Polyphosphoinositide phosphatase also known as phosphatidylinositol 3,5-bisphosphate 5-phosphatase or SAC domain-containing protein 3 (Sac3) is an enzyme that in humans is encoded by the FIG4gene. Fig4 is an abbreviation for Factor-Induced Gene.
Sac3 protein belongs to a family of human phosphoinositide phosphatases that contain a Sac1-homology domain. The Sac1 phosphatase domain encompasses approximately 400 amino acids and consists of seven conserved motifs, which harbor the signature CX5R(T/S) catalytic sequence also found in other lipid and protein tyrosine phosphatases. The founding protein, containing this evolutionarily-conserved domain, has been the first gene product isolated in a screen for Suppressors of yeast ACtin mutations and therefore named Sac1. There are 5 human genes containing a Sac1 domain. Three of these genes (gene symbols SACM1L,INPP5F and FIG4), harbor a single Sac1 domain. In the other two genes, synaptojanin 1 and 2, the Sac1 domain coexists with another phosphoinositide phosphatase domain, with both domains supporting phosphate hydrolysis. The human Sac3 cDNA that predicts a 907 aminoacid protein and gene localization to chromosome 6 has been reported in 1996. Sac3 is characterized as a widespread 97-kDa protein that displays in vitro phosphatase activity towards a range of 5’-phosphorylated phosphoinositides. Sac3 forms a hetero-oligomer with ArPIKfyve (gene symbol, VAC14) and this binary complex associates with the phosphoinositide kinase PIKFYVE in a ternary PAS complex (from the first letters of PIKfyve-ArPIKfyve-Sac3), which is required to maintain proper endosomal membrane dynamics. This unique physical association of two enzymes with opposing functions leads to activation of the phosphoinositide kinase PIKfyve and increased PtdIns(3,5)P2 production. Sac3 is active in the triple complex and responsible for turning over PtdIns(3,5)P2 to PtdIns3P. The PAS complex function is critical for life, because the knockout of each of the 3 genes encoding the PIKfyve, ArPIKfyve or Sac3 protein causes early embryonic, perinatal, or early juvenile lethality in mice. Ectopically expressed Sac3 protein has a very short half-life of only ~18 min due to fast degradation in the proteasome. Co-expression of ArPIKfyve markedly prolongs Sac3 half-life, whereas siRNA-mediated ArPIKfyve knockdown profoundly reduces Sac3 levels. The Sac3 cellular levels are critically dependent on Sac3 physical interaction with ArPIKfyve. The C-terminal part of Sac3 is essential for this interaction. Insulin treatment of 3T3L1 adipocytes inhibits the Sac3 phosphatase activity as measured in vitro. Small interfering RNA-mediated knockdown of endogenous Sac3 by ~60%, resulting in a slight but significant elevation of PtdIns(3,5)P2 in 3T3L1 adipocytes, increases GLUT4 translocation and glucose uptake in response to insulin. In contrast, ectopic expression of Sac3, but not that of a phosphatase-deficient point-mutant, decreases GLUT4 plasma membrane abundance in response to insulin. Thus, Sac3 is an insulin-sensitive lipid phosphatase whose down-regulation improves insulin responsiveness.
Mutations in the FIG4 gene cause the rare autosomal recessive Charcot-Marie-Tooth peripheral neuropathy type 4J (CMT4J). FIG4 mutations are also found (without proven causation) in patients with amyotrophic lateral sclerosis (ALS). Most CMT4J patients (15 out of the reported 16) are compound heterozygotes, i.e., the one FIG4 allele is null whereas the other encodes a mutant protein with threonine for isoleucine substitution at position 41. The Sac3I41T point mutation abrogates the protective action of ArPIKfyve on Sac3 half-life yet the association between the two is largely preserved. Consequently, the Sac3I41T protein level in patient fibroblasts is very low due to mutant degradation in the proteasome. Clinically, the onset and severity of CMT4J symptoms vary markedly, suggesting an important role of genetic background in the individual course of disease. In two siblings, with severe peripheral motor deficits and moderate sensory symptoms, the disease had relatively little impact on the central nervous system. How the initial molecular defect, affecting all cells of the body, results in selective peripheral neuropathy is unknown.
