Exploring the ESCRTing machinery in eukaryotes

Abstract

The profile of protein sorting into multivesicular bodies (MVBs) has risen recently with the identification of three heteromeric complexes known as ESCRT-I,-II,-III (Endosomal Sorting Complex Required for Transport). Genetic analyses in yeast have identified up to 15 soluble class E VPS (vacuolar protein sorting) proteins that have been assigned to the ESCRT machinery and function in cargo recognition and sorting, complex assembly, vesicle formation and dissociation. Despite their functional importance in yeast and mammalian cells, little is known about their presence and function in other organisms including plants. We have made use of the fully sequenced genomes of Arabidopsis thaliana and Oryza sativa, Drosophila melanogaster and Caenorhabditis elegans to explore the identity, structural characteristics and phylogenetic relationships of proteins assigned to the ESCRT machinery.Multivesicular bodies (MVBs) are late endosomal organelles with up to several hundred interluminal vesicles that originate by invagination and budding of the limiting endosomal membrane (Figure 1) [1,2]. MVBs are also prevacuolar compartments involved in the retrograde endocytotic pathway where, upon fusion to vacuoles or lysosomes, the interluminal vesicles are released [3][4][5][6]. In yeast and mammals, MVBs are involved in sorting and degradation of transmembrane surface proteins as transporters (e.g. GAP1, STE6 and PDR5), receptors (e.g. EGFR, GHR, STE3 and STE2), in budding of viruses (e.g. HIV, MuLV and RSV) and in lipid partitioning [7,8]. However, MVBs are also central for sorting vacuolar proteins directly from the late Golgi to the anterograde and biosynthetic pathway ( Figure 1) [9]. Furthermore, MVBs operate in the exocytotic pathway and the secreted interluminal vesicles are known as exosomes (Figure 1) [10][11][12]. Thus, MVBs act as an intermediate station for cargo on the way to degradation, intracellular recycling and secretion ( Figure 1). Although MVBs have been described in plants, little is known about their biogenesis, cargos and function [2,4,[13][14][15][16]. These studies show that plant MVBs are prevacuolar compartments and are labeled by vacuolar sorting receptor (VSR) antibodies such as BP80 [5,17]. Plant MVBs are not only involved in the transport of proteins to lytic but also to protein-storage vacuoles. As plasmalemmasomes in tobacco, MVBs have been implicated in the internalization of arabinogalactanrich plasma membrane proteins that are sequestered within the vacuole [18] Upstream cargo recognition and sorting system of the ESCRT machineryThe entry of membrane proteins into the MVB pathway starts with the binding of monoubiquitinated cargos to the outer endosomal membrane and the recruitment of the ESCRT-I complex. This membrane association is mediated by ubiquitin-binding proteins that contain one or two UIMs or a GAT domain in combination with protein domains that had been implicated in binding to membranes such as the phosphatidylinositol-3 phosphate (PI3P) binding FYVE finger domain (Figure...

Evidence for a sorting endosome in Arabidopsis root cells

“…Interestingly, a comparative analysis of fully sequenced genomes revealed that components of the ESCRT machinery are highly conserved in yeast, mammals, plants (e.g. A. thaliana and Oryza sativa), Drosophila melanogaster and Caenorhabditis elegans (Winter and Hauser, 2006). Recently, the Arabidopsis homologue of the yeast Vps23 protein (Vps23p), which is a key component of ESCRT-I in yeast and animals, has been characterized (Spitzer et al, 2006).…”

Vesicle Trafficking during Somatic Cytokinesis

“…Endocytosis from the plate to ARA7-and ARA6-positive MVBs (Haas et al, 2007;Jaillais et al, 2008) was recently suggested as an alternative degradation pathway to remove monoubiquitinated proteins by the ESCRT machinery (Winter and Hauser, 2006). In favor of this hypothesis, an Arabidopsis mutant in one of the ESCRT-I components (elch) was reported that produces multinucleated cells in several cell types.…”

Multivesicular Bodies Mature from the Trans-Golgi Network/Early Endosome in Arabidopsis

“…Plants also possess ESCRT proteins (Winter and Hauser, 2006;Schellmann and Pimpl, 2009;Shahriari et al, 2011), but there are no clear homologs to the ESCRT-0 complex (Leung et al, 2008), and the exact location of the other complexes is unclear. Nevertheless, several studies with mutated ESCRT proteins point to their presence at least at the LE.…”