The Ca2+- and phospholipid-dependent protein kinase, protein kinase C (PKC), was purified from bovine brain by a modified procedure that provided sufficient quantities of stable protein for analysis of physical properties of protein-membrane binding. The binding of PKC to phospholipid vesicles of various compositions was investigated by light-scattering and fluorescence energy transfer measurements. The binding properties for membranes of low phosphatidylserine (PS) content were consistent with a peripheral membrane association; PKC showed Ca2+-dependent binding to phospholipid vesicles containing phosphatidylserine, phosphatidylinositol, or phosphatidylglycerol. Membranes containing 0-20% PS (the remainder of the phospholipid was phosphatidylcholine) bound less protein than membranes containing greater than 20% PS; the factor limiting protein binding to membranes containing low PS appeared to be the availability of acidic phospholipids. Increasing the PS content above 20% did not increase the amount of membrane-bound protein at saturation, and the limiting factor was probably steric packing of protein on the membrane surface. The membranes bound about 1 g of protein/g of phospholipid at steric saturation. Binding was of relatively high affinity (Kd < 5 nM), and the association rate was rapid on the time scale of the experiments. Addition of ethylene glycol bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid to phospholipid-bound PKC caused dissociation of the complex, and the properties of this dissociation indicated an equilibrium binding of protein to membrane. However, only partial dissociation of PKC was achieved when the PS content of the vesicles exceeded 20%. A number of comparisons revealed that binding of protein to the membrane, even in the presence of phorbol esters, was insufficient for development of enzyme activity. For example, PS was much more effective in promoting activity than the other acidic phospholipids but was not greatly superior in protein binding. Diolein (DAG) and phorbol esters, potent activators of PKC activity, had little effect on the binding of the enzyme to phospholipid vesicles or on the Ca2+ requirement of binding. Binding required only equal weight concentrations of phospholipid and protein while activity always required a large excess of phospholipid. These studies suggest that factors other than formation of a PKC-PS-Ca2+-DAG complex are critical to development of enzyme activity.
|Original language||English (US)|
|Number of pages||8|
|State||Published - 1987|