Myotubularian (MTM) related 5 and 13 proteins (MTMR5 and MTMR13) Pleckstrin Homology-Glucosyltransferases, Rab-like GTPase activators and Myotubularins (PH-GRAM) domain
MTMR5 is a catalytically inactive phosphatase that plays a role as an adapter for the phosphatase myotubularin to regulate myotubularintracellular location. It lacks several amino acids in the dsPTPase catalytic pocket which renders it catalytically inactive as a phosphatase. MTMR5 is the most well-studied inactive member of this family and has been implicated in cellular growth control and oncogenic transformation. MTMR13 is a catalytically inactive phosphatase that plays a role as an adapter for the phosphatase myotubularin to regulate myotubularintracellular location. It contains a Leu residue instead of a conserved Cys residue in the dsPTPase catalytic loop which renders it catalytically inactive as a phosphatase. MTMR13 has high sequence similarity to MTMR5 and has recently been shown to be a second gene mutated in type 4B Charcot-Marie-Tooth syndrome. Both MTMR5 and MTMR13 contain an N-terminal DENN domain, a PH-GRAM domain, an inactive PTP domain, a SET interaction domain, a coiled-coil domain, and a C-terminal PH domain. Myotubularin-related proteins are a subfamily of protein tyrosine phosphatases (PTPs) that dephosphorylate D3-phosphorylated inositol lipids. Mutations in this family cause the human neuromuscular disorders myotubular myopathy and type 4B Charcot-Marie-Tooth syndrome. 6 of the 13 MTMRs (MTMRs 5, 9-13) contain naturally occurring substitutions of residues required for catalysis by PTP family enzymes. Although these proteins are predicted to be enzymatically inactive, they are thought to function as antagonists of endogenous phosphatase activity or interaction modules. Most MTMRs contain a N-terminal PH-GRAM domain, a Rac-induced recruitment domain (RID) domain, a PTP domain (which may be active or inactive), a SET-interaction domain, and a C-terminal coiled-coil region. In addition some members contain DENN domain N-terminal to the PH-GRAM domain and FYVE, PDZ, and PH domains C-terminal to the coiled-coil region. The GRAM domain, found in myotubularins, glucosyltransferases, and other putative membrane-associated proteins, is part of a larger motif with a pleckstrin homology (PH) domain fold. The PH domain family possesses multiple functions including the ability to bind phosphoinositides via its beta1/beta2, beta3/beta4, and beta6/beta7 connecting loops and to other proteins. However, no phosphoinositide binding sites have been found for the MTMRs to date. Although the majority of the sequences are MTMR 5 and 13, this cd also contains MTM5 nematode sequences.
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