No abstract

CD1 proteins present lipid antigens to T cells. The antigens are acquired in the endosomal compartments. This raises the question of how the large hydrophobic CD1 pockets are preserved between the moment of biosynthesis in the endoplasmic reticulum and arrival to the endosomes. To address this issue, the natural ligands associated with a soluble form of human CD1b have been investigated. Using isoelectric focusing, native mass spectrometry and resolving the crystal structure at 1.8 Å resolution, we found that human CD1b is simultaneously associated with endogenous phosphatidylcholine (PC) and a 41-44 carbon atoms-long spacer molecule. The two lipids appear to work in concert to stabilize the CD1b groove, their combined size slightly exceeding the maximal groove capacity. We propose that the spacer serves to prevent binding of ligands with long lipid tails, whereas short-chain lipids might still displace the PC, which is exposed at the groove entrance. The data presented herein explain how the CD1b groove is preserved, and provide a rationale for the in vivo antigen-binding properties of CD1b.

The species-specific phenolic glycolipid 1 (PGL-1) is suspected to play a critical role in the pathogenesis of leprosy, a chronic disease of the skin and peripheral nerves caused by Mycobacterium leprae. Based on studies using the purified compound, PGL-1 was proposed to mediate the tropism of M. leprae for the nervous system and to modulate host immune responses. However, deciphering the biological function of this glycolipid has been hampered by the inability to grow M. leprae in vitro and to genetically engineer this bacterium. Here, we identified the M. leprae genes required for the biosynthesis of the species-specific saccharidic domain of PGL-1 and reprogrammed seven enzymatic steps in M. bovis BCG to make it synthesize and display PGL-1 in the context of an M. leprae-like cell envelope. This recombinant strain provides us with a unique tool to address the key questions of the contribution of PGL-1 in the infection process and to study the underlying molecular mechanisms. We found that PGL-1 production endowed recombinant BCG with an increased capacity to exploit complement receptor 3 (CR3) for efficient invasion of human macrophages and evasion of inflammatory responses. PGL-1 production also promoted bacterial uptake by human dendritic cells and dampened their infection-induced maturation. Our results therefore suggest that M. leprae produces PGL-1 for immune-silent invasion of host phagocytic cells.

Ubiquitin-mediated protein degradation is the main mechanism for controlled proteolysis, which is crucial for muscle development and maintenance. The ankyrin repeat-containing protein with a suppressor of cytokine signaling box 2 gene (ASB2) encodes the specificity subunit of an E3 ubiquitin ligase complex involved in differentiation of hematopoietic cells. Here, we provide the first evidence that a novel ASB2 isoform, ASB2b, is important for muscle differentiation. ASB2b is expressed in muscle cells during embryogenesis and in adult tissues. ASB2b is part of an active E3 ubiquitin ligase complex and targets the actin-binding protein filamin B ( The ubiquitin-proteasome system (UPS) is one of the major mechanisms for controlled proteolysis, which is a crucial determinant of many cellular events in eukaryotes. Degradation of a protein by the ubiquitin-proteasome pathway entails two successive events: the covalent attachment of ubiquitin chains to lysine residues in a substrate protein leading to its recognition and ATP-dependent proteolysis by the proteasome. Ubiquitylation of protein substrates occurs through the sequential action of distinct enzymes: a ubiquitin-activating enzyme E1, a ubiquitin-conjugating enzyme E2 and a ubiquitin ligase E3 responsible for the specific recognition of substrates through dedicated interaction domains. 1 ASB2 is one of 18 members of the ankyrin repeat-containing suppressor of cytokine signaling (SOCS) box protein family (ASB) that are characterized by variable numbers of N-terminal ankyrin repeats. 2 The ASB2 gene was originally identified as an retinoic acid-inducible gene involved in induced differentiation of myeloid leukemia cells. 3,4 We have previously demonstrated that, by interacting with the elongin BC complex, ASB2 can assemble with a Cullin5/Rbx module to form an E3 ubiquitin ligase complex that stimulates polyubiquitylation by the E2 ubiquitin-conjugating enzyme UbcH5a. 5,6 This strongly suggests that ASB2 targets specific proteins to destruction by the proteasome during differentiation of hematopoietic cells. We have recently shown that ASB2 ubiquitin ligase activity drives proteasome-mediated degradation of the ubiquitously expressed actin-binding protein filamins (FLNs), FLNa and b, and can regulate integrin-mediated cell spreading. 6 During muscle development, dramatic changes in protein expression and cell morphology rely on the turnover of regulatory and structural components. Indeed, myogenic transcription factors such as MyoD and its E2A partner or negative Id regulator as well as myofibrillar proteins were shown to be degraded by the UPS. 7-11 Although some E3 ubiquitin ligases active during myogenesis have been identified, 12-23 a precise understanding of the function of ubiquitylation in muscle development and the identities of specific ubiquitin ligases and their potential substrates is lacking.Here we show that ASB2 expression is not restricted to hematopoietic cells but is also expressed and regulated in muscle cells during mouse and chick embryogenesis.

