Fig4

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 FIG4 gene.^[5] Fig4 is an abbreviation for Factor-Induced Gene.^[6]

FIG4
Identifiers
Aliases	FIG4, ALS11, CMT4J, KIAA0274, SAC3, YVS, dJ249I4.1, BTOP, Fig4, FIG4 phosphoinositide 5-phosphatase
External IDs	OMIM: 609390 MGI: 2143585 HomoloGene: 6713 GeneCards: FIG4
Gene location (Human) Chr. Chromosome 6 (human)^[1] Band 6q21 Start 109,690,609 bp^[1] End 109,878,098 bp^[1]
Gene location (Mouse) Chr. Chromosome 10 (mouse)^[2] Band 10|10 B1 Start 41,064,168 bp^[2] End 41,179,256 bp^[2]
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in middle temporal gyrus endothelial cell monocyte Brodmann area 23 parotid gland Achilles tendon nucleus accumbens external globus pallidus prefrontal cortex putamen Top expressed in interventricular septum right ventricle myocardium of ventricle spinal ganglia facial motor nucleus atrioventricular valve medial ganglionic eminence atrium Paneth cell nucleus accumbens More reference expression data BioGPS n/a
Gene ontology Molecular function phosphatidylinositol-3-phosphatase activity phosphatidylinositol-4-phosphate phosphatase activity phosphatidylinositol-3,5-bisphosphate 5-phosphatase activity protein binding phosphatidylinositol bisphosphate phosphatase activity phosphoric ester hydrolase activity hydrolase activity Cellular component Golgi membrane late endosome membrane endosome recycling endosome membrane endoplasmic reticulum early endosome membrane endosome membrane Golgi apparatus lipid droplet intracellular membrane-bounded organelle Biological process locomotory behavior phosphatidylinositol biosynthetic process neuron development phosphatidylinositol metabolic process positive regulation of neuron projection development dephosphorylation pigmentation myelin assembly negative regulation of myelination vacuole organization phosphatidylinositol-3-phosphate biosynthetic process Sources:Amigo / QuickGO
Orthologs Species Human Mouse Entrez 9896 103199 Ensembl ENSG00000112367 ENSMUSG00000038417 UniProt Q92562 Q91WF7 RefSeq (mRNA) NM_014845 NM_133999 RefSeq (protein) NP_055660 NP_598760 Location (UCSC) Chr 6: 109.69 – 109.88 Mb Chr 10: 41.06 – 41.18 Mb PubMed search ^[3] ^[4]
Wikidata
View/Edit Human View/Edit Mouse

Function

Sac3 protein belongs to a family of human phosphoinositide phosphatases containing a Sac1-homology domain. The Sac1 phosphatase domain encompasses approximately 400 amino acids and consists of seven conserved motifs. It harbors the signature CX5R (T/S) catalytic sequence also found in other lipid and protein tyrosine phosphatases.^[7] 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.^[8] There are 5 human genes containing a Sac1 domain. Three of these genes (gene symbols SACM1L, INPP5F and FIG4), harbor a single Sac1 domain.^[9] In the other two genes, synaptojanin 1 and 2, the Sac1 domain coexists with another phosphoinositide phosphatase domain, with both domains supporting phosphate hydrolysis.^[10][11][12] In humans, the FIG4 gene localizes on chromosome 6 and encodes a Sac3 protein of 907 amino acids.^[13] Sac3 is characterized as a widespread 97-kDa protein that, in in vitro assays, displays phosphatase activity towards a range of 5’-phosphorylated phosphoinositides.^[14][15] 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.^[16][17] This unique physical association of two enzymes with opposing functions leads to activation of the phosphoinositide kinase PIKfyve and increases of PIKfyve-catalized PtdIns(3,5)P2 and PtdIns5P production. Sac3 is active as a phosphatase in the triple complex and is responsible for turning over PtdIns(3,5)P2 to PtdIns3P.^[16][17] 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,^[18] perinatal,^[19] or early juvenile lethality^[20] 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. Thus, the Sac3 cellular levels are critically dependent on Sac3 physical interaction with ArPIKfyve.^[16][21] The C-terminal part of Sac3 is essential for this interaction.^[17] 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.^[22]

Medical significance

Mutations in the FIG4 gene cause a rare autosomal recessive Charcot-Marie-Tooth peripheral neuropathy type 4J (CMT4J).^[20] 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.^[23] The Sac3I41T point mutation abrogates the protective action of ArPIKfyve on Sac3 half-life. As a result mutant Sac3 is rapidly degraded in the proteasome.^[21] Consequently, the Sac3I41T protein level in patient fibroblasts is from very low to undetectable.^[24][25] Clinically, the onset and severity of CMT4J symptoms vary markedly, suggesting an important role of genetic background in the individual course of disease.^[25] In two siblings, with severe peripheral motor deficits and moderate sensory symptoms, the disease had relatively little impact on the central nervous system.^[26] Phosphoinositide profiling in fibroblasts derived from the largest CMT4J cohort reported in USA thus far reveals decreased steady-state levels of both PtdIns(3,5)P2 and PtdIns5P. This unexpected direction of the changes is a result of impaired activation of the PIKFYVE kinase under the condition of Sac3 protein deficiency and a failure of the PAS complex assembly.^[27] The reduction in PtdIns(3,5)P2 and PtdIns5P levels is reportedly unrelated to gender or the disease onset, suggesting that the pathological decline in levels of the two lipids might precede the disease symptoms.^[27] FIG4 mutations are also found (without proven causation) in patients with amyotrophic lateral sclerosis (ALS)^[28] as well as in other spectrum of phenotypes such as Yunis-Varon syndrome, cortical malformation with seizures and psychiatric co-morbidities, and cerebral hypomyelination.

