PIKfyve: Partners, significance, debates and paradoxes

Abstract

Key components of membrane trafficking and signaling machinery in eukaryotic cells are proteins that bind or synthesize phosphoinositides. PIKfyve, a product of an evolutionarily conserved single-copy gene has both these features. It binds to membrane phosphatidylinositol (PtdIns)3P and synthesizes PtdIns(3,5)P2 and PtdIns5P. Molecular functions of PIKfyve are elusive but recent advances are consistent with a key role in the course of endosomal transport. PIKfyve dysfunction induces endosome enlargement and profound cytoplasmic vacuolation, likely as a result of impaired normal endosome processing and membrane exit out of endosomes. Multicellular organisms with genetically impaired function of PIKfyve or that of the PIKfyve protein partners regulating PtdIns(3,5)P2 homeostasis display severe disorders, including embryonic/perinatal death. This review describes recent advances on PIKfyve functionality in higher eukaryotes, with particular reference to biochemical and genetic insights in PIKfyve protein partners.

Figure 1

Schematic diagram of the domain structure of the evolutionarily conserved mammalian and yeast counterparts engaged in PtdIns(3,5)P₂ metabolism (http://scansite.mit.edu/cgi-bin/motifscan and http://pfam.janelia.org). The DEP domain region (349–487) is also homologous to “winged helix” DNA-binding domain by http://supfam.org/SUPERFAMILY (not shown). CHK-Hom indicates homologous region enriched in Cys, His and Lys. Parts of CHK-Hom show homology to spectrin repeats (not shown). H in ArPIKfyve are heat repeats. Presented are the products of PIKfyve or Fab1, the substrates of Sac3 or Fig4, and the affected PI by ArPIKfyve or Vac14, confirmed both in vitro and in vivo. See text for further details and references.

Figure 2. General scheme of endocytosis and proposed model for the locus and mode of action of PIKfyve and its physically associated partners

The PIKfyve-ArPIKfyve complex assembles on PtdIns3P-enriched early endosome membranes and catalyzes PtdIns3-to-PtdIns(3,5)P₂ conversion. This switch triggers formation/detachment or maturation of transport intermediates from early endosomes and likely decreases the rate of endosome fusion due to consumption of PtdIns3P and acquisition of PtdIns(3,5)P₂. PIKfyve-ArPIKfyve complex then recruits Sac3 which turns over PtdIns(3,5P)₂ to PtdIns3P, allowing membrane fusion may occur. The described events may occur on other PtdIns3P-containing endosomal structures (i.e., ECV/MVB or late endosomes). Impaired endosome plasticity controlled by the PIKfyve-ArPIKfyve-Sac3 complex affects exit from early/post-early endosomal structures to other destinations (purple line), evidenced by impaired post-early endosomal traffic of solutes and endosome-to-TGN membrane transport., endosomal carrier vesicles/multivesicular bodies.