CIS-ACTIVE RAS G2-LIKE SEQUENCE IMPLICATED IN THE HETEROTROPIC ACTIVATION OF THE DEOXYADENOSINE KINASE OF LACTOBACILLUS-ACIDOPHILUS R-26

Deoxyadenosine kinase (dAK) forms a heterodimer with either deoxyguano sine kinase (dGK) or deoxycytidine kinase (dCK), and is heterotropical ly activated 3-5 times by dGuo or dCyd. Expressed alone, dAK is inacti ve and exhibits no response to dGuo or dCyd; activity and heterotropic response are fully restored upon reassociation with dGK or dCK. Howev er, turnover of independently expressed dGK or dCK: is nearly maximal, being further activated only 50-100% upon reassociation with dAK, In neither case is the heterotropic activation due to ligand-induced hete rodimer formation. A proline/alanine substitution within a dAK segment homologous to loop G2 of Has proteins yielded a heterodimer with dAK permanently cis-activated a-fold, with a corresponding reduction in he terotropic activation by dGuo, A chimeric dAK, with 25% of its C termi nus substituted by the homologous sequence from dGK, was inactive alon e, and its characteristics were unchanged in the reconstituted heterod imer, Superimposing the Pro/Ala substitution on this chimera also redu ced heterotropic activation by half, Cross-linking the dimer by 1,5-di fluoro-2,4-dinitrobenzene was inhibited by ATP, dATP, dGTP, and dAdo, suggesting the proximity of the active site(s) to the interface, These data suggest that dAK depends on dGK or dCK in a manner resembling th e reliance of Has upon GTPase activating protein.