Spontaneous FIG4 knockout leads to mutant mice with smaller size, selectively reduced PtdIns(3,5)P2 levels in isolated fibroblasts, diluted pigmentation, central and peripheral neurodegeneration, hydrocephalus, abnormal tremor and gait, and eventually juvenile lethality, hence the name pale tremor mouse (plt). Neuronal autophagy has been suggested as an important consequence of the knockout, however, its primary relevance is disputed. Interestingly, the plt mice show distinct morphological defects in motor and central neurons on the one hand, and sensory neurons - on the other. Transgenic mice with one spontaneously null allele and another encoding several copies of mouse Sac3I41T mutant (i.e., the genotypic equivalent of human CMT4J), are dose-dependently rescued from the lethality, neurodegeneration, and brain apoptosis observed in the plt mice. However, the hydrocephalus and diluted pigmentation seen in plt mice are not corrected.
Genes encoding orthologs of human Sac3 are found in all eukaryotes. The most studied is the S. cerevisae gene, discovered in a screen for yeast pheromone (Factor)-Induced Genes, hence the name Fig, with the number 4 reflecting the serendipity of isolation. Yeast Fig4p is a specific PtdIns(3,5)P2 5’-phosphatase, which physically interacts with Vac14p (the ortholog of human ArPIKfyve), and the PtdIns(3,5)P2-producing enzyme Fab1p (the ortholog of PIKfyve). The yeast Fab1p-Vac14p-Fig4p complex also involves Vac7p and potentially Atg18p. Deletion of Fig4p in budding yeast has relatively little effect on growth, basal PtdIns(3,5)P2 levels and the vacuolar size in comparison with the deletions of Vac14p or Fab1p. In brief, in evolution Sac3/Fig4 retained the Sac1 domain, phosphoinositide phosphatase activity, and the protein interactions from yeast. In mice, the protein is essential in early postnatal development. In humans, its I41T point mutation in combination with a null allele causes a neurodegenerative disorder.
^Novick P, Osmond BC, Botstein D. Suppressors of yeast actin mutations. Genetics. 1989 Apr;121(4):659-74. PMID2656401
^Minagawa T, Ijuin T, Mochizuki Y, Takenawa T. Identification and characterization of a sac domain-containing phosphoinositide 5-phosphatase. J Biol Chem. 2001 Jun 22;276(25):22011-5. Epub 2001 Mar 26. PMID11274189
^Majerus PW, York JD. Phosphoinositide phosphatases and disease. J Lipid Res. 2009 Apr;50 Suppl:S249-54. Epub 2008 Nov 11.PMID19001665
^Sasaki T, Takasuga S, Sasaki J, Kofuji S, Eguchi S, Yamazaki M, Suzuki A. Mammalian phosphoinositide kinases and phosphatases. Prog Lipid Res. 2009 Nov;48(6):307-43. Epub 2009 Jul 4. PMID19580826
^Liu Y, Bankaitis VA. Phosphoinositide phosphatases in cell biology and disease. Prog Lipid Res. 2010 Jul;49(3):201-17. Epub 2010 Jan 5. PMID20043944
