Phosphatidylinositol 3,5-bisphosphate: low abundance, high significance

Abstract

Recent studies of the low abundant signaling lipid, phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2 ), reveal an intriguingly diverse list of downstream pathways, the intertwined relationship between PI(3,5)P2 and PI5P, as well as links to neurodegenerative diseases. Derived from the structural lipid phosphatidylinositol, PI(3,5)P2 is dynamically generated on multiple cellular compartments where interactions with an increasing list of effectors regulate many cellular pathways. A complex of proteins that includes Fab1/PIKfyve, Vac14, and Fig4/Sac3 mediates the biosynthesis of PI(3,5)P2 , and mutations that disrupt complex function and/or formation cause profound consequences in cells. Surprisingly, mutations in this pathway are linked with neurological diseases, including Charcot-Marie-Tooth syndrome and amyotrophic lateral sclerosis. Future studies of PI(3,5)P2 and PI5P are likely to expand the roles of these lipids in regulation of cellular functions, as well as provide new approaches for treatment of some neurological diseases.

Keywords: Fab1; Fig4; PIKfyve; Vac14; Vac7; phosphatidylinositol 3,5-bisphosphate; phosphoinositide lipid.

Figure 1

Interconversion among the seven known phosphoinositide lipids occurs via action of specific lipid kinases (red arrows) and phosphatases (blue arrows). Selected kinases and phosphatases are shown. While controversial, direct conversion of PI to PI5P via PIKfyve activity may contribute to the PI5P pool (gray arrows). INPP4A phosphatase, which causes neurodegeneration in mice [118], and the type II PI5P 4-kinase [119,120], which has a role in the regulation of PI5P levels, were not discussed in this review.

Figure 2

Fab1/PIKfyve, Vac14 and Fig4 are conserved in most eukaryotes. Domains of S. cerevisiae and human Fab1/PIKfyve, Vac14 and Fig4/Sac3 are shown. A: Fab1 domains include FYVE (binds PI3P), DEP (unknown function; present in chordate and insect Fab1), CCT (homologous to the chaperone Cpn60/TCP-1 family; mediates interactions with Vac14), CCR (a conserved cysteine rich domain found only in Fab1/PIKfyve; part of the Vac14 binding region), kinase (catalytic site for conversion of PI3P to PI(3,5)P₂). B: Vac14 is composed of tandem HEAT repeats, which are rod-like helical structures that mediate protein-protein interactions. C: Fig4 contains a Sac domain, which is a module found in several lipid phosphatases. Note, the number of amino acids in mouse PIKfyve and human PIKfyve are not identical. The catalytically impaired mutation in mouse PIKfyve, K1831E, is indicated on the schematic of human PIKfyve, K1877E. The boundaries for FYVE, CCT, CCR, kinase, and Sac domains were identified as follows: 1) conserved in multiple sequence alignments and 2) contained unbroken secondary structure elements predicted by the program Jpred. Sequences for Fab1/PIKfyve were from the following species: Saccharomyces cerevisiae (budding yeast, NP_116674), Schizosaccharomyces pombe (fission yeast, NP_596090), Candida albicans (human pathogen, CAC42810), Ashbya gossypii (cotton pathogen, NP_985045), Arabidopsis thaliana (plant, NP_001078484), Drosophila melanogaster (fly, NP_611269), Apis mellifera (honey bee, XP_393666), Anopheles gambiae (mosquito, XP_314118), Caenorhabditis elegans (worm, CAA19436), and Homo sapiens (human, NP_055855). The Sac domain in Fig4 was defined through alignment of the following Sac domain proteins in S. cerevisiae: Inp51, Inp52, Inp53, Sac1 and Fig4.

Figure 3

Schematic of the Fab1/PIKfyve, Vac14, Fig4/Sac3 complex. Vac14 oligomerizes with itself and nucleates the complex through direct interactions with Fab1/PIKfyve and Fig4/Sac3. In yeast, Vac14 also directly interacts with Atg18 and Vac7. The yeast Vac14 point mutants, H56Y (HEAT repeat loop 2), R61K (HEAT repeat loop 2) and Q101R (HEAT repeats loop 3), each disrupt binding of Atg18 and Vac7. Thus, Atg18 and Vac7 may bind overlapping or identical sites of Vac14 [26]. The Vac14-L156R mutation, found in ingls mice, and corresponding mutation Vac14-L149R in yeast, disrupts Vac14 interaction with Atg18, Vac7 and Fab1. This suggests that all three proteins bind overlapping sites on Vac14. The point mutation, Fig4-I41T found in patients with CMT4J, disrupts the interaction between Fig4 and Vac14, although the major portion of human Fig4 that interacts with Vac14 resides within residues 478–907 [28]. In mammalian cells, myotubularin related proteins (MTMRs) can convert PI(3,5)P₂ to PI5P and may provide the majority of cellular PI5P.

Figure 4

A: Localization of PI(3,5)P₂ and PI5P inferred from the localization of PIKfyve and Vac14. PI(3,5)P₂ localizes on early endosomes, late endosomes and lysosomes. Localization of PI(3,5)P₂ on autophagosomes is less clear. PI5P may also be present at all or some of these locations. To further establish the locations of these lipids, suitable lipid probes need to be developed. PI(3,5)P₂ effectors and trafficking pathways affected in PIKfyve/Vac14/Fig4 deficient cells are also indicated. Purple: known PI(3,5)P₂ effectors. Blue: proteins affected by PI(3,5)P₂ and/or PI5P. B: The size of a yeast vacuole or mammalian lysosome is dependent on ion and water homeostasis, as well as the net sum of anterograde traffic, retrograde traffic, membrane fusion and membrane fission.