Mouse models

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).^[20][24] Neuronal autophagy has been suggested as an important consequence of the knockout,^[29] however, its primary relevance is disputed.^[30] The plt mice show distinct morphological defects in motor and central neurons on the one hand, and sensory neurons - on the other.^[30] 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.^[24]

Evolutionary biology

Genes encoding orthologs of human Sac3 are found in all eukaryotes. The most studied is the S. cerevisiae gene, discovered in a screen for yeast pheromone (Factor)-Induced Genes, hence the name Fig, with the number 4 reflecting the serendipity of isolation.^[31] Yeast Fig4p is a specific PtdIns(3,5)P2 5’-phosphatase, which physically interacts with Vac14p (the ortholog of human ArPIKfyve),^[32] and the PtdIns(3,5)P2-producing enzyme Fab1p (the ortholog of PIKfyve).^[33] The yeast Fab1p-Vac14p-Fig4p complex also involves Vac7p and potentially Atg18p.^[34] 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.^[35] 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.

References

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2. GRCm38: Ensembl release 89: ENSMUSG00000038417 - Ensembl, May 2017
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23. 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 (Jul 2011). "Distinctive genetic and clinical features of CMT4J: a severe neuropathy caused by mutations in the PI(3,5)P2 phosphatase FIG4". Brain. 134 (7): 1959–71. doi:10.1093/brain/awr148. PMC 3122378. PMID 21705420.
24. Lenk GM, Ferguson CJ, Chow CY, Jin N, Jones JM, Grant AE, Zolov SN, Winters JJ, Giger RJ, Dowling JJ, Weisman LS, Meisler MH (Jun 2011). "Pathogenic mechanism of the FIG4 mutation responsible for Charcot-Marie-Tooth disease CMT4J". PLOS Genetics. 7 (6): e1002104. doi:10.1371/journal.pgen.1002104. PMC 3107197. PMID 21655088.
25. Hu B, McCollum M, Ravi V, Arpag S, Moiseev D, Castoro R, Mobley B, Burnette B, Suskind C, Day J, Yawn R, Feely S, Li Y, Yan Q, Shy M, Li J (Apr 2018). "Myelin abnormality in Charcot-Marie-Tooth type 4J recapitulates features of acquired demyelination". Annals of Neurology. 83 (4): 756–770. doi:10.1002/ana.25198. PMC 5912982. PMID 29518270.
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31. Erdman S, Lin L, Malczynski M, Snyder M (Feb 1998). "Pheromone-regulated genes required for yeast mating differentiation". Journal of Cell Biology. 140 (3): 461–83. doi:10.1083/jcb.140.3.461. PMC 2140177. PMID 9456310.
32. Rudge SA, Anderson DM, Emr SD (Jan 2004). "Vacuole size control: regulation of PtdIns(3,5)P2 levels by the vacuole-associated Vac14-Fig4 complex, a PtdIns(3,5)P2-specific phosphatase". Molecular Biology of the Cell. 15 (1): 24–36. doi:10.1091/mbc.E03-05-0297. PMC 307524. PMID 14528018.
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Further reading

• Maruyama K, Sugano S (1994). "Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides". Gene. 138 (1–2): 171–4. doi:10.1016/0378-1119(94)90802-8. PMID 8125298.
• 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. doi:10.1016/S0378-1119(97)00411-3. PMID 9373149.
• Strausberg RL, Feingold EA, Grouse LH, et al. (2003). "Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences". Proceedings of the National Academy of Sciences. 99 (26): 16899–903. Bibcode:2002PNAS...9916899M. doi:10.1073/pnas.242603899. PMC 139241. PMID 12477932.
• Zhong R, Ye ZH (2003). "The SAC domain-containing protein gene family in Arabidopsis". Plant Physiology. 132 (2): 544–55. doi:10.1104/pp.103.021444. PMC 166996. PMID 12805586.
• Mungall AJ, Palmer SA, Sims SK, et al. (2003). "The DNA sequence and analysis of human chromosome 6". Nature. 425 (6960): 805–11. Bibcode:2003Natur.425..805M. doi:10.1038/nature02055. PMID 14574404.
• Gerhard DS, Wagner L, Feingold EA, et al. (2004). "The status, quality, and expansion of the NIH full-length cDNA project: the Mammalian Gene Collection (MGC)". Genome Research. 14 (10B): 2121–7. doi:10.1101/gr.2596504. PMC 528928. PMID 15489334.

External links

• GeneReviews/NCBI/NIH/UW entry on Charcot-Marie-Tooth Neuropathy Type 4