Caveolae are flask-shaped micro-invaginations associated with the plasma membrane of a wide variety of cell types.
Caveolin, an integral membrane component of caveolae, was first identified as the major
phosphoprotein whose phosphorylation was elevated in v-Src transformed cells. As both v-Src transformation and elevated
caveolin phosphorylation were dependent on membrane attachment of v-Src, it has been suggested that
caveolin is a critical target in v-Src transformation. Although an increase in
tyrosine phosphorylation of
caveolin was evident, the increase in
caveolin phosphorylation was predominantly on
serine residues. In accordance with these in vivo observations, isolated
caveolin-rich membrane domains undergo phosphorylation in vitro predominantly on
serine and contain an unidentified
serine kinase activity. Here, we have identified this
serine kinase activity as a
casein kinase II-like
enzyme, since the phosphorylation of
caveolin-rich membrane domains is stimulated and inhibited by known effectors of
casein kinase II (poly-
L-lysine, endogenous
polyamines, and a
casein kinase II inhibitor
peptide), but is unaffected by modulators of other known
kinases. In support of these observations,
caveolin contains a consensus sequence for
casein kinase II phosphorylation in its cytoplasmic N-terminal domain (Ser-88). A
peptide containing this sequence inhibits the in vitro phosphorylation of
caveolin-rich membrane domains, while many other
peptides derived from the N-terminal domain of
caveolin do not affect phosphorylation.
Caveolin-rich membrane domains were also a substrate for exogenously added purified
casein kinase II, but not
casein kinase I. Finally, immunoblotting of these domains with an antibody directed against the alpha and alpha' subunits of
casein kinase II reveals two bands with apparent molecular weights consistent with the known molecular weights of the alpha and alpha' subunits of
casein kinase II. As
casein kinase II appears to play a role in mitogenic signalling events and
casein kinase II activators (endogenous
polyamines) are required for v-Src transformation, our results may have implications for understanding the mechanism of v-Src
oncogenesis.