Charcot-Marie-Tooth disease type 4B (
CMT4B) is a severe, demyelinating
peripheral neuropathy characterized by distinctive, focally folded myelin sheaths.
CMT4B is caused by recessively inherited mutations in either
myotubularin-related 2 (MTMR2) or MTMR13 (also called SET-binding factor 2). MTMR2 encodes a member of the
myotubularin family of phosphoinositide-3-phosphatases, which dephosphorylate
phosphatidylinositol 3-phosphate (PI(3)P) and bisphosphate
PI(3,5)P2. MTMR13 encodes a large, uncharacterized member of the
myotubularin family. The MTMR13
phosphatase domain is catalytically inactive because the essential Cys and Arg residues are absent. Given the genetic association of both MTMR2 and MTMR13 with
CMT4B, we investigated the biochemical relationship between these two
proteins. We found that the endogenous MTMR2 and MTMR13
proteins are associated in human embryonic kidney 293 cells. MTMR2-MTMR13 association is mediated by coiled-coil sequences present in each
protein. We also examined the cellular localization of MTMR2 and MTMR13 using fluorescence microscopy and subcellular fractionation. We found that (i) MTMR13 is a predominantly
membrane-associated protein; (ii) MTMR2 and MTMR13 cofractionate in both a light membrane fraction and a cytosolic fraction; and (iii) MTMR13 membrane association is mediated by the segment of the
protein which contains the pseudophosphatase domain. This work, which describes the first cellular or biochemical investigation of the MTMR13 pseudophosphatase
protein, suggests that MTMR13 functions in association with MTMR2. Loss of MTMR13 function in
CMT4B2 patients may lead to alterations in MTMR2 function and subsequent alterations in 3-phosphoinositide signaling. Such a mechanism would explain the strikingly similar phenotypes of patients with recessive mutations in either MTMR2 or MTMR13.