title: The role of the calponin homology domain of smoothelin-like 1 (SMTNL1) in myosin phosphatase inhibition and smooth muscle contraction.
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 beta-escin permeabilized rabbit, ileal smooth muscle with SMTNL1 had no effect on the time required to reach half-maximal force (t(1/2)) 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 LC(20) 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.
authors.affiliation: Faculty of Medicine, Department of Biochemistry & Molecular Biology, University of Calgary, 3330 Hospital Drive N.W., Calgary, AB T2N4N1, Canada.
authors.forename: Meredith A
authors.initials: MA
authors.lastname: Borman
authors.forename: Tiffany A
authors.initials: TA
authors.lastname: Freed
authors.forename: Timothy A J
authors.initials: TA
authors.lastname: Haystead
authors.forename: Justin A
authors.initials: JA
authors.lastname: Macdonald
mesh_terms: D000818:Animals; D002135:Calcium-Binding Proteins / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008840:Microfilament Proteins / Q000737:chemistry* / Q000378:metabolism; D009119:Muscle Contraction / Q000502:physiology*; D009124:Muscle Proteins / Q000737:chemistry* / Q000378:metabolism; D009130:Muscle, Smooth / Q000502:physiology*; D009219:Myosin-Light-Chain Kinase / Q000378:metabolism; D043263:Myosin-Light-Chain Phosphatase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D017434:Protein Structure, Tertiary; D011817:Rabbits; D000096985:Calponins
chemical_list: D002135:Calcium-Binding Proteins; D008840:Microfilament Proteins; D009124:Muscle Proteins; D009219:Myosin-Light-Chain Kinase; D043263:Myosin-Light-Chain Phosphatase; D000096985:Calponins; C543629:SMTNL1 protein, human