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Originally published In Press as doi:10.1074/jbc.M207100200 on September 23, 2002
J. Biol. Chem., Vol. 277, Issue 48, 46622-46631, November 29, 2002
Identification of Acidic Residues in the Extracellular Loops of
the Seven-transmembrane Domain of the Human Ca2+ Receptor
Critical for Response to Ca2+ and a Positive Allosteric
Modulator*
Jianxin
Hu §,
Guadalupe
Reyes-Cruz ,
Wangzhong
Chen¶,
Kenneth A.
Jacobson¶, and
Allen M.
Spiegel
From the Molecular Pathophysiology Section, NIDCD,
and the ¶ Laboratory of Bioorganic Chemistry, NIDDK, National
Institutes of Health, Bethesda, Maryland 20892
We investigated the role of the eight
acidic residues in the extracellular loops (exo-loops) of the
seven-transmembrane domain of the human Ca2+ receptor
(hCaR) in receptor activation by Ca2+ and in response to a
positive allosteric modulator, NPS R-568. Both in the context of the
full-length receptor and of a truncated receptor lacking the
extracellular domain (Rho-C-hCaR), we mutated each acidic
residue to alanine, singly and in combination, and tested the effect on
expression of the receptor, on activation by Ca2+, and on
NPS R-568 augmentation of sensitivity to Ca2+. Of the eight
acidic residues, mutation of any of three in exo-loop 2, Asp758, Glu759, and Glu767,
increased the sensitivity of both the full-length hCaR and of Rho-C-hCaR to activation by Ca2+. Mutation of all five
acidic residues in exo-loop 2, whether in the full-length receptor or
in Rho-C-hCaR, impaired cell surface expression of the mutant receptor
and thereby largely abolished response to Ca2+. Mutation of
Glu837 in exo-loop 3 to alanine did not alter
Ca2+ sensitivity of the full-length receptor, but in both
the latter context and in Rho-C-hCaR, alanine substitution of
Glu837 drastically reduced sensitivity to NPS R-568. Our
data point to a key role of three specific acidic residues in exo-loop
2 in hCaR activation and to Glu837 at the junction between
exo-loop 3 and transmembrane helix seven in response to NPS R-568. We
speculate on the basis of these results that the three acidic residues
we identified in exo-loop 2 help maintain an inactive conformation of
the seven-transmembrane domain of the hCaR.
*
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
§
To whom correspondence should be addressed: Bldg. 10, Rm. 8C-101,
National Institutes of Health, 9000 Rockville Pike, Bethesda, MD
20892. Tel.: 301-496-9212; Fax: 301-402-0374; E-mail:
jianxinh@ intra.niddk.nih.gov.
Copyright © 2002 by The American Society for Biochemistry and Molecular Biology, Inc.

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Copyright © 2002 by the American Society for Biochemistry and Molecular Biology.
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