ROLE OF PHOSPHOPROTEIN PHOSPHATASES IN THE CORPUS-LUTEUM .1. IDENTIFICATION AND CHARACTERIZATION OF SERINE THREONINE PHOSPHOPROTEIN PHOSPHATASES IN ISOLATED RAT LUTEAL CELLS/

Although the role of protein kinases and phosphorylation in steroidoge nesis has received much attention, very little is known about the acti vities of phosphoprotein phosphatases (PP) and dephosphorylation in st eroidogenic tissues. The aims of the present study were therefore to i dentify which of those serine/threonine PPs more commonly involved in intracellular signalling are expressed in rat luteal cells; to quantif y, in vitro, the effects of inhibitors on PP activity extracted from p urified rat luteal cells; and to measure the effects of PP inhibitors on the phosphorylation of endogenous luteal cell proteins. Polyclonal antibodies raised against the catalytic subunits of PP types 1 and 2A, and a monoclonal antibody raised against the Ca2+-binding subunit of PP2B, were used to identify immunoreactive proteins that migrated on S DS-PAGE with approximate molecular masses of 37, 34 and 16 kDa, corres ponding well with the reported molecular mass of PP1, PP2A and PP2B re spectively. Five selective inhibitors of PP1/PP2A: okadaic acid, calyc ulin A, cantharidin, tautomycin and microcystin-RR, each caused a dose -dependent decrease in the activity of PPs in luteal cell homogenates, and also enhanced P-32 incorporation into numerous luteal cell protei ns; most notably, proteins with approximate molecular masses of 20 and 22 kDa. The results of this study suggest that PPs may play an import ant role in the regulation of rat luteal cell functions.