The Vacuolar Protein Sorting-38 Subunit of the Arabidopsis Phosphatidylinositol-3-Kinase Complex Plays Critical Roles in Autophagy, Endosome Sorting, and Gravitropism.

Fen Liu,Weiming Hu,Richard D Vierstra

doi:10.3389/fpls.2018.00781

Abstract

The family of phosphatidylinositols (PtdIns) plays essential roles in membrane identity and intracellular trafficking events. In animals and yeast, PtdIn-3-phosphate, which is particularly important for endosomal sorting, lysosomal/vacuolar transport and autophagy, is assembled by two conserved kinase complexes comprised of the catalytic VACUOLAR PROTEIN SORTING (VPS)-34 subunit, along with VPS15, AUTOPHAGY-RELATED (ATG)-6, and either ATG14 (complex I) or VPS38 (complex II). Here, we describe the Arabidopsis ortholog of VPS38 and show by interaction assays that it assembles into a tetrameric PtdIn-3 kinase complex II. Plants missing VPS38 are viable but have dampened pollen germination and heightened seed abortion, and display a dwarf rosette phenotype, with defects in leaf and vascular development and sucrose sensing. vps38 seeds accumulate irregular protein storage vesicles and suppress processing of storage proteins into their mature forms. Consistent with a role for PtdIn-3-phosphate in autophagy, vps38 mutants are hypersensitive to nitrogen and fixed-carbon starvation and show reduced autophagic transport of cargo into vacuoles. vps38 seedlings also have dampened root gravitropism, which is underpinned by aberrant vectoral auxin transport likely caused by defects in plasma membrane/endosome cycling of the PIN-FORMED family of auxin transporters necessary for asymmetric cell elongation. Collectively, this study places VPS38 and its class-III PtdIn-3 kinase complex at the nexus of numerous endosomal trafficking events important to plant growth and development.

Highlights

Cellular trafficking and membrane/organelle identity are determined by the combined actions of specific lipids and proteins on each membrane surface
Whereas Atg14 has been connected to autophagy in yeast (Kihara et al, 2001; Diao et al, 2015), we found that autophagy is compromised in Arabidopsis mutants missing VPS38, indicating that the plant complex-II isoform is engaged in this recycling process
Given the potential importance of PtdIn-3P to intracellular trafficking in plants and the essential nature of the Arabidopsis VPS34, VPS15, and ATG6 (VPS30/Beclin1) subunits that generate this PtdIn, we sought to investigate its functions through the analysis of Arabidopsis counterparts of the human and yeast ATG14 and Vps38/UVRAG subunits that define the class-III PtdIn-3 kinase complexes I and II, respectively (Kihara et al, 2001; Itakura et al, 2008)

Summary

Introduction

Cellular trafficking and membrane/organelle identity are determined by the combined actions of specific lipids and proteins on each membrane surface. Through the combined action of kinases and phosphatases, a dynamic set of mono-, bis-, and tris-phosphate-containing PtdIns modified at the 3 , 4 , and/or 5 positions are assembled that provide spatial and temporal control of protein trafficking and endosomal sorting. These species are recognized by specific PtdIn-binding proteins that promote vesicle enclosure, transport and fusion, and eventually help distinguish specific cytoplasmic compartments. The vesicles enter multivesicular bodies (MVBs) that sort and release them as late endosomes for transport of the cargo to appropriate compartments where they are deposited by vesicle fusion

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