Role of vimentin phosphorylation at Ser‐56 in mediating tetramer formation and spatial reorganization of vimentin filaments in smooth muscle cells

Intermediate filament protein vimentin has been implicated in mediating tension development in smooth muscle. Our previous studies have shown that vimentin undergoes phosphorylation on Ser‐56 and spatial reorganization in cultured smooth muscle cells upon contractile stimulation. However, the mechanism by which vimentin phosphorylation affects its spatial remodeling is not well understood. We hypothesized that vimentin phosphorylation at Ser‐56 may mediate vimentin tetramer formation (disassembly) and the structural reorganization. Wild type vimentin and the vimentin mutant S56A (alanine substitution at serine‐56) were reacted with P21‐activated kinase in vitro. Vimentin phosphorylation at Ser‐56 and tetramer formation were than assessed. Phosphorylation at Ser‐56 and the amount of tetramers were higher in wild type vimentin than in S56A vimentin mutant. Furthermore, stimulation of cells expressing wild type vimentin with 5‐hydroxytryptamine (5‐HT) increased the tetramer amount and initiated the spatial reorganization of vimentin filaments, whereas the tetramer formation and vimentin filament remodeling were inhibited in cells expressing S56A vimentin mutant. Our results suggest that vimentin phosphorylation at Ser‐56 plays an essential role in mediating the tetramer formation and spatial remodeling of vimentin filaments in smooth muscle cells during 5‐HT stimulation.