Phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) protein is associated with bovine luteal oxytocin exocytosis

The ruminant corpus luteum, in addition to producing progesterone, synthesi zes and secretes oxytocin (OT) during the estrous cycle. Secretion of oxyto cin occurs by exocytosis of membrane-encapsulated granules of this hormone, Exocytosis of oxytocin involves transport of granules through a cytoskelet al matrix including an actin cortex closely associated with the plasma memb rane (PM). Actin filaments crosslinked by various proteins give rise to the structural integrity of the cortex. Myristoylated alanine-rich C kinase su bstrate (MARCKS), a protein specifically phosphorylated by protein kinase C (PKC), crosslinks actin filaments and anchors the actin network to the inn er leaflet of the PM. There is evidence that the intact actin cortex may se rve as a barrier, precluding fusion of transport vesicles with the PM. In s ome secretory cells, phosphorylation of MARCKS has resulted in its transloc ation from the PM to the cytoplasm with an associated disassembly of the ac tin cortex. Prostaglandin F-2 alpha (PGF(2 alpha)) stimulation of the bovin e corpus luteum during the midluteal phase of the estrous cycle activates P KC, which is associated with an increase in OT secretion in vivo and in vit ro. Data are presented demonstrating that stimulation of bovine luteal cell s with PGF(2 alpha) on Day 8 of the cycle promotes rapid phosphorylation of MARCKS protein and causes its translocation from the PM to the cytoplasm a nd concomitant, enhanced exocytosis of OT These data are consistent with th e premise that MARCKS plays a role in the exocytotic process.