Identification of cysteine 656 as the amino acid of hepatoma tissue culture cell glucocorticoid receptors that is covalently labeled by dexamethasone 21-mesylate.

Recent results using proteases suggest that dexamethasone 21-mesylate (Dex-Mes) labeling of the rat hepatoma tissue culture (HTC) cell glucocorticoid receptor occurs at one or a few closely grouped cysteine residues (Simons, S.S., Jr. (1987) J. Biol. Chem. 262, 9669-9675). In this study, a more direct approach was used both to establish that only one cysteine is labeled by [3H]Dex-Mes and to identify the amino acid sequence containing this labeled cysteine. Various analytical procedures did not provide the purification of the extremely hydrophobic Staphylococcus aureus V8 protease digestion fragment that is required for unique amino acid sequencing data. Therefore, Edman degradation was performed on the limit protease digest mixtures which appeared to contain only one 3H-labeled peptide. These degradation experiments revealed the number of amino acid residues between the NH2 terminus of each peptide and the [3H]Dex-Mes-labeled cysteine. A comparison of these amino acid spacings with the published amino acid sequence of the HTC cell glucocorticoid receptor (Miesfeld, R., Rusconi, S., Godowski, P. J., Maler, B. A., Okret, S., Wikstom, A-C., Gustafsson, J-A., and Yamamoto, K. R. (1986) Cell 46, 389-399) indicated that the one cysteine labeled by [3H]Dex-Mes is Cys-656. Further analysis of the receptor sequence for the presence of the observed grouping of proteolytic cleavage sites, but without any preconditions as to which amino acid was labeled, gave Asp-122 and Cys-656 as the only two possibilities. Potential labeling of Asp-122 could be eliminated on the basis of immunological and genetic evidence. We, therefore, conclude that the single Dex-Mes-labeled site of the HTC cell glucocorticoid receptor has been identified as Cys-656. Since several lines of evidence indicate that [3H]Dex-Mes labeling of the receptor occurs in the steroid binding site, Cys-656 is the first amino acid which can be directly associated with a particular property of the glucocorticoid receptor.