The content of phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2) in endosomal membranes changes dynamically with fission and fusion events that generate or absorb intracellular transport vesicles. The ArPIKfyve protein scaffolds a trimolecular complex to tightly regulate the level of PtdIns(3,5)P2. Other components of this complex are the PtdIns(3,5)P2-synthesizing enzyme PIKFYVE and the Sac1-domain-containing PtdIns(3,5)P2 5-phosphatase Sac3, encoded by the human gene FIG4. VAC14 functions as an activator of PIKFYVE.[1][4] Studies in VAC14 knockout mice indicate that, in addition to increasing the PtdIns(3,5)P2-producing activity of PIKfyve, VAC14 also controls the steady-state levels of another rare phosphoinositide linked to PIKfyve enzyme activity – phosphatidylinositol 5-phosphate.
In addition to the formation of the ternary complex with PIKfyve and Sac3, ArPIKfyve is engaged in a number of other interactions. ArPIKfyve forms a stable complex with the PtdIns(3,5)P2-specific phosphatase Sac3, thereby protecting Sac3 from rapid degradation in the proteasome.[5] ArPIKfyve forms a homooligomer through its carboxyl terminus. However, the number of monomers in the ArPIKfyve homooligomer, ArPIKfyve-Sac3 heterodimer or PIKfyve-ArPIKfyve-Sac3 heterotrimer is unknown.[6] Human Vac14/ArPIKfyve also interacts with the PDZ (post-synaptic density) domain of neuronal nitric oxide synthase [7] but the functional significance of this interaction is still unclear. ArPIKfyve facilitates insulin-regulated GLUT4 translocation to the cell surface.[8]
Lessons from VAC14 mouse models
VAC14 knock-out mice die at, or shortly after birth and exhibit massive neurodegeneration. Fibroblasts from these mice display ~50% lower levels of PtdIns(3,5)P2 and PtdIns(5)P.[9] A spontaneous mouse VAC14-point mutation (with arginine substitution of leucine156) is associated with reduced life span (up to 3 weeks), body size, enlarged brain ventricles, 50% decrease in PtdIns(3,5)P2 levels, diluted pigmentation, tremor and impaired motor function.[10]
VAC14 and human disease
The VAC14 gene is yet to be linked convincingly to human disease.[11]
↑Davy BE, Robinson ML (May 2003). "Congenital hydrocephalus in hy3 mice is caused by a frameshift mutation in Hydin, a large novel gene". Hum. Mol. Genet. 12 (10): 1163–70. doi:10.1093/hmg/ddg122. PMID12719380.
↑Sbrissa D, Ikonomov OC, Fu Z, Ijuin T, Gruenberg J, Takenawa T, Shisheva A (August 2007). "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. 282 (33): 23878–91. doi:10.1074/jbc.M611678200. PMID17556371.
↑Lemaire JF, McPherson PS (December 2006). "Binding of Vac14 to neuronal nitric oxide synthase: Characterisation of a new internal PDZ-recognition motif". FEBS Lett. 580 (30): 6948–54. doi:10.1016/j.febslet.2006.11.061. PMID17161399.
