Alkylation of sulfhydryl groups on Gαs/olf subunits by N-ethylmaleimide: regulation by guanine nucleotides

AbstractIn rat striatum A2A adenosine receptors activate adenylyl cyclase through coupling to Gs-like proteins, mainly Golf that is expressed at high levels in this brain region. In this study we report that the sulfhydryl alkylating reagent, N-ethylmaleimide (NEM), causes a concentration- and time-dependent inhibition of [3H] 2-p-(2-carboxyethyl)phenylethylamino)-5′-N-ethylcarboxamido adenosine ([3H]CGS21680) binding to rat striatal membranes. Membrane treatment with [14C]N-ethylmaleimide ([14C]NEM) labels numerous proteins while addition of 5′-guanylylimidodiphosphate (Gpp(NH)p) reduces labeling of only three protein bands that migrate in SDS-polyacrylamide gel electrophoresis with apparent molecular masses of ∼52, 45 and 39 kDa, respectively. The 52- and 45-kDa labeled bands show electrophoretic motilities as Gαs-long and Gαs-short/Gαolf subunits. An anti-Gαs/olf antiserum immunoprecipitates two 14C labeled bands of 44 and 39 kDa. The band density decreases by 21–26% when membranes are treated with NEM in the presence of Gpp(NH)p. An anti-A2A receptor antibody also immunoprecipitates two 14C labeled bands of 40 and 38 kDa, respectively. However, such protein bands do not show any decrease of their density upon membrane treatment with NEM plus Gpp(NH)p. These results indicate that in rat striatal membranes NEM alkylates sulfhydryl groups of both Gαs/olf subunits and A2A adenosine receptors. In addition, cysteine residues of Gαs/olf are easily accessible to modification when the subunit is in the GDP-bound form. The 39- and 38-kDa labeled proteins may represent proteolytic fragments of Gαs/olf and A2A adenosine receptor, respectively