The v-ras oncogene of Harvey murine sarcoma virus encodes a 21,000-dalton protein, p21, which mediates transformationH produced by that virus. Previous work has shown that both p21V-/t6I. and the cellular homolog p2l"cr appear to bind guanine nucleotides. We report here the expression in Escherichia coli of v-rasH to produce a biochemically active p21 fusion protein which retains both guanine nucleotide binding and autophosphorylating activity. Furthermore, direct interaction of this protein with GTP is unequivocally demonstrated by photoaffinity labeling it with bo-32P]GTP.