Affinity purification and characterization of myristoylated alanine-rich protein kinase C substrate (MARCKS) from bovine brain. Comparison of the cytoplasmic and the membrane-bound forms.

A major in vivo substrate of Ca(2+)-phospholipid-dependent protein kinase (myristoylated alanine-rich C-kinase substrate (MARCKS)) has been purified to apparent homogeneity from the particulate as well as from the cytoplasmic fractions of calf brain using a calmodulin affinity column. The two preparations were characterized and compared with various biochemical and biophysical techniques. Although they behave similarly in various chromatographic procedures during purification, their elution positions from the gel filtration column are markedly different. Stokes radii of 85 and 45 A were measured for the cytoplasmic and membrane MARCKS, respectively. Once purified, however, they show a similar small Stokes radius (45 A), suggesting the dissociation of a component or a drastic conformational change in the cytoplasmic preparation during purification. The electrospray mass spectroscopic analysis of the two preparations revealed the existence of at least three major subpopulations with molecular mass differences of 80 daltons, which suggests the presence of protein phosphorylated in different degrees. The cytoplasmic preparation contains more phosphorylated species compared with the membrane preparation, whereas the calculated molecular weight of each peak was indistinguishable between the two preparations. Correspondingly, when the two preparations were phosphorylated by purified protein kinase C in vitro, more phosphate groups were transferred to the membrane preparation (4 mol/mol) than to the cytoplasmic preparation (2.9 mol/mol). A significant difference was also observed in the inhibition of calmodulin of the phosphorylation reaction. On the other hand, the circular dichroism of the two preparations showed similar spectra rich in random coil with little contribution of alpha-helix (approximately 10%), suggesting that there is not a significant difference in the overall conformation. These results clearly established that the two preparations are the same protein coded by a single gene but they differ in their degree of phosphorylation, and that the difference observed in their Stokes radius is due to the presence of an unidentified factor that is removed from the cytoplasmic MARCKS during purification.