title: Telokin mediates Ca2+-desensitization through activation of myosin phosphatase in phasic and tonic smooth muscle.
abstract: Telokin, a 17 kDa smooth muscle specific protein, consists of the C-terminal domain of MLCK, is phosphorylated by PKA and PKG at Ser13 in vivo (Wu et al. (1998) J Biol Chem 273: 11362-11369; Walker et al. (2001) J. Biol Chem 276: 24519-24524) and is proposed to induce Ca2+-desensitization through activation of myosin phosphatase (Wu et al. (1998) J. Biol Chem 273: 11362-11369). Telokin is reported to be highly expressed in phasic with only trace amounts in tonic smooth muscle. In alpha-toxin permeabilized femoral artery, 5 microM 8-Br-cGMP induced a two-fold increase in telokin phosphorylation and a maximal 30% relaxation of Ca2+-activated force compared to a 90% relaxation in phasic ileum muscle consistent with the relative amounts of telokin expressed in ileum, 27+/-4.6 microM SEM compared to 6+/-1.7 microM SEM, in femoral artery. Recombinant Wt telokin and the phospho-telokin mutant, S13D relaxed telokin-depleted femoral artery, by 38+/-8% SEM and 60+/-20% SEM, respectively. 8-Br-cGMP increased the rate and decreased the amplitude of force development initiated by photolysis of caged ATP in alpha-toxin permeabilized ileum and femoral artery smooth muscle, consistent with a cGMP-induced increase in phosphatase activity. Similarly, in telokin depleted ileum, recombinant S13D mutant telokin significantly increased the rate (0.08+/-0.01 s-1 vs. 014+/-0.02 s-1) and decreased force amplitude. In conclusion, our data support a role for telokin in cyclic nucleotide-induced relaxation of not only phasic, but also tonic smooth muscle and that this relaxation is mediated by activation of myosin phosphatase activity leading to a decrease in myosin light chain phosphorylation.
authors.affiliation: Department of Molecular Physiology, University of Virginia, Health Sciences System, Charlottesville, VA 22908-0736, USA.
authors.forename: Nandini
authors.initials: N
authors.lastname: Choudhury
authors.forename: Alexander S
authors.initials: AS
authors.lastname: Khromov
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Somlyo
authors.forename: Avril V
authors.initials: AV
authors.lastname: Somlyo
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000502:physiology*; D006152:Cyclic GMP / Q000031:analogs & derivatives / Q000494:pharmacology; D004789:Enzyme Activation; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D009130:Muscle, Smooth / Q000187:drug effects / Q000201:enzymology / Q000502:physiology*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D009219:Myosin-Light-Chain Kinase / Q000096:biosynthesis / Q000378:metabolism / Q000494:pharmacology / Q000502:physiology*; D043263:Myosin-Light-Chain Phosphatase / Q000378:metabolism*; D010446:Peptide Fragments; D010455:Peptides / Q000378:metabolism / Q000494:pharmacology / Q000502:physiology*; D010766:Phosphorylation; D011817:Rabbits; D011994:Recombinant Proteins / Q000494:pharmacology; D013997:Time Factors
chemical_list: D010446:Peptide Fragments; D010455:Peptides; D011994:Recombinant Proteins; C016276:8-bromocyclic GMP; C060640:telokin; D000255:Adenosine Triphosphate; D009219:Myosin-Light-Chain Kinase; D043263:Myosin-Light-Chain Phosphatase; D006152:Cyclic GMP; D002118:Calcium