The p6 region of HIV-1 Gag contains two late (L) domains, PTAP and LYPXnL, that bind the cellular proteins Tsg101 and Alix, respectively. These interactions are thought to recruit members of the host fission machinery (ESCRT) to facilitate HIV-1 release. Here we report a new role for the p6-adjacent nucleocapsid (NC) domain in HIV-1 release. The mutation of basic residues in NC caused a pronounced decrease in virus release from 293T cells, although NC mutant Gag proteins retained the ability to interact with cellular membranes and RNAs. Remarkably, electron microscopy analyses of these mutants revealed arrested budding particles at the plasma membrane, analogous to those seen following the disruption of the PTAP motif. This result indicated that the basic residues in NC are important for virus budding. When analyzed in physiologically more relevant T-cell lines (Jurkat and CEM), NC mutant viruses remained tethered to the plasma membrane or to each other by a membranous stalk, suggesting membrane fission impairment. Remarkably, NC mutant release defects were alleviated by the coexpression of a Gag protein carrying a wild-type (WT) NC domain but devoid of all L domain motifs and by providing alternative access to the ESCRT pathway, through the in trans expression of the ubiquitin ligase Nedd4.2s. Since NC mutant Gag proteins retained the interaction with Tsg101, we concluded that NC mutant budding arrests might have resulted from the inability of Gag to recruit or utilize members of the host ESCRT machinery that act downstream of Tsg101. Together, these data support a model in which NC plays a critical role in HIV-1 budding.

SUMMARY Interactions of the CHMP protein carboxyl terminal tails with effector proteins play important roles in retroviral budding, cytokinesis, and multivesicular body biogenesis. Here we demonstrate that hydrophobic residues at the CHMP4B C-terminal amphipathic α-helix bind a concave surface of Brox, a mammalian paralog of Alix. Unexpectedly, CHMP5 was also found to bind Brox and specifically recruit endogenous Brox to detergent-resistant membrane fractions through its C-terminal 20 residues. Instead of an α-helix, the CHMP5 C-terminal tail adopts a tandem β-hairpin structure that binds Brox at the same site as CHMP4B. Additional Brox:CHMP5 interface is furnished by a unique CHMP5 hydrophobic pocket engaging the Brox residue Y348 that is not conserved among the Bro1 domains. Our studies thus unveil a novel conformation of the CHMP protein C-terminal tails, and provide new insights into the overlapping but distinct binding profiles of ESCRT-III and the Bro1 domain proteins.

