EFFECT OF MODIFICATION OF THE BETA-HAIRPIN AND LONG LOOPS SIMULTANEOUSLY IN BOTH ALPHA- AND BETA-SUBUNITS ON THE FUNCTION OF HUMAN CHORIOGONADOTROPIN .2.

According to the X-ray diffraction, human choriogonadotropin has four beta-hairpin and two long loops, equally distributed in each of the al pha and beta subunits. Radical mutations such as the replacement of al pha 18Phe and alpha 74Phe with Thr in the alpha(1) and alpha(3) loops respectively and the replacement of alpha 45Lys with Asp in the alpha( 2) loop in the alpha-subunit were introduced while the loop sequences in the beta-subunit were replaced with the corresponding sequences in hFSH beta. Nine different double mutants with simultaneous mutations i n both the alpha and beta loops including hCG alpha(1) beta(1), hCG al pha(1) beta(2), hCG alpha(1) beta(3), hCG alpha(2) beta(1), hCG alpha( 2) beta(2), hCG alpha(2) beta(3), hCG alpha(3) beta(1), hCG alpha(3) b eta(2) and hCG alpha(3) beta(3) were partially purified from insect Hi gh-Five cells. As previously reported (Shao et al., 1996, Mel. Cell. E ndocrinol. 122, 173-182), the mutation in the alpha(1) loop in the mut ant, hCG alpha(1) beta, the mutants hCG alpha(1) beta(1) and hCG alpha (1) beta(3) caused 200% increase in the receptor binding, cAMP and pro gesterone stimulation. The mutant, hCG alpha(1) beta(2) and all other mutants behaved like the recombinant hCG (rehCG) in the receptor bindi ng and post-receptor signaling activities. The molecular cause for thi s increase is probably due to a conformational change in the heterodim ers caused by the mutation in the alpha(1) loop. This conclusion is ba sed on the results of the dissociation studies of the mutants heterodi mers which indicated a decreased affinity between the subunits. The fi rst order rate constants for the dissociation of the mutants hCG alpha (1) beta(1), hCG alpha(1) beta(2) and hCG alpha(1) beta(3) were 3.7 x 10(-2) min(-1), 1.4 x 10(-2) min(-1) and 4.6 x 10(-2) min(-1) respecti vely, as compared with 4.6 x 10(-3) min(-1) for the rehCG. It seems fr om the data that alpha 18Phe is located in, or in proximity to the rec eptor binding site and probably plays a critical role in maintaining e ither directly or indirectly its conformational integrity. (C) 1997 El sevier Science Ireland Ltd.