Phosphorylated HSP27 essential for acetylcholine-induced association of RhoA with PKCα

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Abstract
Reorganization of the cytoskeleton and association of contractile proteins are important steps in modulating smooth muscle contraction. Heat shock protein (HSP) 27 has significant effects on actin cytoskeletal reorganization during smooth muscle contraction. We investigated the role of phosphorylated HSP27 in modulating acetylcholine-induced sustained contraction of smooth muscle cells from the rabbit colon by transfecting smooth muscle cells with phosphomimic (3D) or nonphosphomimic (3G) HSP27. In 3G cells, the initial peak contractile response at 30 s was inhibited by 25% (24.0 ± 4.5% decrease in cell length, n = 4). The sustained contraction was greatly inhibited by 75% [9.3 ± .9% decreases in cell length ( n = 4)]. Furthermore, in 3D cells, translocation of both PKCα and of RhoA was greatly enhanced and resulted in a greater association of PKCα-RhoA in the membrane fraction. In 3G transfected cells, PKCα and RhoA failed to translocate in response to stimulation with acetylcholine, resulting in an inhibition of association of PKCα-RhoA in the membrane fraction. Studies using GST-RhoA fusion protein indicate that there is a direct association of RhoA with PKCα and with HSP27. The results suggest that phosphorylated HSP27 plays a crucial role in the maintenance of association of PKCα-RhoA in the membrane fraction and in the maintenance of acetylcholine-induced sustained contraction.