VAC14

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.^[5][8] 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. It is seen that VAC14 is scaffold protein that acts in complex with the lipid kinase PIKfyve which works to phosphorylate phosphatidylinositol-3-phosphate, as well as the counteracting phosphatase FIG4, which removes a phosphate group.^[9]

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.^[10] 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.^[11] Human Vac14/ArPIKfyve also interacts with the PDZ (post-synaptic density) domain of neuronal nitric oxide synthase ^[12] but the functional significance of this interaction is still unclear. ArPIKfyve facilitates insulin-regulated GLUT4 translocation to the cell surface.^[13]

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.^[14] 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.^[15]

In 2016, a new condition caused by mutations of the gene was discovered and named childhood-onset striatonigral degeneration (OMIM 617054).^[16] It is thought that the PIKfyve-VAC14-FIG4 complex plays an important role on the maturation of early endosomes to late endosomes/lysosomes. These organelles play critical roles in vesicular trafficking, which move cargo from donor membrane cells to target membranes within the body.^[17]