The subcellular distribution of the regulatory (RII) subunit of cyclic AMP-dependent protein kinase (PK-A) was analyzed at the electron-microscopical level using thawed cryo-sections of Madin Darby Bovine Kidney (MDBK) cells. The highest density of labelling for RII was found on membranes of the prelysosomal compartment (PLC; marked with the cation-independent mannose 6-phosphate receptor, MPR) and the trans-Golgi network, TGN (at 20 degrees C, marked with the G protein of vesicular stomatitis virus, VSV), as well as in coated buds on the latter. Significant labelling was also localized to the cytoplasmic surface of the plasma membrane, including clathrin-coated pits and microvilli, and to early endosomes (identified using internalized HRP). In contrast, no significant label was seen over the Golgi compartments proximal to the TGN, the endoplasmic reticulum (ER) or over lysosomes. From these results we conclude that PK-A type II is associated with the membranes of precisely those subcellular compartments that are active in endocytosis and recycling of cell surface receptors. We believe these findings to be related to the well-established role of cyclic AMP in signal transduction. In particular, we propose that activation of PK-A in endocytic compartments may contribute to regulation (via phosphorylation) of the subcellular distribution of internalized surface receptors or their functional coupling to effector systems involved in signal propagation.

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