HUMAN-XENOPUS CHIMERAS OF G(S)ALPHA REVEAL A NEW REGION IMPORTANT FORITS ACTIVATION OF ADENYLYL-CYCLASE

G proteins are heterotrimeric GTPases that play a key role in signal t ransduction. The alpha subunit of G(s) bound to GTP is capable of acti vating adenylyl cyclase. The amino acid sequences derived from two X. laevis cDNA clones that apparently code for G(s) alpha subunits are 92 % identical to those found in the short form of human G(s) alpha. Desp ite this high homology, the X. laevis G(s) alpha clones expressed in v itro, yielded a protein that are not able to activate the adenylyl cyc lase present in S49 cy $$($) over bar c membranes in contrast with hum an G(s) alpha similarly expressed. This finding suggested that the few amino acid substitutions found in the amphibian subunit are important in defining the functionality of the human G(s) alpha. The constructi on of chimeras composed of different fractions of the cDNAs of the two species was adopted as an approach in determining the regions of the molecule important in its functionality in this assay. Four pairs of c himeras were constructed using reciprocal combinations of the cDNAs co ding for human and Xenopus G(s) alpha. These eight constructs were exp ressed in vitro and equivalent amounts of the resulting proteins were assayed in the activation of adenylyl cyclase with GTP gamma s and iso proterenol. The results obtained here clearly indicate that the G alph a sequence that extends from amino acid 70 to 140, is important for th e functionality of human G(s) alpha in activating adenylyl cyclase.