STUDIES OF THE RAS-GDP AND RAS-GTP NONCOVALENT COMPLEXES BY ELECTROSPRAY MASS-SPECTROMETRY

A novel MS based methodology utilizing electrospray ionization is desc ribed for the detection of the noncovalent interaction between a host protein (ras) and its guest ligands (GDP and GTP). The ras proteins ar e regulatory guanine nucleotide binding proteins which serve as signal transducers controlling cell proliferation or differentiation. They c an exist in two interconvertible states of GDP-bound (inactive form) a nd GTP-bound (active form). The presence of the noncovalent complexes of ras-GDP and ras-GTP, as well as the unbound apo-ras protein, in var ious sample solutions containing biological buffers were confirmed by the observed average molecular weights of 19295, 19374, and 18852 Da, respectively. The stability of the observed GDP-bound and GTP-bound co mplexes is a combined function of solution pH and organic modifier con tent.