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1 Pediatrics, University of Michigan, Ann Arbor, MI, USA
* To whom correspondence should be addressed. E-mail: bitar{at}umich.edu.
Displacement of the contractile protein tropomyosin from actin filament exposes the myosin binding sites on actin, resulting in actin-myosin interaction and muscle contraction. The objective of the present study was to better understand the interaction of tropomyosin with HSP27 in contraction of smooth muscle cells of the colon. We have investigated the possibility of a direct protein-protein interaction of tropomyosin with HSP27 and the role of phosphorylated HSP27 in this interaction. Immunoprecipitation studies on rabbit smooth muscle cells indicate that upon acetylcholine-induced contraction, tropomyosin shows increased association with HSP27 phosphorylated at ser82 and ser78. Transfection of smooth muscle cells with HSP27 phosphorylation mutants indicated that the association of tropomyosin with HSP27 could be affected by HSP27 phosphorylation. In vitro binding studies with GST-tagged HSP27 mutant proteins show that tropomyosin has greater direct interaction to phosphomimic HSP27 mutant compared to wild type and non-phosphomimic HSP27. Our data suggest that in response to a contractile agonist, HSP27 undergoes a rapid phosphorylation that may strengthen its interaction with tropomyosin.
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