Truncated form of tenascin-X, XB-S, interacts with mitotic motor kinesin Eg5.

XB-S is a protein with an amino-terminal-truncated form of tenascin-X (TNXB). However, the precise roles of XB-S in vivo are unknown. In this study, to determine the role of XB-S in vivo, we screened XB-S-binding proteins. FLAG-tagged XB-S was transiently introduced into 293T cells. Then its associated proteins were purified by immunoprecipitation using an anti-FLAG antibody and its components were identified by mass spectrometric analyses. Mitotic motor kinesin Eg5 was identified in the immunoprecipitates. XB-S and Eg5 proteins were co-localized in the cytoplasm in interphase and mitosis, but XB-S did not localize on mitotic spindle microtubules, on which Eg5 prominently localized in mitosis. As for Eg5 binding to XB-S, glutathione S-transferase-fused XB-S expressed in vitro directly bound to full-length Eg5 translated in reticulocyte lysate, and the XB-S-binding region was located in the motor domain of Eg5. Furthermore, during cell cycle progression XB-S showed a similar expression profile to that of Eg5. These results suggest possible involvement of XB-S in the function of Eg5.