Association of hnRNP S1 proteins with vimentin intermediate filaments in migrating cells.

S1 proteins C2 and D2 are multifunctional hnRNP proteins acting as transcriptional regulators in the nucleus. Immunofluorescence staining of various cells in culture revealed that S1 proteins also occur in the cytoplasm, often in association with vimentin intermediate filaments (VFs). Here, we verified the association of S1 proteins with vimentin using vimentin-deficient cells, crosslinking and immunoprecipitation, and further investigated the biological significance of this association. S1 proteins on VFs, referred to here as S1 fibers, were lost in highly confluent cells, where cell proliferation and cellular metabolic activity greatly decreased owing to cell density-dependent arrest. However, the disappearance of S1 fibers was not related to these reduced activities, but to inhibited cell migration. Although undetected in cells of non-migratory tissues as well as in confluent cultured cells, S1 fibers were found in all migratory cells examined, such as cultured cells in scratch/wound experiments, blood neutrophils and monocytes, and fibroblasts engaging in tissue healing. In addition, S1 fibers reappeared even in confluent cells when VFs were induced to reorganize with okadaic acid. We propose that S1 proteins occur in association with VFs in migratory cells. Possible participation of S1 proteins in the formation/reorganization of VFs is discussed.