The amino-terminal non-catalytic region of Salmonella typhimurium SigD affects actin organization in yeast and mammalian cells

The internalization of Salmonella into epithelial cells relies on the function of bacterial proteins which are injected into the cell by a specialized type III secretion system. Such bacterial effectors interfere with host cell signalling and induce local cytoskeletal rearrangements. One of such effectors is SigD/SopB, which shares homology with mammalian inositol phosphatases. We made use of the Saccharomyces cerevisiae model for elucidating new aspects of SigD function. Endogenous expression of SigD in yeast caused severe growth inhibition. Surprisingly, sigD alleles mutated in the catalytic site or even deleted for the whole C-terminal phosphatase domain still inhibited yeast growth by inducing loss of actin polarization and precluding the budding process. Accordingly, when expressed in HeLa cells, the same sigD alleles lost the ability of depleting phosphatidylinositol 4,5-bisphosphate from the plasma membrane, but still caused disappearance of actin fibres and loss of adherence. We delineate a region of 25 amino acids (residues 118-142) that is necessary for the effect of SigD on actin in HeLa cells. Our data indicate that SigD exerts a toxic effect linked to its N-terminal region and independent of its phosphatase activity.