M. Medkova et Wh. Cho, DIFFERENTIAL MEMBRANE-BINDING AND ACTIVATION MECHANISMS OF PROTEIN-KINASE-C-ALPHA AND PROTEIN-KINASE-C-EPSILON, Biochemistry, 37(14), 1998, pp. 4892-4900
To elucidate the mechanisms of membrane binding and activation of conv
entional and novel protein kinase C (PKC), we measured the interaction
s of rat PKC-alpha and -epsilon with phospholipid monolayers and vesic
les of various compositions. Besides the established difference in cal
cium requirement, the two isoforms showed major differences in their m
embrane-binding and activation mechanisms. For PKC-alpha, diacylglycer
ol (DG) specifically enhanced the binding of PKC-alpha to phosphatidyl
serine (PS)-containing vesicles by 2 orders of magnitude, allowing PKC
-alpha high specificity for PS. Also, PKC-alpha could penetrate into t
he phospholipid monolayer with a packing density comparable to that of
the cell membrane only in the presence of Ca2+ and PS. When compared
to PKC-alpha, PKC-epsilon had lower binding affinity for PS-containing
vesicles both in the presence and in the absence of DG, As a result,
PKC-epsilon did not show pronounced specificity for PS. Also, PKC-epsi
lon showed reduced penetration into PS-containing monolayers, which wa
s comparable to the Ca2+-independent penetration of PKC-alpha into the
same monolayers. Taken together, these results suggest the following:
(1) The role of Ca2+ in the membrane binding of PKC-alpha is to expos
e a specific PS-binding site. (2) Once bound to membrane surfaces, PS
specifically induces the partial membrane penetration of PKC-alpha tha
t allows its optimal interactions with DG, hence the enhanced membrane
binding and activation. (3) PKC-epsilon, due to the lack of Ca2+ bind
ing, cannot specifically interact with PS and DG, which implies the pr
esence of other physiological activator(s) for this isoform.