Thymosin beta 4 expression and nuclear transport are regulated by hMLH1.

For hMLH1, a key enzyme of DNA mismatch repair and frequently mutated in human cancers, several additional functions have been suggested. We now identified Thymosin beta4 (Tbeta4), an actin-binding and cell motility regulating protein, by bacterial two-hybrid screening. Interaction was confirmed by coimmunoprecipitation. Tbeta4 was weakly expressed in the hMLH1-deficient cell lines 293T and HCT-116. Reconstitution of hMLH1 resulted in strong expression of Tbeta4. Confocal laser microscopy revealed nuclear colocalization of both proteins. Reconstitution with hMLH1 mutants lacking a functional nuclear localization sequence resulted in cytoplasmatic retention of both proteins. After Tbeta4- or hMLH1-siRNA treatment, cell migration of hMLH1-proficient cells was markedly decreased. Our results show that hMLH1 interacts with Tbeta4 and regulates its expression and nuclear transport. Moreover, loss of hMLH1 causes Tbeta4 deprivation and results in reduced migratory activity in vitro. These data give insight into novel functions of hMLH1 and probably disease related dysregulated mechanisms.