The Bro1-related protein HD-PTP/PTPN23 is required for endosomal cargo sorting and multivesicular body morphogenesis

A crescent-shaped ALIX dimer targets ESCRT-III CHMP4 filaments

ALIX recruits ESCRT-III CHMP4 and is involved in membrane remodeling during endosomal receptor sorting, budding of some enveloped viruses, and cytokinesis. We show that ALIX dimerizes via the middle domain (ALIX(-V)) in solution. Structural modeling based on small angle X-ray scattering (SAXS) data reveals an elongated crescent-shaped conformation for dimeric ALIX lacking the proline-rich domain (ALIX(BRO1-V)). Mutations at the dimerization interface prevent dimerization and induce an open elongated monomeric conformation of ALIX(-V) as determined by SAXS modeling. ALIX dimerizes in vivo and dimeric ALIX colocalizes with CHMP4B upon coexpression. We show further that ALIX dimerization affects HIV-1 budding. C-terminally truncated activated CHMP4B retaining the ALIX binding site forms linear, circular, and helical filaments in vitro, which can be bridged by ALIX. Our data suggest that dimeric ALIX represents the active form that interacts with ESCRT-III CHMP4 polymers and functions as a scaffolding protein during membrane remodeling processes.

Multivesicular bodies associate with components of miRNA effector complexes and modulate miRNA activity

In animals, P-bodies or GW-bodies appear to cause the congregation of proteins involved in microRNA (miRNA)-mediated post-transcriptional silencing. The localization of P-bodies does not overlap with that of known organelles and are thus considered independent of lipid bilayers. Nonetheless, an miRNA effector protein, argonaute 2 (AGO2), was initially identified as membrane-associated, and some miRNAs have been found in secreted vesicles (exosomes) that derive from endo-lysosomal compartments called multivesicular bodies (MVBs). Proteins can be sorted in a ubiquitin-dependent manner into MVBs by three heteromeric subcomplexes, collectively termed ESCRT (endosomal sorting complex required for transport), to be further secreted in exosomes and/or degraded by the lysosome. Here we show that GW-bodies containing GW182 and AGO2, two main components of the RNA-induced silencing complex (RISC), are distinct from P-bodies due to their congregation with endosomes and MVBs. Moreover, miRNAs and miRNA-repressible mRNAs are enriched at these cellular membranes, suggesting that endosomes and/or MVBs are sites of miRNA-loaded RISC (miRISC) accumulation and, possibly, action. We further show that purified exosome-like vesicles secreted by MVBs are considerably enriched in GW182, but not P-body components, AGO2 or miRNA-repressible mRNA. Moreover, cells depleted of some ESCRT components show compromised miRNA-mediated gene silencing and over-accumulate GW182, which associates with ubiquitylated proteins. Therefore, GW182, possibly in association with a fraction of miRNA-loaded AGO2, is sorted into MVBs for secretion and/or lysosomal degradation. We propose that this process promotes continuous assembly or disassembly of membrane-associated miRISCs, which is possibly required for miRNA loading or target recognition and subsequent silencing.

Divergent Bro1 domains share the capacity to bind human immunodeficiency virus type 1 nucleocapsid and to enhance virus-like particle production

To promote the release of infectious virions, human immunodeficiency virus type 1 (HIV-1) exploits the endosomal sorting complex required for transport (ESCRT) pathway by engaging Tsg101 and ALIX through late assembly (L) domains in p6 Gag. An LYPx(n)L motif in p6 serves as docking site for the central V domain of ALIX and is required for its ability to stimulate HIV-1 budding. Additionally, the nucleocapsid (NC) domain of Gag binds to the N-terminal Bro1 domain of ALIX, which connects ALIX to the membrane-deforming ESCRT-III complex via its CHMP4 subunits. Since the isolated Bro1 domain of ALIX is sufficient to markedly stimulate virus-like particle (VLP) production in a minimal Gag rescue assay, we examined whether the Bro1 domains of other human proteins possess a similar activity. We now show that the Bro1 domain-only protein Brox and the isolated Bro1 domains of HD-PTP and rhophilin all bind to HIV-1 NC. Furthermore, all shared the capacity to stimulate VLP production by a minimal HIV-1 Gag molecule, and Brox in particular was as potent as the Bro1 domain of ALIX in this assay. Unexpectedly, Brox retained significant activity even if its CHMP4 binding site was disrupted. Thus, the ability to assist in VLP production may be an intrinsic property of the boomerang-shaped Bro1 domain.

