Characterization of organic solvent extracted photoreceptor protein from stentor coeruleus

Abstract

The organic solvent soluble compound of the photoreceptor pigment in the protozoan S. coeruleus (stentorin I) was extracted with acetone. SDS-polyacrylamide gel electrophoresis of this integral protein gave three fluorescent bands and one non fluorescent blue green band which can also be stained with Coomassie brilliant blue. The estimated molecular weights of three fluorescent bands were 39,000, 30,000 and 8,000 daltons. The three bands showed the same protein with same chromophore. After a prolonged treatment with SDS, the high molecular components disappeared and only the intensity of the low molecular fluorescent band largely was increased. The multiplicity of a single polypeptide in the presence of SDS was compared with other integral membrane proteins, proteolipids. The multiple bands of stentorin I was the aggregator of a single polypeptide(8K). Thus, it appears that the acetone extracted stentorin I has a single homogeneous polypeptide unit of molecular weight of approximately 8,000 daltons. This is in sharp contrast to the values (64K or 16K) given in the literature. Acetone extracted stentorin I was purified by the chromatography of Sephadex-G 100 column in sodium deoxycholate solution and Sephadex LH 60 column in organic solution. The content of lipid was determined on the purified stentorin I free from lipids of the impurified form. The amino acid composition showed a high content of hydrophobic side chains. The hydrophobicity, discriminant function(Z) was found to be 0.867, the highest value reported so far. The minimum molecular weight from amino acid composition was 9,600. The size of the single polypeptide and its amino acid compositions are similar to those of proteolipis from the Fo factor in the mitochondrial, chloroplast and bacterial ATPase. A discussion on the solubility of stentorin I in various solvent systems and the artifect of circular dichroism spectrum is given.