Serum-activated T51B rat liver cells transiently accumulate cyclic AMP-dependent protein kinases on their surfaces during the G1 phase.

Confluent T51B rat liver epithelial cells promptly began accumulating cyclic AMP-binding sites on their surfaces when they were stimulated from quiescence by serum growth factors in medium containing 1.8 mM Ca2+, but they began losing the accumulated binding sites shortly before initiating DNA replication. When the medium contained only 0.02 mM Ca2+, the cells still accumulated surface cyclic AMP-binding sites, but they did not initiate DNA replication and tended to continue accumulating the binding sites. The cyclic AMP-binding sites were eliminated completely by treating intact cells for 5 minutes with 0.005% trypsin (which did not damage the cells), and cyclic AMP caused them to be released from intact, undamaged cells into the medium. The binding sites also comigrated electrophoretically with purified regulatory subunits of type I cyclic AMP-dependent protein kinase, and to a lesser extent the regulatory subunit of type II cyclic AMP-dependent protein kinase. Therefore, it is likely that a transient accumulation of cyclic AMP-dependent protein kinases on the outer surface of the plasma membrane is part of the T51B rat liver cell's prereplicate program.