HD-PTP is a catalytically inactive tyrosine phosphatase due to a conserved divergence in its phosphatase domain

Background

The HD-PTP protein has been described as a tumor suppressor candidate and based on its amino acid sequence, categorized as a classical non-transmembrane protein tyrosine phosphatase (PTP). To date, no HD-PTP phosphorylated substrate has been identified and controversial results concerning its catalytic activity have been recently reported.

Methodology and Results

Here we report a rigorous enzymatic analysis demonstrating that the HD-PTP protein does not harbor tyrosine phosphatase or lipid phosphatase activity using the highly sensitive DiFMUP substrate and a panel of different phosphatidylinositol phosphates. We found that HD-PTP tyrosine phosphatase inactivity is caused by an evolutionary conserved amino acid divergence of a key residue located in the HD-PTP phosphatase domain since its back mutation is sufficient to restore the HD-PTP tyrosine phosphatase activity. Moreover, in agreement with a tumor suppressor activity, HD-PTP expression leads to colony growth reduction in human cancer cell lines, independently of its catalytic PTP activity status.

Conclusion

In summary, we demonstrate that HD-PTP is a catalytically inactive protein tyrosine phosphatase. As such, we identify one residue involved in its inactivation and show that its colony growth reduction activity is independent of its PTP activity status in human cancer cell lines.

The nucleocapsid region of HIV-1 Gag cooperates with the PTAP and LYPXnL late domains to recruit the cellular machinery necessary for viral budding

HIV-1 release is mediated through two motifs in the p6 region of Gag, PTAP and LYPX(n)L, which recruit cellular proteins Tsg101 and Alix, respectively. The Nucleocapsid region of Gag (NC), which binds the Bro1 domain of Alix, also plays an important role in HIV-1 release, but the underlying mechanism remains unclear. Here we show that the first 202 residues of the Bro1 domain (Bro(i)) are sufficient to bind Gag. Bro(i) interferes with HIV-1 release in an NC-dependent manner and arrests viral budding at the plasma membrane. Similar interrupted budding structures are seen following over-expression of a fragment containing Bro1 with the adjacent V domain (Bro1-V). Although only Bro1-V contains binding determinants for CHMP4, both Bro(i) and Bro1-V inhibited release via both the PTAP/Tsg101 and the LYPX(n)L/Alix pathways, suggesting that they interfere with a key step in HIV-1 release. Remarkably, we found that over-expression of Bro1 rescued the release of HIV-1 lacking both L domains. This rescue required the N-terminal region of the NC domain in Gag and the CHMP4 binding site in Bro1. Interestingly, release defects due to mutations in NC that prevented Bro1 mediated rescue of virus egress were rescued by providing a link to the ESCRT machinery via Nedd4.2s over-expression. Our data support a model in which NC cooperates with PTAP in the recruitment of cellular proteins necessary for its L domain activity and binds the Bro1-CHMP4 complex required for LYPX(n)L-mediated budding.

ESCRT proteins, endosome organization and mitogenic receptor down-regulation

Mitogenic tyrosine kinase receptors such as the EGFR (epidermal growth factor receptor) are endocytosed once they are activated at the cell surface. After reaching the early endosome, they are ubiquitinated within their cytosolic domain and are consequently sorted away from recycling receptors. They are then incorporated into intraluminal vesicles within the MVB (multivesicular body) en route to the lysosome, where they are degraded. MVB formation requires the stabilization of the vacuolar domain of the early endosome, the segregation of degradative cargo within this domain (with subsequent incorporation of receptors such as EGFR into intraluminal vesicles) and the physical separation and movement of this domain away from the tubular regions of the early endosome. How these different aspects of MVB biogenesis are coupled is unknown, but ESCRTs (endosomal sorting complexes required for transport) have been identified as key molecular players in driving mitogenic receptor sequestration and formation of intraluminal vesicles. The present review summarizes recent findings within the field and from our laboratory regarding the detailed function of ESCRTs and associated proteins in driving the ubiquitin-dependent sorting of EGFR and in maintaining the domain organization of the early endosome.

Structure and function of ESCRT-III

ESCRT-III (endosomal sorting complex required for transport III) is required for the formation and abscission of intraluminal endosomal vesicles, which gives rise to multivesicular bodies, budding of some enveloped viruses and cytokinesis. ESCRT-III is composed of 11 members in humans, which, except for one, correspond to the six ESCRT-III-like proteins in yeast. At least CHMP (charged multivesicular body protein) 2A and CHMP3 assemble into helical tubular structures that provide a platform for membrane interaction and VPS (vacuolar protein sorting) 4-catalysed effects leading to disassembly of ESCRT-III CHMP2A-CHMP3 polymers in vitro. Progress towards the understanding of the structures and function of ESCRT-III, its activation, its regulation by accessory factors and its role in abscission of membrane enveloped structures in concert with VPS4 are discussed.