^Nagase T, Seki N, Ishikawa K, Ohira M, Kawarabayasi Y, Ohara O, Tanaka A, Kotani H, Miyajima N, Nomura N. Prediction of the coding sequences of unidentified human genes. VI. The coding sequences of 80 new genes (KIAA0201-KIAA0280) deduced by analysis of cDNA clones from cell line KG-1 and brain. DNA Res. 1996 Oct 31;3(5):321-9, 341-54. PMID9039502
^Sbrissa D, Ikonomov OC, Fu Z, Ijuin T, Gruenberg J, Takenawa T, Shisheva A. Core protein machinery for mammalian phosphatidylinositol 3,5-bisphosphate synthesis and turnover that regulates the progression of endosomal transport. Novel Sac phosphatase joins the ArPIKfyve-PIKfyve complex. J Biol Chem. 2007 Aug 17;282(33):23878-91. Epub 2007 Jun 7. PMID17556371
^Yuan Y, Gao X, Guo N, Zhang H, Xie Z, Jin M, Li B, Yu L, Jing N. rSac3, a novel Sac domain phosphoinositide phosphatase, promotes neurite outgrowth in PC12 cells. Cell Res. 2007 Nov;17(11):919-32. PMID17909536
^ abcSbrissa D, Ikonomov OC, Fenner H, Shisheva A. ArPIKfyve homomeric and heteromeric interactions scaffold PIKfyve and Sac3 in a complex to promote PIKfyve activity and functionality. J Mol Biol. 2008 Dec 26;384(4):766-79. Epub 2008 Oct 11. PMID18950639
^ abcIkonomov OC, Sbrissa D, Fenner H, Shisheva A. PIKfyve-ArPIKfyve-Sac3 core complex: contact sites and their consequence for Sac3 phosphatase activity and endocytic membrane homeostasis. J Biol Chem. 2009 Dec 18;284(51):35794-806. Epub. PMID19840946
^Ikonomov OC, Sbrissa D, Delvecchio K, Xie Y, Jin JP, Rappolee D, Shisheva A. The phosphoinositide kinase PIKfyve is vital in early embryonic development: preimplantation lethality of PIKfyve-/- embryos but normality of PIKfyve+/- mice. J Biol Chem. 2011 Apr 15;286(15):13404-13. Epub 2011 Feb 24.PMID21349843
^Zhang Y, Zolov SN, Chow CY, Slutsky SG, Richardson SC, Piper RC, Yang B, Nau JJ, Westrick RJ, Morrison SJ, Meisler MH, Weisman LS. Loss of Vac14, a regulator of the signaling lipid phosphatidylinositol 3,5-bisphosphate, results in neurodegeneration in mice. Proc Natl Acad Sci U S A. 2007 Oct 30;104(44):17518-23. Epub 2007 Oct 23.PMID17956977
^ abcChow CY, Zhang Y, Dowling JJ, Jin N, Adamska M, Shiga K, Szigeti K, Shy ME, Li J, Zhang X, Lupski JR, Weisman LS, Meisler MH. Mutation of FIG4 causes neurodegeneration in the pale tremor mouse and patients with CMT4J. Nature. 2007 Jul 5;448(7149):68-72. Epub 2007 Jun 17.PMID17572665
^ abIkonomov OC, Sbrissa D, Fligger J, Delvecchio K, Shisheva A. ArPIKfyve regulates Sac3 protein abundance and turnover: disruption of the mechanism by Sac3I41T mutation causing Charcot-Marie-Tooth 4J disorder. J Biol Chem. 2010 Aug 27;285(35):26760-4. Epub 2010 Jul 14.PMID20630877
^Ikonomov OC, Sbrissa D, Ijuin T, Takenawa T, Shisheva A. Sac3 is an insulin-regulated phosphatidylinositol 3,5-bisphosphate phosphatase: gain in insulin responsiveness through Sac3 down-regulation in adipocytes. J Biol Chem. 2009 Sep 4;284(36):23961-71. Epub 2009 Jul 3.PMID19578118
