Slow cycling of unphosphorylated myosin is inhibited by calponin, thus keeping smooth muscle relaxed

UMMS Affiliation

Biomedical Imaging Group

Publication Date


Document Type



Animals; Bufo marinus; Calcium-Binding Proteins; Cells, Cultured; Microfilament Proteins; Muscle Contraction; Muscle, Smooth; Myosin Light Chains; Phosphorylation


Life Sciences | Medicine and Health Sciences


A key unanswered question in smooth muscle biology is whether phosphorylation of the myosin regulatory light chain (RLC) is sufficient for regulation of contraction, or if thin-filament-based regulatory systems also contribute to this process. To address this issue, the endogenous RLC was extracted from single smooth muscle cells and replaced with either a thiophosphorylated RLC or a mutant RLC (T18A/S19A) that cannot be phosphorylated by myosin light chain kinase. The actin-binding protein calponin was also extracted. Following photolysis of caged ATP, cells without calponin that contained a nonphosphorylatable RLC shortened at 30% of the velocity and produced 65% of the isometric force of cells reconstituted with the thiophosphorylated RLC. The contraction of cells reconstituted with nonphosphorylatable RLC was, however, specifically suppressed in cells that contained calponin. These results indicate that calponin is required to maintain cells in a relaxed state, and that in the absence of this inhibition, dephosphorylated cross-bridges can slowly cycle and generate force. These findings thus provide a possible framework for understanding the development of latch contraction, a widely studied but poorly understood feature of smooth muscle.


Proc Natl Acad Sci U S A. 1997 Jul 8;94(14):7655-60.

Journal/Book/Conference Title

Proceedings of the National Academy of Sciences of the United States of America

Related Resources

Link to Article in PubMed

PubMed ID