Detection of a 65 kDaras binding protein in rat and sheep brain cytosol using a chemical cross linking agent

The ability of aras protein to associate with proteins present in rat brain cytosolin vitro was investigated using chemical cross-linking agents and the¹²⁵I-labelled v-H-ras protein. Two iodinated protein complexes with apparent molecular weights of 40 and 85 kDa were observed when a mixture of rat brain cytosol and ¹²⁵I]ras was treated with the cross-linking agent disuccinimidyl suberate and subjected to SDS-PAGE. Formation of the ¹²⁵I] 85 kDa complex was enhanced by a high concentration of EDTA while generation of the 40 kDa species was abolished by this treatment. Formation of the ¹²⁵I] 85 kDa complex was inhibited by unlabelledras protein, GTP, GTPS, and GDP but not by ATPS and GMP.Chromatography of the cross-linked brain cytosol-¹²⁵I]ras mixture on DEAE cellulose partially resolved the ¹²⁵I] 85 kDa complex from the ¹²⁵I]ras protein. The ¹²⁵I] 85 kDa complex (formed using ethyleneglycolbis (succinimidylsuccinate) as the cross-linking agent) could be immunoprecipitated using a rabbit anti-ras polyclonal antibody. Treatment of the immunoprecipitate with hydroxylamine to cleave the cross-link yielded ¹²⁵I]-labelledras. A substantial enrichment of the proportion of the ¹²⁵I] 85 kDa complex in the cross-linked extract was achieved by preparative SDS-PAGE. It is concluded that thein vitro chemical cross-linking approach employed here has detected tworas binding proteins in rat brain cytosol: a 65 kDa heat-sensitive and a 20 kDa heat-stable protein. The possibility that the 65 kDaras binding protein is aras regulatory orras effector protein which has not so far been characterised is briefly discussed.Abbreviations DSS disuccinimidyl suberate - EGS ethyleneglycolbis (succinimidylsuccinate) - GTPS guanosine 5--thio] triphosphate - ATPS adenosine 5--thio] triphosphate