[Show abstract][Hide abstract] ABSTRACT: Two different human diseases, X-linked myotubular myopathy and Charcot-Marie-Tooth disease, result from mutant MTM1 or MTMR2 lipid phosphatases. Although events involved in endosomal PI(3)P and PI(3,5)P(2) synthesis are well established and pivotal in receptor signaling and degradation, enzymes involved in phosphoinositide degradation and their roles in trafficking are incompletely characterized. Here, we dissect the functions of the MTM1 and MTMR2 myotubularins and establish how they contribute to endosomal PI(3)P homeostasis. By mimicking loss of function in disease through siRNA-mediated depletion of the myotubularins, excess PI(3)P accumulates on early (MTM1) and late (MTMR2) endosomes. Surprisingly, the increased PI(3)P blocks the egress of epidermal growth factor receptors from early or late endosomes, suggesting that the accumulation of signaling receptors in distinct endosomes may contribute to the unique disease etiologies when MTM1 or MTMR2 are mutant. We further demonstrate that direct myotubularin binding to the type III PI 3-kinase complex hVps34/hVps15 leads to phosphatase inactivation. The lipid kinase-phosphatase interaction also precludes interaction of the PI 3-kinase with Rab GTPase activators. Thus, unique molecular complexes control kinase and phosphatase activation and locally regulate PI(3)P on discrete endosome populations, thereby providing a molecular rationale for related human myo- and neuropathies.
Full-text · Article · Aug 2008 · Molecular biology of the cell
[Show abstract][Hide abstract] ABSTRACT: Myotubularins constitute a ubiquitous family of phosphatidylinositol (PI) 3-phosphatases implicated in several neuromuscular disorders. Myotubularin [myotubular myopathy 1 (MTM1)] PI 3-phosphatase is shown associated with early and late endosomes. Loss of endosomal phosphatidylinositol 3-phosphate [PI(3)P] upon overexpression of wild-type MTM1, but not a phosphatase-dead MTM1C375S mutant, resulted in altered early and late endosomal PI(3)P levels and rapid depletion of early endosome antigen-1. Membrane-bound MTM1 was directly complexed to the hVPS15/hVPS34 [vacuolar protein sorting (VPS)] PI 3-kinase complex with binding mediated by the WD40 domain of the hVPS15 (p150) adapter protein and independent of a GRAM-domain point mutation that blocks PI(3,5)P(2) binding. The WD40 domain of hVPS15 also constitutes the binding site for Rab7 and, as shown previously, contributes to Rab5 binding. In vivo, the hVPS15/hVPS34 PI 3-kinase complex forms mutually exclusive complexes with the Rab GTPases (Rab5 or Rab7) or with MTM1, suggesting a competitive binding mechanism. Thus, the Rab GTPases together with MTM1 likely serve as molecular switches for controlling the sequential synthesis and degradation of endosomal PI(3)P. Normal levels of endosomal PI(3)P and PI(3,5)P(2) are crucial for both endosomal morphology and function, suggesting that disruption of endosomal sorting and trafficking in skeletal muscle when MTM1 is mutated may be a key factor in precipitating X-linked MTM.
[Show abstract][Hide abstract] ABSTRACT: The Rab7 GTPase is a key regulator of late endocytic membrane transport and autophagy. Rab7 exerts temporal and spatial control over late endocytic membrane transport through interactions with various effector proteins. Among Rab7 effectors, the hVPS34/p150 phosphatidylinositol (PtdIns) 3-kinase complex serves to regulate late endosomal phosphatidylinositol signaling that is important for protein sorting and intraluminal vesicle sequestration. In this chapter, reagents and methods for the characterization of the interactions and regulation of the Rab7/hVPS34/p150 complex are described. Using these methods we demonstrate the requirement for activated Rab7 in the regulation of hVPS34/p150 PtdIns 3-kinase activity on late endosomes in vivo.
Full-text · Article · Feb 2005 · Methods in Enzymology