MTMR2 is a member of the myotubularin family of inositol lipid phosphatases, a 
large protein-tyrosine phosphatase subgroup that is conserved from yeast to 
humans. Furthermore, the peripheral neuromuscular disease Charcot-Marie Tooth 
disease type 4B has been attributed to mutations in the mtmr2 gene. Because the 
molecular mechanisms regulating MTMR2 have been poorly defined, we investigated 
whether reversible phosphorylation might regulate MTMR2 function. We used mass 
spectrometry-based methods to identify a high stoichiometry phosphorylation site 
on serine 58 of MTMR2. Phosphorylation at Ser(58), or a phosphomimetic S58E 
mutation, markedly decreased MTMR2 localization to endocytic vesicular 
structures. In contrast, a phosphorylation-deficient MTMR2 mutant (S58A) 
displayed constitutive localization to early endocytic structures. This 
localization pattern was accompanied by displacement of a PI(3)P-specific sensor 
protein and an increase in signal transduction pathways. Thus, MTMR2 
phosphorylation is likely to be a critical mechanism by which MTMR2 access to 
its lipid substrate(s) is temporally and spatially regulated, thereby 
contributing to the control of downstream endosome maturation events.