Telokin mediates Ca2+-desensitization through activation of myosin phosphatase in phasic and tonic smooth muscle

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 α-toxin permeabilized femoral artery, 5 μM 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 μM SEM compared to 6 ± 1.7 μM 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 α-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.