Properties of a monoclonal antibody directed to the calmodulin-binding domain of rabbit skeletal muscle myosin light chain kinase

Activation and inhibition of phosphorylase kinase by monospecific antibodies raised against peptides from the regulatory domain of the gamma-subunit

The C terminus of the catalytic gamma-subunit of phosphorylase kinase comprises a regulatory domain that contains regions important for subunit interactions and autoinhibitory functions. Monospecific antibodies raised against four synthetic peptides from this region, PhK1 (362-386), PhK5 (342-366), PhK9 (322-346), and PhK13 (302-326), were found to have significant effects on the catalytic activities of phosphorylase kinase holoenzyme and the gamma delta complex. Antibodies raised against the very C terminus of the gamma-subunit, anti-PhK1 and anti-PhK5, markedly activated both holoenzyme and the gamma delta complex, in the presence and absence of Ca2+. In the presence of Ca2+ at pH 8.2, anti-PhK1 activated the holoenzyme more than 11-fold and activated the gamma delta complex 2.5-fold. Activation of the holoenzyme and the gamma delta complex by anti-PhK5 was 50-70% of that observed with anti-PhK1. Prior phosphorylation of the holoenzyme by the cAMP-dependent protein kinase blocked activation by both anti-PhK1 and anti-PhK5. Antibodies raised against the peptides from the N terminus of the regulatory domain, anti-PhK9 and anti-PhK13, were inhibitory, with their greatest effects on the gamma delta complex. These data demonstrate that the binding of antibodies to specific regions within the regulatory domain of the gamma-subunit can augment or inhibit structural changes and subunit interactions important in regulating phosphorylase kinase activity.

The solution structures of calmodulin and its complexes with synthetic peptides based on target enzyme binding domains

Small-angle X-ray and neutron scattering experiments have given important information on the solution structures of calmodulin and its complexes with synthetic peptides used to model target enzyme interactions. In combination with crystallographic data, site directed mutagenesis and various spectroscopic studies, these experiments have contributed to our understanding of the solution structure of calmodulin in different functional states. We have gained important insights into the conformational flexibility in calmodulin that appears to be crucial to its regulatory functions. Specifically, flexibility in the interconnecting helix region of calmodulin has been shown to play a critical role in facilitating calmodulin's binding to a wide variety of target enzymes whose activities are thus regulated. This review will focus mainly on the contributions small-angle scattering has made to our understanding of the solution structure of calmodulin in the context of other studies, with particular regard to circular dichroism and Fourier transform infrared studies that complement the small-angle scattering data.

Activation of smooth muscle myosin light chain kinase activity by a monoclonal antibody which recognizes the calmodulin-binding region

The regulatory domain of smooth muscle myosin light chain kinase (MLCK) was studied using monoclonal antibodies. Of the 22 monoclonal antibodies tested, a monoclonal antibody designated LKH-18 was found to activate MLCK in the absence of Ca2+/calmodulin. This activation was even greater when an Fab fragment of LKH-18 was used. Consequently, the actin-dependent smooth muscle myosin ATPase activity and the superprecipitation of actomyosin were significantly activated by MLCK plus LKH-18, even in the absence of Ca2+/calmodulin. The antibody-binding site was studied using proteolytic fragments and synthetic peptide analogues of MLCK. Immunoblot analysis revealed that LKH-18 reacted with the 66 kDa calmodulin-dependent active fragment but not with the 64 kDa inactive fragment or with the 61 kDa calmodulin-independent active fragment. Furthermore, LKH-18 reacted with MLCK-(796-815)-peptide but not with MLCK-(786-801)-peptide or with MLCK-(796-807)-peptide. Therefore the LKH-18-binding site was assigned to amino acid residues 808-815 of MLCK, which are thought to be a part of the calmodulin-binding site. The present results suggest that the binding of ligand to this region induces a conformation change in MLCK and that this abolishes the action of the inhibitory region which exists next to the N-terminal side of the calmodulin-binding site.

A monoclonal antibody against brain calmodulin-dependent protein kinase type II detects putative conformational changes induced by Ca2+-calmodulin

A mouse monoclonal IgG1 antibody has been generated against the soluble form of the calmodulin-dependent protein kinase type II. This antibody recognizes both the soluble and cytoskeletal forms of the enzyme, requiring Ca2+ (EC50 = 20 microM) for the interaction. Other divalent cations such as Zn2+, Mn2+, Cd2+, Co2+, and Ni2+ will substitute for Ca2+, while Mg2+ and Ba2+ will not. The antibody reacts with both the alpha- and beta-subunits on Western blots in a similar Ca2+-dependent fashion but with a lower sensitivity. The affinity of the antibody for the kinase is 0.13 nM determined by displacement of 125I Bolton-Hunter-labeled kinase with unlabeled enzyme. A variety of other proteins including tubulin do not compete for antibody binding. The Mr 30,000 catalytic fragment obtained by proteolysis of either the soluble or the cytoskeletal form of the kinase fails to react with the antibody. Calmodulin and antibody reciprocally potentiate each other's interaction with the enzyme. This is illustrated both by direct binding studies and by a decrease of the Kmapp for calmodulin and an increase in the Vmax for the autophosphorylation reaction of the enzyme. The antibody thus appears to recognize and stabilize a conformation of the kinase which favors calmodulin binding although it does not itself activate the kinase in the absence of calmodulin. Since the Mr 30,000 catalytic fragment of the kinase is not immunoreactive, either the antibody combining site of the kinase must be present in the noncatalytic portion of the protein along with the calmodulin binding site or proteolysis interferes with the putative Ca2+-dependent conformational change.(ABSTRACT TRUNCATED AT 250 WORDS)