beta-Subunit 102-104 residues are crucial to confer FSH activity to equine LH/CG but are not sufficient to confer FSH activity to human CG

Horse LH/CG (eLH/CG) and donkey LH/CG (dkLH/ CG) are strictly LH-specific in their respective homologous species. However, both bind to the FSH receptors from non-equid species, whereas the zebra hormone (zbLH/CG) does not. The FSH/LH ratio of eLH/CG and of the dke hybrid is about tenfold higher than that of dkLH/CG and of the edk hybrid, showing that the e subunit contains the structural features responsible for the high FSH activity of eLH/CG. Only six amino acid positions (51, 94, 95, 102, 103 and 106) are unique to the e subunit when compared with the dk and zb subunits. The Gly-Pro and Val-Phe sequences in positions 102‐103 of dk and e respectively were swapped by site-directed mutations and the mutated -subunits cDNAs were cotransfected in COS cells with either eo r dk subunit cDNA. Other mutations were also introduced in 102‐103 dkLH/CG -subunit: Ala-Ala, Gly-Ala or Ala-Pro. These mutations with Ala-Ala, Gly-Ala or Ala-Pro in the 102‐103 dkLH/CG subunit did not change the FSH/LH ratio of dkLH/CG but the Gly 102 Pro 103 Val 102 -Phe 103 mutation promoted a marked increase in the FSH/LH activity ratio. This was observed with the two heterodimers containing eo rdk. Con