin-chlorophyll a proteins with pI values of 7.6 and 6.4 were purified and found to have amino acid and chromophore composition essentially identical with the pI 7.5 protein. Peridinin-chlorophyll a protein, pI 7.5, after treatment at alkaline pH was transformed into several more acid pI forms of the protein, strongly suggesting that the naturally occurring proteins are deamidation products of a single protein. Fluorescence excitation and emission spectra demonstrate that light energy absorbed by peridinin induces chlorophyll a fluorescence presumably by intramolecular energy transfer. The peridinin-chlorophyll a proteins presumably function in vivo as photosynthetic light-harvesting pigments.