IDENTIFICATION OF NOVEL PHOSPHORYLATION SITES IN HORMONE-SENSITIVE LIPASE THAT ARE PHOSPHORYLATED IN RESPONSE TO ISOPROTERENOL AND GOVERN ACTIVATION PROPERTIES IN-VITRO

Hormone-sensitive lipase (HSL) is the rate-limiting enzyme in lipolysi s, Stimulation of rat adipocytes with isoproterenol results in phospho rylation of HSL and a 50-fold increase in the rate of lipolysis, In th is study, we used site-directed mutagenesis and two-dimensional phosph opeptide mapping to show that phosphorylation sites other than the pre viously identified Ser-563 are phosphorylated in HSL in response to is oproterenol stimulation of P-32-labeled rat adipocytes, Phosphorylatio n of HSL in adipocytes in response to isoproterenol and in vitro phosp horylation of HSL containing Ser --> Ala mutations in residues 563 and 565 (S563A,S565A) with protein kinase A (PKA), followed by tryptic ph osphopeptide mapping resulted in two tryptic phosphopeptides, These tr yptic phosphopeptides co-migrated with the phosphopeptides released by the same treatment of F(654)HPRRSSQGVLHMPLYSSPIVK(675) phosphorylated with PKA, Analysis of the phosphorylation site mutants, S659A, S660A, and S659A,S660A disclosed that mutagenesis of both Ser-659 and Ser-66 0 was necessary to abolish the activation of HSL toward a triolein sub strate after phosphorylation with PKA. Mutation of Ser-563 to alanine did not cause significant change of activation compared with wild-type HSL, Hence, our results demonstrate that in addition to the previousl y identified Ser-563, two other PKA phosphorylation sites, Ser-659 and Ser-660, are present in HSL and, furthermore, that Ser-659 and Ser-66 0 are the major activity controlling sites in vitro.