Protein Phosphorylation in Luteal Membrane Fraction

Abstract

In corpus luteum, LH plays a key role in regulating various cellular functions. It is well established that interaction of LH with its receptor leads to the activation of adenylate cyclase and the formation of cyclic AMP which then activates the protein kinase A (Hunzicker-Dunn et al 1985). Several substrate proteins in soluble fractions prepared from rat (Richards & Kirschick 1984) and ovine (Hoyer & Kong 1989) luteal cells are known to be phosphorylated by a cAMP dependent mechanism. In addition to LH, the luteal steroidogenesis can also be stimulated by the phorbol ester PMA or a diacylglycerol (Brunswig et al.,1986), via putative activation of a Ca²⁺/phospholipid- dependent protein kinase (PKC). Furthermore the presence of substrates in luteal cell cytosolic compartments for Ca²⁺/phospholipid- or calmodulin- dependent protein kinases have been reported (Maizels & Jungman 1983; Hoyer & Kong 1989). However, most of the studies reported so far on this subject have dealt with the substrates present in the cytosolic compartments. Since protein and peptide hormones act on cells via interactions with the cell-surface receptors, we considered it to be of interest to examine whether the particulate fractions contain protein kinase activities amenable to hormonal manipulation and whether the presence of endogenous substrates in the particulate fraction for these kinases can be demonstrated.