^Chow CY, Landers JE, Bergren SK, Sapp PC, Grant AE, Jones JM, Everett L, Lenk GM, McKenna-Yasek DM, Weisman LS, Figlewicz D, Brown RH, Meisler MH. Deleterious variants of FIG4, a phosphoinositide phosphatase, in patients with ALS. Am J Hum Genet. 2009 Jan;84(1):85-8.PMID19118816
^Nicholson G, Lenk GM, Reddel SW, Grant AE, Towne CF, Ferguson CJ, Simpson E, Scheuerle A, Yasick M, Hoffman S, Blouin R, Brandt C, Coppola G, Biesecker LG, Batish SD, Meisler MH. Distinctive genetic and clinical features of CMT4J: a severe neuropathy caused by mutations in the PI(3,5)P2 phosphatase FIG4. Brain. 2011 Jul;134(Pt 7):1959-71. PMID21705420
^ abcLenk GM, Ferguson CJ, Chow CY, Jin N, Jones JM, Grant AE, Zolov SN, Winters JJ, Giger RJ, Dowling JJ, Weisman LS, Meisler MH. Pathogenic mechanism of the FIG4 mutation responsible for Charcot-Marie-Tooth disease CMT4J. PLoS Genet. 2011 Jun;7(6):e1002104. Epub 2011 Jun 2.PMID21655088
^Zhang X, Chow CY, Sahenk Z, Shy ME, Meisler MH, Li J. Mutation of FIG4 causes a rapidly progressive, asymmetric neuronal degeneration. Brain. 2008 Aug;131(Pt 8):1990-2001. Epub 2008 Jun 12. PMID18556664
^Ferguson CJ, Lenk GM, Meisler MH. Defective autophagy in neurons and astrocytes from mice deficient in PI(3,5)P2. Hum Mol Genet. 2009 Dec 15;18(24):4868-78. Epub 2009 Sep 29.PMID19793721
^ abKatona I, Zhang X, Bai Y, Shy ME, Guo J, Yan Q, Hatfield J, Kupsky WJ, Li J. Distinct pathogenic processes between Fig4-deficient motor and sensory neurons. Eur J Neurosci. 2011 Apr;33(8):1401-10. doi:10.1111/j.1460-9568.2011.07651.x. Epub 2011 Mar 17. PMID21410794
^Erdman S, Lin L, Malczynski M, Snyder M. Pheromone-regulated genes required for yeast mating differentiation. J Cell Biol. 1998 Feb 9;140(3):461-83. PMID9456310
^Rudge SA, Anderson DM, Emr SD. Vacuole size control: regulation of PtdIns(3,5)P2 levels by the vacuole-associated Vac14-Fig4 complex, a PtdIns(3,5)P2-specific phosphatase. Mol Biol Cell. 2004 Jan;15(1):24-36. Epub 2003 Oct 3. PMID14528018
^Botelho RJ, Efe JA, Teis D, Emr SD. Assembly of a Fab1 phosphoinositide kinase signaling complex requires the Fig4 phosphoinositide phosphatase. Mol Biol Cell. 2008 Oct;19(10):4273-86. Epub 2008 Jul 23.PMID18653468
^Jin N, Chow CY, Liu L, Zolov SN, Bronson R, Davisson M, Petersen JL, Zhang Y, Park S, Duex JE, Goldowitz D, Meisler MH, Weisman LS. VAC14 nucleates a protein complex essential for the acute interconversion of PI3P and PI(3,5)P(2) in yeast and mouse. EMBO J. 2008 Dec 17;27(24):3221-34. Epub 2008 Nov 27. PMID19037259
^Duex JE, Nau JJ, Kauffman EJ, Weisman LS. Phosphoinositide 5-phosphatase Fig 4p is required for both acute rise and subsequent fall in stress-induced phosphatidylinositol 3,5-bisphosphate levels. Eukaryot Cell. 2006 Apr;5(4):723-31.PMID16607019
Suzuki Y, Yoshitomo-Nakagawa K, Maruyama K, et al. (1997). "Construction and characterization of a full length-enriched and a 5'-end-enriched cDNA library.". Gene. 200 (1-2): 149–56. PMID9373149. doi:10.1016/S0378-1119(97)00411-3.