Myotubularin-related protein (MTMR) 9 determines the enzymatic activity, substrate specificity, and role in autophagy of MTMR8

Abstract

The myotubularins are a large family of inositol polyphosphate 3-phosphatases that, despite having common substrates, subsume unique functions in cells that are disparate. The myotubularin family consists of 16 different proteins, 9 members of which possess catalytic activity, dephosphorylating phosphatidylinositol 3-phosphate [PtdIns(3)P] and phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P 2 ] at the D-3 position. Seven members are inactive because they lack the conserved cysteine residue in the CX 5 R motif required for activity. We studied a subfamily of homologous myotubularins, including myotubularin-related protein 6 (MTMR6), MTMR7, and MTMR8, all of which dimerize with the catalytically inactive MTMR9. Complex formation between the active myotubularins and MTMR9 increases their catalytic activity and alters their substrate specificity, wherein the MTMR6/R9 complex prefers PtdIns(3,5)P 2 as substrate; the MTMR8/R9 complex prefers PtdIns(3)P. MTMR9 increased the enzymatic activity of MTMR6 toward PtdIns(3,5)P 2 by over 30-fold, and enhanced the activity toward PtdIns(3)P by only 2-fold. In contrast, MTMR9 increased the activity of MTMR8 by 1.4-fold and 4-fold toward PtdIns (3,5)P 2 and PtdIns(3)P, respectively. In cells, the MTMR6/R9 complex significantly increases the cellular levels of PtdIns(5)P, the product of PI(3,5)P 2 dephosphorylation, whereas the MTMR8/R9 complex reduces cellular PtdIns(3)P levels. Consequentially, the MTMR6/R9 complex serves to inhibit stress-induced apoptosis and the MTMR8/R9 complex inhibits autophagy.I nositol lipids play important roles in a variety of intracellular signaling pathways. In response to stimuli, the phosphoinositide profile is regulated by phospholipases, lipid kinases, and phosphatases. Understanding the roles of inositol signaling has expanded during the last decade and a number of these enzymes have been shown to cause diseases when mutated (1). The tumor-suppressor PTEN was discovered through positional cloning as being mutated in several types of cancer (2, 3). PTEN was subsequently shown to be a phosphatase, which dephosphorylates phosphatidylinositol 3,4,5-trisphosphate to generate phosphatidylinositol 4,5-bisphosphate, an activity that is lost in patients with PTEN mutations (4, 5). Mutations in the inositol polyphosphate 5-phosphatase OCRL cause the X-linked disorder Lowe syndrome, which is associated with mental retardation, blindness, and renal failure (6). Mutations in myotubularin cause myotubular myopathy (7), and mutations in myotubularin-related protein 2 (MTMR2) and MTMR13 cause a form of Charcot Marie Tooth disease type 4B, a demyelinating neurodegenerative disorder (8, 9).The myotubularin family consists of 16 different proteins, 9 members of which possess catalytic activity (10, 11) and 7 members that are inactive. Myotubularin proteins are not redundant and have unique functions within cells by regulating a specific pool of dephosphorylating phosphatidylinositol 3-phosphate [PtdIns(3)P] and phosphatidylinositol 3,5-bisphosphate [PtdIns (3...

Myotubularin and PtdIns3P remodel the sarcoplasmic reticulum in muscle in vivo

“…6B). Protein extracts from muscles injected with AAVMtm1-WT-GFP and AAV-Mtm1-C375S-GFP displayed a tendency of higher protein levels for the endogenous MTM1 compared to the muscle expressing GFP, suggesting a stabilization of the endogenous MTM1 protein through the oligomerization with MTM1-WT-GFP and MTM1-C375S-GFP proteins, as suggested for the MTMR8-MTMR9 myotubularins dimer (Zou et al, 2012).…”

Phosphatidylinositol 3-Phosphatase Myotubularin-related Protein 6 (MTMR6) Is Regulated by Small GTPase Rab1B in the Early Secretory and Autophagic Pathways

“…This evidence suggests that the function of MTMR6 is different from that of the other MTM phosphatases and that MTMR6 dephosphorylates the specific pool of PI(3)P in the case of autophagy. Very recently, Zou et al (60) reported that the MTMR8-MTMR9 complex is also involved in autophagosome formation. Because rodents do not possess the MTMR8 gene, MTMR6 may compensate for some functions of MTMR8 in rodents.…”

Myotubularin-related protein 7 inhibits insulin signaling in colorectal cancer

“…Heterodimers are formed between catalytically active and inactive enzymes, e.g. MTMR6/7/8 with MTMR9 [12], which dephosphorylate phosphatidyl-inositol-3-monophosphate (PI(3)P) and −3,5-bisphosphate (PI(3,5)P2) at the 3′-position of inositol. MTMs localize to endosomal-lysosomal membranes, initiate vesicle trafficking and autophagy [10] and are mutated in human congenital neuromuscular diseases (such as MTM1) [11].…”