A dileucine determinant in the carboxyl terminal sequence of the LHβ subunit is implicated in the regulated secretion of lutropin from transfected GH3 cells

LH and FSH are essential for control of gonadal function. They are synthesized in the same gonadotrope but differ in their mode of secretion. LH release is regulated, while FSH is secreted constitutively. One unique feature of LHβ is a carboxyl terminal hydrophobic heptapeptide. We demonstrated that deleting the heptapeptide diverted the truncated LH dimer to the constitutive pathway in vitro. To examine if the residues of this heptapeptide play a role in LH sorting, leucines 118-119 were substituted with alanine (L118A and L119A, respectively). The intracellular pool of the L118A mutant protein decreased with a corresponding increase in constitutive secretion. Moreover, immunofluorescence microscopy revealed that the L118A mutant exhibited fewer puncta as compared to wild-type LH. L119A behaved similar to wild-type LH, indicating that a single leucine residue at position 118, rather than a dileucine motif, contributes to the process that sorts LH into the regulated pathway.