The second cytoplasmic loop of metabotropic glutamate receptor functions at the third loop position of rhodopsin.
G protein-coupled receptors identified so far are classified into at least three major families based on their amino acid sequences. For the family of receptors homologous to rhodopsin (family 1), the G protein activation mechanism has been investigated in detail, but much less for the receptors of other families. To functionally compare the G protein activation mechanism between rhodopsin and metabotropic glutamate receptor (mGluR), which belong to distinct families, we prepared a set of bovine rhodopsin mutants whose second or third cytoplasmic loop was replaced with either the second or third loop of Gi/Go- or Gq-coupled mGluR (mGluR6 or mGluR1). Among these mutants, the mutants in which the second or third loop was replaced with the corresponding loop of mGluR exhibited no G protein activation ability. In contrast, the mutant whose third loop was replaced with the second loop of Gi/Go-coupled mGluR6 efficiently activated Gi but not Gt: this activation profile is almost identical with those of the mutant rhodopsins whose third loop was replaced with those of the Gi/Go-coupled receptors in family 1 [Yamashita et al. (2000) J. Biol. Chem. 275, 34272-34279]. The mutant whose third loop was replaced with the second loop of Gq-coupled mGluR1 partially retained the Gi coupling ability of rhodopsin, which is in contrast to the fact that all the rhodopsin mutants having the third loops of Gq-coupled receptors in family 1 exhibit no detectable Gi activation. These results strongly suggest that the molecular architectures of rhodopsin and mGluR are different, although the G protein activation mechanism involving the cytoplasmic loops is common.
Yamashita
T
T
Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
Terakita
A
A
Shichida
Y
Y
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002417:Cattle; D004789:Enzyme Activation; D020962:Heterotrimeric GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D051381:Rats; D018094:Receptors, Metabotropic Glutamate / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012243:Rhodopsin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
D018094:Receptors, Metabotropic Glutamate; D011993:Recombinant Fusion Proteins; D012243:Rhodopsin; D020962:Heterotrimeric GTP-Binding Proteins