IDENTIFICATION OF 2 17-KDA RAT PAROTID-GLAND PHOSPHOPROTEINS, SUBJECTS FOR DEPHOSPHORYLATION UPON BETA-ADRENERGIC STIMULATION, AS DESTRIN-LIKE AND COFILIN-LIKE PROTEINS

We previously reported that when P-32(i)-loaded rat parotid slices are incubated with the beta-adrenergic agonist isoproterenol, the level of a soluble P-32-labeled 17-kDa protein (pp17) decreases rapidly (Kanamori, T., and Hayakawa, T. (1982) Biochem. Int. 4, 517-523). Here we show that pp17 consists of two distinct phosphoproteins (pp17a and pp17b), identify their unphosphorylated forms (p17a and p17b, respectively), and provide evidence for their beta-adrenergic stimulation-induced dephosphorylation. Since p17a and p17b were predominant forms even in nonstimulated cells, peptides were generated from them with Staphylococcus aureus V8 protease or cyanogen bromide; subsequent sequencing of these peptides and homology search allowed identification of p17a and p17b as destrin- and cofilin-like proteins, respectively. Interestingly, they were also dephosphorylated in response to cholinergic stimulation. Because destrin and cofilin are actin-depolymerizing proteins whose activities are possibly regulated by their phosphorylation/dephosphorylation, the two parotid proteins reported here might be involved in cortical F-actin disruption observed in parallel with exocytotic amylase secretion.