The Phosphoinositide-3-phosphatase MTMR2 Associates with MTMR13, a Membrane-associated Pseudophosphatase Also Mutated in Type 4B Charcot-Marie-Tooth Disease

Fred L Robinson,Jack E Dixon

doi:10.1074/jbc.m505159200

Fred L Robinson, Jack E Dixon

Open Access

https://doi.org/10.1074/jbc.m505159200

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Abstract

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.

Highlights

Charcot-Marie-Tooth disease (CMT)1 is the most common inherited neurological disorder, affecting about 1 in 2,000 people in the United States.2 The disease is a heterogeneous group of peripheral neuropathies that lead to progressive degeneration of the muscles of the extremities and loss of sensory function [1]
Given the genetic association of both mutations in either myotubularin-related 2 (MTMR2) and MTMR13 with Charcot-Marie-Tooth disease type 4B (CMT4B), we investigated the biochemical relationship between these two proteins
CMT4B1 and CMT4B2 are both recessively inherited, strongly suggesting that it is a loss of function of either MTMR2 or MTMR13 that leads to demyelination [7,8,9,10,11]

Summary

Introduction

Charcot-Marie-Tooth disease (CMT)1 is the most common inherited neurological disorder, affecting about 1 in 2,000 people in the United States.2 The disease is a heterogeneous group of peripheral neuropathies that lead to progressive degeneration of the muscles of the extremities and loss of sensory function [1]. In addition to a phosphatase domain, MTMR proteins contain a PH-GRAM domain and sequences predicted to form coiled-coils (Fig. 1A) [18, 19]. To examine whether MTMR2 and MTMR13 might associate in vivo, we used an anti-MTMR2 serum to immunoprecipitate the endogenous MTMR2 protein from HEK293 cells (Fig. 2A).

Results

Conclusion