Presence of Rhodopsin-like Proteins in Sorghum bicolor and Pisum sativum

Antiserum raised against squid (Loligo forbesi) rhodopsin cross-reacted with proteins from microsomal and plasma membranes isolated from Sorghum bicolor and Pisum sativum. In S. bicolor the main crossreacting protein (SbR) presented a molecular mass of 38 kDa. SbR appeared to be more abundant in mesocotyl microsomal membranes than in leaf or root membranes. SbR showed similarities to G-protein coupled receptors (GPCRs) in that it required a high detergent concentration for extraction and exhibited diffuse antibody staining on blots similar to that of squid rhodopsin. In P. sativum hypocotyl membranes the strongest immunological signal corresponded to a 35 kDa polypeptide (PsR) that was shown to be enriched in plasma membranes. A P. sativum glycoprotein bearing the same Mr and membrane association behaviour as PsR could be purified by Con A affinity chromatography and elution with α methyl manoside. The chromatographic fractions containing the glycoprotein were shown to possess high and specific GTPγS binding activity, indicating association to G-proteins. The glycoprotein showed weak cross-reaction to anti-rhodopsin serum. N-terminal sequencing of the glycoprotein did not show high homology to other proteins in the databases but indicated a likely amphipathic character.