Phosphatase Subfamily MTMR6

Phosphatase Classification: FoldCC1: Superfamily CC1: Family Myotubularin: Subfamily MTMR6

MTMR6 is a phosphoinositide phosphatase found in holozoan. Its enzymatic activity is regulated by MTMR9.

Evolution

MTMR6 subfamily is found throughout holozoan. Human and most if not all vertebrates has three members per genome: MTMR6, MTMR7 and MTMR8. Other metazoan such as fruit fly and C elegans has a single member.

Domain Structure

Vertebrate MTMR6 subfamily has three domains: PH/GRAM, phosphatase domain and coiled-coil domain. Coiled-coil domain has been shown to mediate the interaction with MTMR9 in human [1, 2, 3].

The GRAM domain is similar to PH domain in structure and is found in membrane-associated proteins. As shown in MTMR3 subfamily, PH/GRAM domain can bind to phosphoinositide lipids. But, in the case of human MTMR6, small GTPase Rab1B interacts with MTMR6 via PH/GRAM domain, and therefore regulates cellular localization in the early secretory and autophagic pathways [4].

Most if not all non-vertebrate metazoan MTMR6 has an additional FYVE domain, which is also found in MTMR3 subfamily. In MTMR3, FYVE domain neither confers endosomal localisation nor binds to the lipid PtdIns3P, and is not required for in vitro enzyme activity of MTMR3. But, its function in non-vertebrate MTMR6 is unclear.

Catalytic activity and functions

The enzymatic activity of all three human MTMR6s are up-regulated by pseudophosphatase MTMR9 through protein interactions [1, 2, 3]. The interaction between MTMR9 and MTMR6 subfamily is also observed in C elegans [5].

Though all of MTMR6, MTMR7 and MTMR8 are 3-phosphatase, they have different preferences of substrates. MTMR6/R9 complex that regulates PtdIns (3, 5)P2 levels and thereby affects apoptosis; MTMR8/R9 complex down-regulates the levels of PthIns(3)P and blocks the autophagic process [2, 3]; MTMR7/MTMR9 dephosphorylates phosphatidylinositol 3-phosphate and Ins(1,3)P2 in neuronal cells [1].

By regulating the levels of different phosphoinositides, MTMR6 subfamily is involved in various processes. For example, MTMR6 is a negative-regulator of the Ca2+-Activated K+ Channel KCa3.1 [6]. MTMR7 has also been reported to regulate T-cell differentiation and Protein kinase B AKT signaling, possibly through regulation of phosphatidylinositol [3,4,5]-trisphosphate activity [7].

References

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