GTP binding protein: properties and lack of activation by phosphorylated rhodopsin.

Taking advantage of the capability of GTP binding protein to bind GTP, we purified the catalytic subunit (G alpha) of bovine rod GTP binding protein by nucleotide-affinity chromatography on Blue Sepharose CL6B. Purified G alpha was essentially free of bound guanine nucleotide and activated by photoactivated rod membranes. Circular dichroism spectra suggested that a significant portion of the protein would be in alpha-helical conformation. No appreciable differences were detected in the circular dichroism spectra when G alpha . GDP and G alpha . GppNp were compared. The extent of G protein activation by rod membranes was reduced moderately by phosphorylation of rhodopsin during photolysis. However, if the pigment had been phosphorylated and regenerated, the ability of rhodopsin to activate G protein was markedly suppressed.