This entry represents a group of polyphosphoinositide phosphatases, including fungal Fig4 and its homologues from animals and plants. They contain a SAC (suppressor of actin) domain. Fig4 is part of the PtdIns(3,5)P2-synthesising complex of yeast at the vacuole membrane []and is involved in regulating both the acute rise and subsequent fall in PI3,5P(2) levels [].In Arabidopsis there are five Fig4 homologues, known as SAC1-5. They play an important role in vacuolar trafficking, vacuolar morphology and function in plants [].In humans, mutations of the FIG4 gene cause Charcot-Marie-Tooth disease 4J (CMT4J), a recessive demyelinating form of Charcot-Marie-Tooth disease, a disorder of the peripheral nervous system [].
Vac14 is a component of the PI(3,5)P2 regulatory complex, which regulates both the synthesis and turnover of phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2) []. This complex is composed of Atg18, Fig4, Fab1, Vac14 and Vac7 in yeast. In mammals it consists of PIKfyve, FIG4 and VAC14. Fab1/PIKfyve (Fab1 in yeast and PIKfyve in mammals) is a kinase that converts PI(3)P into PI(3,5)P2 by phosphorylation at the 5-position; conversely Fig4 dephosphorylates PI(3,5)P2 back to PI(3)P. Vac14 is a scaffolding adaptor protein whose multimerisation is a prerequisite step for PI(3,5)P2 complex assembly and function [].
The PI(3,5)P2 regulatory complex regulates both the synthesis and turnover of phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2). It catalyzes the phosphorylation of phosphatidylinositol 3-phosphate on the fifth hydroxyl of the myo-inositol ring, to form phosphatidylinositol 3,5-bisphosphate. The dynamic interconversion of PI3P and PI(3,5)P2 is regulated by a protein complex which is conserved from yeast to mammals. It includes at least five proteins in yeast, located in the vacuole membrane: the lipid kinase Fab1p, lipid phosphatase Fig4p, the Fab1p activator Vac7p, the Fab1p inhibitor Atg18p, and Vac14p. Homologues of Fab1p, Vac14p and Fig4p are found in all eukaryotes which in mammalian cells, are known as Fab1/PIKfyve/PIP5K3, Fig4/Sac3, and Vac14/ArPIKfyve, located on early and late endosomes. Fab1, Vac14, and Fig4 form a ternary complex where Fab1 lipid kinase and Fig4 lipid phosphatase bind to opposite ends of Vac14. A conformational change of Vac14 allows a quick effect on kinase and phosphatase activities. This complex is required for endocytic-vacuolar pathway and nuclear migration. It plays a role in the biogenesis of endosome carrier vesicles (ECV)/ multivesicular bodies (MVB) transport intermediates from early endosomes in mammals, being essential for nervous tissue function. It has been shown that alterations in this complex leads to reduced intracellular levels of PI(3,5)P2, cytoplasmic vacuolisation and neurodegeneration [, ].This entry represents the 1-phosphatidylinositol-3phosphate-5-kinase (PIKfyve) found in eukaryotes.
The Sac domain is a region of homology between the N terminus of synaptojanin and the otherwise unrelated yeast protein Sac1p. The Sac domain is approximately 400 residues in length, and proteins containing this domain show approximately 35% identity with other Sac domains throughout this region. The Sac domain exhibits phosphatidylinositol polyphosphate phosphatase activity and can hydrolyse phosphate from any of the three positions of inositol that may be phosphorylated (3-, 4- and 5). However, adjacent phosphates are resistant to hydrolysis. Sac domains cannot hydrolyse phosphate from phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2), or PtdIns(3,4)P2, or PtdIns(3,4,5)P3, but can hydrolyse PtdIns(3,5)P2 [].The Sac domain consists of seven highly conserved motifs which appear to define the catalytic and regulatory regions of the phosphatase. The sixth conserved region contains a highly conserved C-x(5)-R-[TS]motif, thought to be the catalytic motif of many metal-independent protein and inositide polyphosphate phosphatases. Interestingly, the Inp51p Sac domain in which the cysteine, arginine and threonine/serine residues within the C-x(5)-R-[TS]motif are absent, does not exhibit any phosphatase activity [].Two classes of Sac domain proteins have been identified in mammals as well as lower eukaryotes []. The first comprises proteins, which, in addition to an N-terminal phosphatase Sac domain, have all the domains associated with type II phosphatidylinositol phosphate 5-phosphatases:Mammalian synaptojanins, type II phosphatidylinositol phosphate 5- phosphatases.Yeast INP51, a 108kDa membrane protein. It is involved in endocytosis and regulation of the actin cytoskeleton under conditions of normal vegetative growth. Although the Sac phosphatase domain of INP51 may be catalytically inactive, the domain may retain other functions.Yeast INP52, a 133kDa membrane protein. It is involved in endocytosis and regulation of the actin cytoskeleton under conditions of normal vegetative growth.Yeast INP53, a 124kDa membrane protein. It appears to have a role in intra-Golgi and Golgi-to-endosomal trafficking.The other class of Sac-containing phosphatases consists of proteins with an N-terminal Sac phosphatase domain and no other recognizable domains:Yeast Sac1p, a 67kDa membrane protein found in the endoplasmic reticulum (ER) and Golgi. It regulates the actin cytoskeleton and phospholipid metabolism.Yeast FIG4, a 101kDa protein encoded by a pheromone regulated or induced gene. FIG4 might function to regulate effector molecules of the actin cytoskeleton during mating.
