title: The Dictyostelium class I myosin, MyoD, contains a novel light chain that lacks high-affinity calcium-binding sites.
abstract: Dictyostelium discoideum MyoD, a long-tailed class I myosin, co-purified with two copies of a 16 kDa light chain. Sequence analysis of the MyoD light chain showed it to be a unique protein, termed MlcD, that shares 44% sequence identity with Dictyostelium calmodulin and 43% sequence identity with Acanthamoeba castellanii myosin IC light chain. MlcD comprises four EF-hands; however, EF-hands 2-4 contain mutations in key Ca2+-co-ordinating residues that would be predicted to impair Ca2+ binding. Electrospray ionization MS of MlcD in the presence of Ca2+ and La3+ showed the presence of one major and one minor metal-binding site. MlcD contains a single tryptophan residue (Trp39), the fluorescence intensity of which was quenched upon addition of Ca2+ or Mg2+, yielding apparent dissociation constants ( K'(d)) of 52 microM for Ca2+ and 450 microM for Mg2+. The low affinity of MlcD for Ca2+ indicates that it cannot function as a sensor of physiological Ca2+. Ca2+ did not affect the binding of MlcD to MyoD or to either of the two MyoD IQ (Ile-Gln) motifs. FLAG-MlcD expressed in Dictyostelium formed a complex with MyoD, but not with the two other long-tailed Dictyostelium myosin I isoenzymes, MyoB and MyoC. Through its specific association with the Ca2+-insensitive MlcD, MyoD may exhibit distinct regulatory properties that distinguish it from myosin I isoenzymes with calmodulin light chains.
authors.affiliation: Department of Biochemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6.
authors.forename: Marc A
authors.initials: MA
authors.lastname: De La Roche
authors.forename: Sheu-Fen
authors.initials: SF
authors.lastname: Lee
authors.forename: Graham P
authors.initials: GP
authors.lastname: Côté
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002147:Calmodulin / Q000378:metabolism; D002148:Calmodulin-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004023:Dictyostelium / Q000737:chemistry* / Q000235:genetics; D008969:Molecular Sequence Data; D017570:MyoD Protein / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018994:Myosin Light Chains / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D024461:Myosin Type I / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010802:Phylogeny; D015800:Protozoan Proteins / Q000737:chemistry / Q000235:genetics
chemical_list: D002135:Calcium-Binding Proteins; D002147:Calmodulin; D002148:Calmodulin-Binding Proteins; D017570:MyoD Protein; D018994:Myosin Light Chains; D015800:Protozoan Proteins; D024461:Myosin Type I; D002118:Calcium