Phosphatase Subfamily MTMR6

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

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

Evolution

MTMR6 is found throughout holozoa. The three human members are MTMR6, MTMR7 and MTMR8. Other metazoa such as Drosophila and C elegans have 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 invertebrate metazoa MTMR6s have an additional FYVE domain at C-terminal. These range from sponge and anemone to the sea urchin. The FYVE domain is absent from vertebrates and choanoflagellates. This suggests that MTMR6 emerged in holozoa, then gained FYVE domain in metazoa, and lost it in vertebrates (see technical notes).

FYVE is also found in another myotubularin subfamily, MTMR3. The function of FYVE in invertebrate metazoa is unclear. The FYVE domain of MTMR3 neither confers endosomal localisation nor binds to the lipid PtdIns3P, and is not required for in vitro enzyme activity of MTMR3.

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

1. Mochizuki Y and Majerus PW. Characterization of myotubularin-related protein 7 and its binding partner, myotubularin-related protein 9. Proc Natl Acad Sci U S A. 2003 Aug 19;100(17):9768-73. DOI:10.1073/pnas.1333958100 |
2. Zou J, Chang SC, Marjanovic J, and Majerus PW. MTMR9 increases MTMR6 enzyme activity, stability, and role in apoptosis. J Biol Chem. 2009 Jan 23;284(4):2064-71. DOI:10.1074/jbc.M804292200 |
3. Zou J, Zhang C, Marjanovic J, Kisseleva MV, Majerus PW, and Wilson MP. Myotubularin-related protein (MTMR) 9 determines the enzymatic activity, substrate specificity, and role in autophagy of MTMR8. Proc Natl Acad Sci U S A. 2012 Jun 12;109(24):9539-44. DOI:10.1073/pnas.1207021109 |
4. Mochizuki Y, Ohashi R, Kawamura T, Iwanari H, Kodama T, Naito M, and Hamakubo T. Phosphatidylinositol 3-phosphatase myotubularin-related protein 6 (MTMR6) is regulated by small GTPase Rab1B in the early secretory and autophagic pathways. J Biol Chem. 2013 Jan 11;288(2):1009-21. DOI:10.1074/jbc.M112.395087 |
5. Silhankova M, Port F, Harterink M, Basler K, and Korswagen HC. Wnt signalling requires MTM-6 and MTM-9 myotubularin lipid-phosphatase function in Wnt-producing cells. EMBO J. 2010 Dec 15;29(24):4094-105. DOI:10.1038/emboj.2010.278 |
6. Srivastava S, Li Z, Lin L, Liu G, Ko K, Coetzee WA, and Skolnik EY. The phosphatidylinositol 3-phosphate phosphatase myotubularin- related protein 6 (MTMR6) is a negative regulator of the Ca2+-activated K+ channel KCa3.1. Mol Cell Biol. 2005 May;25(9):3630-8. DOI:10.1128/MCB.25.9.3630-3638.2005 |
7. Guo L, Martens C, Bruno D, Porcella SF, Yamane H, Caucheteux SM, Zhu J, and Paul WE. Lipid phosphatases identified by screening a mouse phosphatase shRNA library regulate T-cell differentiation and protein kinase B AKT signaling. Proc Natl Acad Sci U S A. 2013 May 14;110(20):E1849-56. DOI:10.1073/pnas.1305070110 |

Technical notes

Gain and loss of FYVE domain

We first noticed the presence of FYVE domain in invertebrate metazoa in the nine genomes in phosphatome.net database. To clarify the evolutionary history of the FYVE domain in MTMR6 subfamily, we obtained the sequences of 256 MTMR6s of 203 eukaryotes in our internal orthology database and searched FYVE domain using Pfam web server (e-value threshold 1.0). We found the FYVE domain in MTMR6s of basal metazoa like sponge, basal eumetazoa like nematostella, protostomes, invertebrate chordates like sea urchin. The FYVE domain is absent from vertebrates and choanoflagellida like monosiga.