Myotubularin, the phosphatase mutated in X-linked myotubular myopathy, was shown 
to dephosphorylate phosphatidylinositol 3-monophosphate (PtdIns3P) and was also 
reported to interact with nuclear transcriptional regulators from the trithorax 
family. We have characterized a panel of specific antibodies and investigated 
the subcellular localization of myotubularin. Myotubularin is not detected in 
the nucleus, and localizes mostly as a dense cytoplasmic network. Overexpression 
of myotubularin does not detectably affect vesicle trafficking in the mammalian 
cells investigated, in contrast to previous observations in yeast models. Both 
mutation of a key aspartate residue of myotubularin and dominant activation of 
Rac1 GTPase lead to the recruitment of myotubularin to specific plasma membrane 
domains. Localization to Rac1-induced ruffles is dependent on the presence of a 
domain highly conserved in the myotubularin family (that we named RID). We thus 
propose that myotubularin may dephosphorylate a subpool of PtdIns3P (or another 
related substrate) at the plasma membrane.