title: Limited trypsin proteolysis of photoreceptor GTP-binding protein. Light- and GTP-induced conformational changes.
abstract: The amphibian photoreceptor rod outer segment contains a guanine nucleotide-binding complex which consists of a 39,000-dalton polypeptide that binds guanine nucleotides (G protein), a 36,000-dalton polypeptide (H protein), and an approximately 6,500-dalton polypeptide. Sensitivity to trypsin proteolysis was utilized as a probe of structure-function relationships for these polypeptides. Digestion of the H protein generated fragments of 26,000 and 15,000 daltons whose proteolytic susceptibility was not altered by guanosine triphosphates, light, or membranes. The approximately 6,500-dalton polypeptide was not trypsin sensitive. When the G protein was eluted from illuminated membranes by GTP, trypsin proteolysis cleaved a terminal 1,000-dalton fragment (G1) to yield a 38,000-dalton fragment (G38). With increased digestion time, a 6,000-dalton fragment (G6) was removed from G38 to yield a 32,000-dalton fragment (G32). G32 was subsequently digested to fragments of 23,000 and 12,000 daltons. However, when the G protein was eluted from illuminated membranes by hydrolysis-resistant analogues of GTP, G32 was protected from further digestion. This is consistent with a GTP-induced conformational change in the G protein which is altered by GTP hydrolysis. Proteolysis of the G protein after covalent labeling with a photoaffinity analogue of GTP demonstrated that the analogue is bound to first G38 and then G32, indicating the GTP-binding site is associated with G32. Fragment G6 was cleaved when the G protein was soluble or bound to unilluminated membranes. However, when bound to illuminated membranes, fragments were generated reflecting the loss of 7,500, 9,000, or 11,000 daltons from the G protein. This light-induced alteration in proteolytic susceptibility indicates there is a light-induced conformational change in the G protein. Fragment G1 was not removed from the G protein when it was membrane bound, suggesting G1 is involved in binding to a membrane structure. These data suggest that the light-induced binding of the G protein to illuminated membranes and the reversal of this binding by GTP are mediated through conformational changes in the G protein and that three conformations exist: 1) a basal, inactive conformation; 2) a primed conformation induced by binding to photolyzed rhodopsin, with a high affinity for GTP; and 3) an active conformation, induced by binding of GTP, which activates the catalytic complex of light-activated phosphodiesterase.
mesh_terms: D000818:Animals; D002024:Bufo marinus; D019204:GTP-Binding Proteins; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D010775:Photic Stimulation; D010786:Photoreceptor Cells / Q000378:metabolism*; D011487:Protein Conformation; D011956:Receptors, Cell Surface / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism; D013873:Thionucleotides / Q000494:pharmacology; D013997:Time Factors; D014357:Trypsin / Q000378:metabolism*
chemical_list: D011956:Receptors, Cell Surface; D013873:Thionucleotides; D006165:Guanylyl Imidodiphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D014357:Trypsin; D019204:GTP-Binding Proteins