Isolation of the non-myristoylated form of a major substrate of protein kinase C (MARCKS) from bovine brain.

A substrate protein of protein kinase C with an apparent molecular mass of 70 kDa has been purified from bovine brain. This protein shares several properties with a major substrate of protein kinase C (myristoylated alanine-rich C kinase substrate; MARCKS). It is heat-stable and copurifies with MARCKS during various steps (ammonium sulfate precipitation and gel filtration). However, its elution from a calmodulin affinity column is different from that of MARCKS. It can be eluted by high ionic strength in the presence of calcium, whereas MARCKS can be eluted only in the absence of calcium. Its earlier elution from a reversed phase column suggests that p70 is less hydrophobic than MARCKS. The electrospray mass spectrum revealed an actual mass of 31,550 +/- 6.5 Da, very far from the apparent molecular mass in SDS-polyacrylamide gel electrophoresis (70,000 Da). This mass is about 200 Da smaller than that of MARCKS determined by mass spectrometry analysis (Manenti, S., Sorokine, O., Van Dorsselaer, A., and Taniguchi, H. (1992) J. Biol. Chem. 267, 22310-22315), close to the value expected for the change due to N-terminal myristoylation (210 Da). N-terminal amino acid sequencing showed that the N terminus is not blocked, and the sequence found for the 10 first amino acids is identical to that deduced from the cDNA sequence of bovine MARCKS. These data clearly establish that this protein is a non-myristoylated form of MARCKS and that the absence of the myristoyl moiety at the N terminus lowers the affinity to calmodulin. The purification performed both from the membrane and the cytoplasmic fractions of bovine brain indicated that this non-myristoylated form represents 20-30% of the MARCKS protein in the cytoplasmic fraction, and less than 5% in the membrane one.