Molecular assemblies and membrane domains in multivesicular endosome dynamics

Along the degradation pathway, endosomes exhibit a characteristic multivesicular organization, resulting from the budding of vesicles into the endosomal lumen. After endocytosis and transport to early endosomes, activated signaling receptors are incorporated into these intralumenal vesicles through the action of the ESCRT machinery, a process that contributes to terminate signaling. Then, the vesicles and their protein cargo are further transported towards lysosomes for degradation. Evidence also shows that intralumenal vesicles can undergo "back-fusion" with the late endosome limiting membrane, a route exploited by some pathogens and presumably followed by proteins and lipids that need to be recycled from within the endosomal lumen. This process depends on the late endosomal lipid lysobisphosphatidic acid and its putative effector Alix/AIP1, and is presumably coupled to the invagination of the endosomal limiting membrane at the molecular level via ESCRT proteins. In this review, we discuss the intra-endosomal transport routes in mammalian cells, and in particular the different mechanisms involved in membrane invagination, vesicle formation and fusion in a space inaccessible to proteins known to control intracellular membrane traffic.

Autophagy discriminates between Alix and ESCRTs

Alix and ESCRT proteins are required for membrane fission during viral budding and egress and during the abscission stage of cytokinesis. These common roles have suggested that Alix functions as an ESCRT protein, a conclusion challenged by the finding that unlike ESCRTs, which control the formation of multivesicular endosomes, Alix does not influence the degradation of the EGF receptor. We previously showed that Alix controls neuronal death by an unknown mechanism, but dependent on its interaction with ESCRT proteins. Since then, numerous reports have shown that ESCRTs participate in macroautophagy. Given the direct interaction between ESCRTs and Alix, together with the known contribution of autophagy to cell death, it was hypothesized that Alix controls autophagy and thereby cell death. Our recent published results show that this is not the case. ESCRT protein activity therefore needs Alix for viral budding and cytokinesis but not for autophagy. The function of ESCRT can thus be clearly be disconnected from that of Alix.

HD-PTP inhibits endothelial migration through its interaction with Src

Endothelial migration, early step in angiogenesis, is tightly regulated by the coordinated action of tyrosine kinases and tyrosine phosphatases. HD-PTP contributes to endothelial motility, since endothelial cells silencing HD-PTP after transfection with iRNA acquire a scattered and spindle-shaped phenotype and migrate faster than controls. Since (i) the proto-oncogene Src contributes to the regulation of cell motility and (ii) HD-PTP has a potential binding site for Src, we investigated whether an interplay exists between these two proteins. We found that Src binds HD-PTP and this interaction is enhanced after exposure to basic fibroblast growth factor. While HD-PTP does not modulate the levels of Src phosphorylation both in vitro and in vivo, we found that Src phosphorylates HD-PTP on tyrosine residues. Here we show for the first time that (i) HD-PTP has a tyrosine phosphatase activity; (ii) HD-PTP phosphorylation by Src inhibits its enzymatic activity. Interestingly, pharmacological and genetic inhibition of Src abrogates the migratory phenotype of endothelial cells silencing HD-PTP. On these bases, and because we have previously demonstrated that HD-PTP binds and dephosphorylates focal adhesion kinase (FAK), another crucial regulator of cell migration, we hypothesize that HD-PTP participates to the regulation of endothelial motility through its interactions with Src and FAK.

Alix differs from ESCRT proteins in the control of autophagy

Alix/AIP1 is a cytosolic protein that regulates cell death through mechanisms that remain unclear. Alix binds to two protein members of the so-called Endosomal Sorting Complex Required for Transport (ESCRT), which facilitates membrane fission events during multivesicular endosome formation, enveloped virus budding and cytokinesis. Alix itself has been suggested to participate in these cellular events and is thus often considered to function in the ESCRT pathway. ESCRT proteins were recently implicated in autophagy, a process involved in bulk degradation of cytoplasmic constituents in lysosomes, which can also participate in cell death. In this study, we shown that, unlike ESCRT proteins, Alix is not involved in autophagy. These results strongly suggest that the capacity of several mutants of Alix to block both caspase-dependent and independent cell death does not relate to their capacity to modulate autophagy. Furthermore, they reinforce the conclusion of other studies demonstrating that the role of Alix is different from that of classical ESCRT proteins.