CLONING, EXPRESSION, AND PURIFICATION OF RAT-BRAIN PROTEIN L-ISOASPARTYL METHYLTRANSFERASE

Protein L-isoaspartyl methyltransferase (PIMT) methylates isoaspartyl residues in peptides and proteins using S-adenosyl-L-methionine as the methyl donor, A cloned source of this enzyme should be useful in the identification of cellular substrates and for quantitation and localiz ation of isoaspartyl sites in purified proteins, including recombinant proteins used as pharmaceuticals. Rat brain PIMT cDNA was amplified u sing the polymerase chain reaction. The reaction product was direction ally cloned into the expression vector p Delta blue (M. E. Brandt and L. E. Vickery, Arch. Biochem. 294, 735-740, 1992). The vector contains the strong promoter lambda pL and allows for the direct expression of cloned genes. After transformation, Escherichia coli cells containing the plasmid constitutively produced recombinant rat brain PIMT (rrPIM T) at levels between 2 and 3% of total soluble protein. Recombinant en zyme was purified to homogeneity by ammonium sulfate precipitation of the crude extract followed by anion-exchange chromatography. The speci fic activity was 14,000 pmol methyl groups transferred/min/mg protein at 30 degrees C using bovine gamma-globulin as the methyl acceptor, A typical yield was 12 mg of purified rrPIMT per liter of bacterial cult ure. Subsequent dye ligand chromatography increased the specific activ ity of the preparation to 16,800 pmol methyl groups transferred/min/mg protein with an overall yield of 5 mg per liter of bacterial culture. Using isoaspartyl delta sleep-inducing peptide as the methyl acceptor , rrPIMT exhibited normal Michaelis-Menten kinetics that yielded the f ollowing constants: K-m (S-adenosyl-L-methionine) = 1.1 mu M, K-m (pep tide) = 16 mu M, V-max = 60,000 pmol/min/mg. (C) 1995 Academic Press, Inc.