The ATPase Activity of Myr3, a Rat Myosin I, Is Allosterically Inhibited by Its Own Tail Domain and by Ca2+ Binding to Its Light Chain Calmodulin

• Published in: The Journal of biological chemistry Vol. 273; no. 23; pp. 14605 - 14611
• Main Authors: Stöffler, Hanns-Eugen, Bähler, Martin
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 05.06.1998; American Society for Biochemistry and Molecular Biology
• Subjects: Actins - metabolism; Actins - ultrastructure; Allosteric Regulation - physiology; Amino Acid Sequence; Animals; Antibodies - pharmacology; Ca(2+) Mg(2+)-ATPase - antagonists & inhibitors; Calcium - pharmacology; Calmodulin - metabolism; Enzyme Activation - physiology; Immunoglobulin Fab Fragments - pharmacology; Liver - chemistry; Male; Microscopy, Electron; Molecular Sequence Data; Myosin Light Chains - metabolism; Myosin Type I; Myosins - chemistry; Myosins - ultrastructure; Peptide Fragments - chemistry; Rats; Rats, Sprague-Dawley; src Homology Domains - physiology
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.273.23.14605

We purified Myr3 (third unconventional myosin from rat), a mammalian “amoeboid” subclass myosin I, from rat liver. The heavy chain of purified Myr3 is associated with a single calmodulin light chain. Myr3 exhibits K/EDTA-ATPase and Mg-ATPase activity. The Mg-ATPase activity is stimulated by increasing F-actin concentrations in a complex triphasic manner similar to the Mg-ATPase activity of myosin I molecules from protozoa. Although purified Myr3 was observed to cross-link actin filaments, it bound in an ATP regulated manner to F-actin, and no evidence for a nucleotide-independent high affinity actin binding site that could explain the triphasic activation pattern was obtained. Micromolar concentrations of free Ca2+ reversibly inhibit the Mg-ATPase activity of Myr3 by binding to its light chain calmodulin, which remains bound to the Myr3 heavy chain irrespective of the free Ca2+ concentration. Polyclonal antibodies and Fab fragments directed against the tail domain were found to stimulate the Mg-ATPase activity. A similar stimulation of the Myr3 Mg-ATPase activity is observed upon proteolytic removal of the very C-terminal SH3 domain. These results demonstrate that Myr3 is subject to negative regulation by free calcium and its own tail domain and possibly positive regulation by a tail-domain binding partner.