TI - Silencing of p21-activated kinase attenuates vimentin phosphorylation on Ser-56 and reorientation of the vimentin network during stimulation of smooth muscle cells by 5-hydroxytryptamine . AB - Vimentin intermediate filaments undergo spatial reorganization in endothelial cells and fibroblasts in response to stimulation with platelet-derived growth factor and epidermal growth factor . In the present study , the vimentin network exhibited a curved filamentous structure in unstimulated smooth muscle cells . Vimentin filaments became straight and were arranged along the long axis of cells upon stimulation with 5-hydroxytryptamine ( 5-HT ; serotonin ) . Stimulation of smooth muscle cells with 5-HT also induced phosphorylation of vimentin on Ser-56 . Treatment of cells with small interfering RNA selectively down-regulated the expression of PAK1 ( p21-activated kinase 1 ) without affecting the content of smooth muscle alpha-actin . The silencing of PAK1 inhibited the site -specific phosphorylation and spatial rearrangement of the vimentin network in response to stimulation with 5-HT . Neither the disruption of stress fibres by cytochalasin D nor the inhibition of protein tyrosine phosphorylation affects the spatial reorganization of vimentin intermediate filaments in response to stimulation with 5-HT . In addition , stimulation of smooth muscle cells with 5-HT increased the ratio of soluble to insoluble vimentin . PAK1 silencing attenuated increases in the ratio of soluble to insoluble vimentin upon stimulation with 5-HT . These results suggest that the PAK -mediated site -specific phosphorylation of vimentin may play a role in regulating the reorganization of vimentin intermediate filaments during stimulation of smooth muscle cells with 5-HT .