Identification of the cysteine residues in the amino-terminal extracellular domain of the human Ca²⁺ receptor critical for dimerization. Implications for function of monomeric Ca²⁺ receptor

We analyzed the effect of substituting serine for each of the 19 cysteine residues within the amino-terminal extracellular domain of the human Ca²⁺ receptor on cell surface expression and receptor dimerization. C129S, C131S, C437S, C449S, and C482S were similar to wild type receptor; the other 14 cysteine to serine mutants were retained intracellularly. Four of these, C60S, C101S, C358S and C395S, were unable to dimerize. A C129S/C131S double mutant failed to dimerize but was unique in that the monomeric form expressed at the cell surface. Substitution of a cysteine for serine 132 within the C129S/C131S mutant restored receptor dimerization. Mutation of residues Cys- 129, Cys-131, and Set-132, singly and in various combinations caused a left shift in Ca²⁺ response compared with wild type receptor. These results identify cysteines 129 and 131 as critical in formation of intermolecular disulfide bond(s) responsible for receptor dimerization. In a 'venus flytrap' model of the receptor extracellular domain, Cys-129 and Cys-131 are located within a region protruding from one lobe of the flytrap. We suggest that this region represents a dimer interface for the receptor and that mutation of residues within the interface causes important changes in Ca²⁺ response of the receptor.