Topography of protein kinases and phosphoproteins in the plasma membrane of 3T3 cells

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Abstract
The plasma membrane of 3T3 cells contains at least two different endogenous cyclic AMP‐dependent protein kinase systems. One catalyzes the phosphorylation of endogenous protein substrates, i.e., PP24 and PP14, whereas the other catalyzes the phosphorylation of exogenous substrates. In this paper the topography of these cyclic AMP‐dependent phosphorylation systems is described. The results show that the kinases which phosphorylate only exogenous substrates are primarily localized to the outer plasma membrane surface whereas the endogenous cyclic AMP‐dependent protein kinase and its two endogenous substrates are localized to the cytoplasmic plasma membrane surface. The data also establish that neither the cytoplasmically orientated kinase nor its substrates has a transmembrane orientation even though factors acting on the outer plasma membrane can affect these proteins. This suggests that functional modulation of the cytoplasmically localized cyclic AMP‐dependent phosphorylation system can be mediated by a transmembrane regulatory mechanism. The importance of determining the topography of such plasma membrane phosphorylation systems is emphasized by recent studies which show that neoplastic transformation can be mediated at least in part by protein kinases and/or phosphoproteins which are localized on the cytoplasmic surface of the plasma membrane.

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