eWe recently reported that human immunodeficiency virus type 1 (HIV-1) carrying PTAP and LYPX n L L domains ceased budding when the nucleocapsid (NC) domain was mutated, suggesting a role for NC in HIV-1 release. Here we investigated whether NC involvement in virus release is a property specific to HIV-1 or a general requirement of retroviruses. Specifically, we examined a possible role for NC in the budding of retroviruses relying on divergent L domains and structurally homologous NC domains that harbor diverse protein sequences. We found that NC is critical for the release of viruses utilizing the PTAP motif whether it functions within its native Gag in simian immunodeficiency virus cpzGAB2 (SIVcpzGAB2) or SIVsmmE543 or when it is transplanted into the heterologous Gag protein of equine infectious anemia virus (EIAV). In both cases, virus release was severely diminished even though NC mutant Gag proteins retained the ability to assemble spherical particles. Moreover, budding-defective NC mutants, which displayed particles tethered to the plasma membrane, were triggered to release virus when access to the cell endocytic sorting complex required for transport pathway was restored (i.e., in trans expression of Nedd4.2s). We also examined the role of NC in the budding of EIAV, a retrovirus relying exclusively on the (L)YPX n L-type L domain. We found that EIAV late budding defects were rescued by overexpression of the isolated Alix Bro1 domain (Bro1). Bro1-mediated rescue of EIAV release required the wild-type NC. EIAV NC mutants lost interactions with Bro1 and failed to produce viruses despite retaining the ability to self-assemble. Together, our studies establish a role for NC in the budding of retroviruses harboring divergent L domains and evolutionarily diverse NC sequences, suggesting the utilization of a common conserved mechanism and/or cellular factor rather than a specific motif.

In addition to PTAP L domains, primate lentiviruses carry Alix-binding motifs that include the recently described type 3 SREKPYKEVTEDLLHLNSLF sequence. We examined the requirements for the type 3 sequence motif in simian immunodeficiency virus SIV smE543 and identified the 499 LNSLF 503 sequence as a key functional determinant. Mutation of distal leucines 499 L and 502 L (LL mutant) caused an inhibitory effect on Alix-dependent SIV smE543 release that was quantitatively similar to that observed following disruption of the type 3 L domain or RNA interference (RNAi) depletion of Alix. Similar results were obtained with the SIV mac239 LL mutant. Thus, distal leucines are key determinants of SIV smE543 and SIV mac239 type 3 L domains.Retroviruses acquire their envelopes at the host cell membrane of infected cells. To this end, they utilize short sequences designated L (late) domains to recruit members of the endosomal sorting complex required for transport (ESCRT) to catalyze membrane modeling events that lead to virus release (1,2,5,11,16). Three types of L domains have so far been identified: the PT/SAP, PPXY, and LYPX n L motifs. They bind the host proteins Tsg101, Nedd4-like ubiquitin ligase family members, and Alix, respectively (6,13,15,23,25,26). These interactions lead to the recruitment of members of the ESCRT pathway and require the activity of the AAA ATPase VPS4 (14,23,24,26).All retroviral Gag proteins contain at least one L domain, though most carry multiple L domain motifs, believed to function synergistically and ensure efficient viral release (4,7,8,13,21,27). Structural studies have been used to determine L domain sequence and functional requirements. Only limited sequence variability was noted in motifs that bind Tsg101, including PTAP, PSAP, and PXXP motifs found in Hrs, the natural partner for Tsg101 (3,10,(18)(19)(20)28). However, more sequence divergence was observed in L domains binding Alix, with the identification of three sequences: YPDL in equine infectious anemia virus (EIAV) (23,26,29), LYPLASLRSLF in human immunodeficiency virus type 1 (HIV-1), and the recently described SREKPYKEVTEDLLHLNSLF sequence in the simian immunodeficiency virus SIV mac239 (30). They have been dubbed types 1, 2, and 3, respectively. The last type contains an anchoring tyrosine, followed by one or two hydrophobic residues (valine and/or leucine) (proximal leucines), and both residues are key functional determinants for Alix (30). We observed that the Alix-binding type 3 L domain also contains a downstream LXXLF sequence carrying additional leucines (distal leucines) that are present in SIV mac239 , SIV agmTan-1 , and SIV smE543 . Similarly, the HIV-1 p6 domain also harbors an LXXLF motif in its LYPLASLRSLF L domain (underlined), which is critical for Alix binding and function (23,26). In this study, we examined the role of distal leucine residues in the LNSLF sequence within the Alix-binding type 3 L domains of both SIV smE543 (9) and SIV mac239 (12).The 499 LNSLF 503 sequence is required for PTAP-independent SI...