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The role of the calponin homology domain of smoothelin-like 1 (SMTNL1) in myosin phosphatase inhibition and smooth muscle contraction

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Abstract

In this study, we provide further insight into the contribution of the smoothelin-like 1 (SMTNL1) calponin homology (CH)-domain on myosin light chain phosphatase (SMPP-1M) activity and smooth muscle contraction. SMTNL1 protein was shown to have inhibitory effects on SMPP-1M activity but not on myosin light chain kinase (MLCK) activity. Treatment of β-escin permeabilized rabbit, ileal smooth muscle with SMTNL1 had no effect on the time required to reach half-maximal force (t1/2) during stimulation with pCa6.3 solution. The addition of recombinant SMTNL1 protein to permeabilized, smooth muscle strips caused a significant decrease in contractile force. While the calponin homology (CH)-domain was essential for maximal SMTNL1-associated relaxation, it alone did not cause significant changes in force. SMTNL1 was poorly dephosphorylated by PP-1C in the presence of the myosin targeting subunit (MYPT1), suggesting that phosphorylated SMTNL1 does not possess “substrate trapping” properties. Moreover, while full-length SMTNL1 could suppress SMPP-1M activity toward LC20 in vitro, truncated SMTNL1 lacking the CH-domain was ineffective. In summary, our findings suggest an important role for the CH-domain in mediating the effects of SMTNL1 on smooth muscle contraction.

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Acknowledgments

This work was supported by research grants from the Heart and Stroke Foundation of Canada (HSFC) to J.A.M. and National Institutes of Health (DK065954-02) to T.A.J.H. M.A.B. was a recipient of Fellowships from the Alberta Heritage Foundation for Medical Research (AHFMR) and HSFC. J.A.M. is recipient of a Canada Research Chair (Tier II) in Smooth Muscle Pathophysiology and a Scholarship from HSFC.

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Correspondence to Justin A. MacDonald.

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Borman, M.A., Freed, T.A., Haystead, T.A.J. et al. The role of the calponin homology domain of smoothelin-like 1 (SMTNL1) in myosin phosphatase inhibition and smooth muscle contraction. Mol Cell Biochem 327, 93–100 (2009). https://doi.org/10.1007/s11010-009-0047-z

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