Studies of the Conformational Change in Human N - p 21 raS Produced by Replacement of Bound GDP with the GTP Analog GTPrSt

lH-Detected 15N-edited N M R in solution was used to study the conformational differences between the GDPand GTPyS-bound forms of human N-p2lraS. The amide protons of I5N-labeled glycine and isoleucine were observed. Resonances were assigned to residues of particular interest, glycines-60 and -75 and isoleucines-21 and -36, by incorporating various 13C-labeled amino acids in addition to [15N]glycine and [15N]iosleucine and by replacing Mg2+ by Co*+. When GTPyS replaced GDP in the active site of p2lraS, only 5 of the 14 glycine amide resonances show major shifts, indicating that the conformational effects are fairly localized. Responsive glycines-10, -12, -13, and -15 are in the active site. Gly-75, located at the far end of a conformationally-active loop and helix, also responds to substitution of GTPyS for GDP, while Gly-77 does not, supporting a role for Gly-75 as a swivel point for the conformational change. The amide proton resonances of isoleucines-36 and -21 and a third unidentified isoleucine also undergo major shifts upon replacement of GDP by GTPyS. Thus, the effector-binding loop containing Ile-36 is confirmed to be involved in the conformational change, and the a-helix containing Ile-21 is also shown to be affected. Human p21 Ias’s are small guanine nucleotide-binding proteins (Bourne et al., 199 1). p21raS-related proteins function in cell signaling, proliferation, and differentiation (Barbacid, 1987; Bourne et al., 1990; Nishimura & Sekiya, 1987). In addition, p2 1 ras is a useful model for guanine nucleotide-binding proteins in general. p2 lras cycles between its active guanosine 5’-triphosphate (GTP) form and its inactive GDP form by hydrolysis of GTP, and by subsequent exchange of GDP for predominant GTP (Bourne et al., 1990; Field et al., 1987). GTPase activating protein (GAP) stimulates p21raS’s GTP hydrolysis rate by 3 orders of magnitude (Trahey & McCormick, 1987), and exchange factor proteins interact with p2 lras to accelerate nucleotideexchange (West et al., 1990; Wolfmanet al., 1990). Thus, various proteins normally regulate the balance between GDPand GTP-bound p21ras in cells. Because these proteins interact specifically with either the GTPor the GDP-bound state of p21ras (Field et al., 1987; Vogel et al., 1988), conformational differences between the two states are essential to the function of p21ras. Amino acids involved in the conformational switch of p2 lras have been identified. Residues 32-40 of the effector-binding loop are required for signal transduction from GTP-bound p21ras (Willumsen et al., 1986; Sigal et al., 1986) and binding This work was supported in part by USPHS Grants CAS1992 and GM20168. A.-F.M.isa Bristol-MyersSquibbFellow oftheLifeSciences Research Foundation and also gratefully acknowledges the Natural Sciences and Engineering Research Council of Canada for support during earlier stages of this research. t Present address: Department of Chemistry, The Johns Hopkins University, Baltimore, MD 21218. Abbreviations: DTT, dithiothreitol; EDTA, ethylenediaminetetraacetic acid; GDP, guanosine S’-diphosphate; GMPPCP, guanosine 5’(&y -methylene)diphosphonate; GMPPNP, 5’-guanylyl imidodiphosphate; GTP, guanosine 5’-triphosphate; GTP-/S, guanosine 5’-0-(3-thiotriphosphate); HEPES, 4-( 2-hydroxyethy1)1 -piperazineethanesulfonic acid; HMQC, heteronuclear multiple quantum coherence; HSMQC, heteronuclear single and multiple quantum coherence; IPTG, isopropyl 8-Dthiogalactopyranoside; OD, optical density at 660 nm, &a; NMR, nuclear magnetic resonance; PAGE, polyacrylamide gel electrophoresis; PMSF, phenylmethanesulfonyl fluoride; SDS, sodium dodecyl sulfate; TLC, thinlayer chromatography; Tris, tris(hydroxymethy1)aminomethane. 0006-2960/92/043 110208%03.00/0 to GAP (Adari et al., 1988; Calts et al., 1988). The crystal structures of the GDP-bound and GTP analog-bound states of p2lraS differ significantly between residues 30-40 and 6076 (Wittinghofer & Pai, 1991; Milburn et al., 1990), but apart from these changes and changes in hydrogen bonds directly involving the additional phosphate present in GTP analogs, the overall structures of GDPand GMPPNP-bound p2lras appear to be similar. NMR studies comparing p21ras bound to GDP and to the GTP analog GTPyS have indicated that interactions between &strands 2 and 3 evident in the GDP-bound state are altered in the GTPyS-bound state (Yamasaki et al., 1989). NMR studies in this laboratory have identified the amide proton resonances of residues 10-1 6 in the phosphate-binding loop of N-p21raS (Campbell-Burk et al., 1989; Redfield & Papastavros, 1990). The resonances of Gly13 and Lys16 occur at unusually low field due to the involvement of these protons in strong hydrogen bonds with the phosphate groups of GDP (Pai et al., 1989; Redfield & Papastavros, 1990). In this report, we have added to these assignments and extended them to residues more distant from the active site. We have used isotope-editing NMR methods (Griffey 8c Redfield, 1987) in conjunction with 15N-labeling of glycine and isoleucine to obtain highly simplified spectra containing only resonances from the amide protons of those amino acids. GTPyS was chosen for this work because of its high affinity for p21ras, approaching those for GDP and GTP (Feuerstein et al., 1989), and because it is hydrolyzed by p2lraS, albeit at a reduced rate which is compatible with NMR experiments (Feuerstein et al., 1989). Thus, we expect that the p21raSGTPyS complex retains key characteristics of the p21rasGTP complex. We have used 15N-labeled glycine and isoleucine to report on conformational effects in the phosphate-binding loop and active site of p2 lras, and the effector-binding loop. Conserved glycines in the active site are essential to the function of p2lraS (Seeburg et al., 1984; Bos et al., 1985; Fasano et al., 1984). Ile-36 is central to the effector-binding loop and is conserved amongp21ras-related proteins (Boumeet al., 1991). According to the X-ray structures, the conformation of Ile-36 is coupled