cAMP dependent actin phosphorylation in developing rat lung and type II epithelial cells.

Cyclic adenosine monophosphate (cAMP) increased in vitro phosphorylation of protein of 43,000 daltons in cytosolic fractions of rat lung and type II epithelial cells. The phosphoprotein was identified as 32P-actin by means of migration in one- and two-dimensional SDS-polyacrylamide gel electrophoresis and by phosphopeptide mapping followed by immunoperoxidase staining of peptides with anti-actin monoclonal antibody. Phosphorylation of actin in lung and type II cell cytosol was entirely cAMP dependent and the phosphorylated amino acid was identified as 32P-serine. Actin phosphorylation increased during the perinatal period of development, was barely detectable between 17 and 20 days gestation, increased prior to birth, and increased dramatically during the first week of life. Actin was the major substrate of cAMP-dependent protein kinase in lung cytosol from postnatal rats. Changes in actin phosphorylation that occur during development were not due to changes in cytosolic actin content or cAMP-dependent protein kinase activity but appeared to be related to the presence of factors inhibiting cAMP-dependent actin phosphorylation in fetal lung cytosol. Actin was also the major cAMP-dependent phosphoprotein identified in cytosolic fractions of purified type II epithelial cells. cAMP-dependent phosphorylation of pulmonary actin is developmentally regulated, occurring in association with other aspects of type II epithelial cell maturation during the perinatal period.