protein-kinase-inhibitor-peptide and kemptide

Affinities of the catalytic subunit (C1) of Saccharomyces cerevisiae cAMP-dependent protein kinase and of mammalian cGMP-dependent protein kinase were determined for the protein kinase inhibitor (PKI) peptide PKI(6-22)amide and seven analogues. These analogues contained structural alterations in the N-terminal alpha-helix, the C-terminal pseudosubstrate portion, or the central connecting region of the PKI peptide. In all cases, the PKI peptides were appreciably less active as inhibitors of yeast C1 than of mammalian C alpha subunit. Ki values ranged from 5- to 290-fold higher for the yeast enzyme than for its mammalian counterpart. Consistent with these results, yeast C1 exhibited a higher Km for the peptide substrate Kemptide. All of the PKI peptides were even less active against the mammalian cGMP-dependent protein kinase than toward yeast cAMP-dependent protein kinase, and Kemptide was a poorer substrate for the former enzyme. Alignment of amino acid sequences of these homologous protein kinases around residues in the active site of mammalian C alpha subunit known to interact with determinants in the PKI peptide [Knighton, D. R., Zheng, J., Ten Eyck, L. F., Xuong, N-h, Taylor, S. S., & Sowadski, J. M. (1991) Science 253, 414-420] provides a structural basis for the inherently lower affinities of yeast C1 and cGMP-dependent protein kinase for binding peptide inhibitors and substrates. Both yeast cAMP-dependent and mammalian cGMP-dependent protein kinases are missing two of the three acidic residues that interact with arginine-18 in the pseudosubstrate portion of PKI. Further, the cGMP-dependent protein kinase appears to completely lack the hydrophobic/aromatic pocket that recognizes the important phenylalanine-10 residue in the N-terminus of the PKI peptide, and binding of the inhibitor by the yeast protein kinase at this site appears to be partially compromised.

Topics: Amino Acid Sequence; Animals; Binding Sites; Cattle; Cyclic AMP; Cyclic GMP; Kinetics; Molecular Sequence Data; Myocardium; Oligopeptides; Peptide Fragments; Peptides; Protein Kinase Inhibitors; Protein Kinases; Saccharomyces cerevisiae; Substrate Specificity

The smooth muscle myosin light chain kinase (smMLCK) catalytic core was modeled by using the crystallographic coordinates of the cyclic AMP-dependent protein kinase catalytic subunit (cAPK) and a bound pseudosubstrate inhibitor peptide, PKI(5-24). Despite only 30% identity in amino acid sequence, the MLCK sequence can be readily accommodated in this structure. With the exception of the short B-helix, all major elements of secondary structure in the core are very likely conserved. The active site of the modeled MLCK complements the known requirements for peptide substrate recognition. MLCK contains a pseudosubstrate sequence that overlaps the calmodulin binding domain and has been proposed to act as an intrasteric inhibitor and occupy the substrate binding site in the absence of Ca(2+)-calmodulin. The pseudosubstrate sequence can be modeled easily into the entire backbone of PKI(5-24). The results demonstrate that the intrasteric model for regulation of MLCK by intramolecular competitive inhibition is structurally plausible.

Topics: Amino Acid Sequence; Binding Sites; Chromosome Mapping; Crystallography; Gene Expression Regulation, Enzymologic; Models, Molecular; Molecular Sequence Data; Molecular Structure; Myosin-Light-Chain Kinase; Oligopeptides; Peptide Fragments; Peptides; Protein Binding; Protein Kinases; Sequence Alignment; Sequence Homology

A synthetic peptide of 18 amino acids corresponding to the inhibitory domain of the heat-stable protein kinase inhibitor was synthesized and shown to inhibit both the C alpha and C beta isoforms of the catalytic (C) subunit of cAMP-dependent protein kinase. Extracts from cells transfected with expression vectors coding for the C alpha or the C beta isoform of the C subunit required 200 nM protein kinase inhibitor peptide for half-maximal inhibition of kinase activity in extracts from these cells. An affinity column was constructed using this synthetic peptide, and the column was incubated with protein extracts from cells overexpressing C alpha or C beta. Elution of the affinity column with arginine allowed single step isolation of purified C alpha and C beta subunits. The C alpha and C beta proteins were enriched 200-400-fold from cellular extracts by this single step of affinity chromatography. No residual inhibitory peptide activity could be detected in the purified protein. The purified C subunit isoforms were used to demonstrate preferential antibody reactivity with the C alpha isoform by Western blot analysis. Furthermore, preliminary characterization showed both isoforms have similar apparent Km values for ATP (4 microM) and for Kemptide (5.6 microM). These results demonstrate that a combination of affinity chromatography employing peptides derived from the heat-stable protein kinase inhibitor protein and the use of cells overexpressing C subunit related proteins may be an effective means for purification and characterization of the C subunit isoforms. Furthermore, this method of purification may be applicable to other kinases which are known to be specifically inhibited by small peptides.

Topics: Adenosine Triphosphate; Amino Acid Sequence; Blotting, Western; Cell Line; Chromatography, Affinity; Cyclic AMP; Electrophoresis, Polyacrylamide Gel; Isoenzymes; Kinetics; Molecular Sequence Data; Molecular Weight; Oligopeptides; Peptides; Protein Kinase Inhibitors; Protein Kinases