Nectins (nectin1-4) and Necls ] are Ig superfamily cell adhesion molecules that regulate cell differentiation and tissue morphogenesis. Adherens junction formation and subsequent cell-cell signaling is initiated by the assembly of higher-order receptor clusters of cognate molecules on juxtaposed cells. However, the structural and mechanistic details of signaling cluster formation remain unclear. Here, we report the crystal structure of poliovirus receptor (PVR)/Nectin-like-5/CD155) in complex with its cognate immunoreceptor ligand T-cell-Ig-and-ITIM-domain (TIGIT). The TIGIT/ PVR interface reveals a conserved specific "lock-and-key" interaction. Notably, two TIGIT/PVR dimers assemble into a heterotetramer with a core TIGIT/TIGIT cis-homodimer, each TIGIT molecule binding one PVR molecule. Structure-guided mutations that disrupt the TIGIT/ TIGIT interface limit both TIGIT/PVR-mediated cell adhesion and TIGIT-induced PVR phosphorylation in primary dendritic cells. Our data suggest a cis-trans receptor clustering mechanism for cell adhesion and signaling by the TIGIT/PVR complex and provide structural insights into how the PVR family of immunoregulators function.N ectins (nectin1-4) and nectin-like (Necl1-5) molecules are members of the large Ig superfamily (IgSF) of cell-surface receptors that play central roles in cell adhesion, cell movement, proliferation, and survival and contribute to the morphogenesis and differentiation of many cell and tissue types by inducing an intracellular signaling cascade (1-5). Nectins and Necls can function as both ligands and receptors and therefore are able to signal bidirectionally into juxtaposed cells (3,6). To mediate the formation of cell adherens junctions, a model suggests that the extracellular domains of these molecules form ligand-dependent homo-or heterodimers in trans (between molecules located on the same or opposite cell surfaces, respectively) and lateral homodimers in cis, creating a tight network of nectin zippers between juxtaposed cells (7,8). To date, structural and functional studies suggest a mechanism whereby the cis-homodimerization of a receptor on the same cell surface is followed by the formation of a trans-dimer between juxtaposed cells using identical protein interfaces. This assembly is noteworthy because it requires a rearrangement and breakup of the cis-homodimer followed by a transdimerization across the adherens junction. The cis-trans clustering is then initiated through another unknown protein interface, likely involving a different receptor domain.Several high-affinity homophilic trans-interactions have been described in detail for nectins/Necls and similar molecules (8-13). However, the structure and function of the presumably weaker lateral homophilic cis-dimers in cell adhesion and their role in intracellular signaling is not known. Because all structures solved to date are homodimers, it is unclear if they represent the cisor the trans-state. Thus, the question of how cis-trans heterodimerization drives cell adhesion and intracellular sign...

Secretory and membrane proteins carry amino-terminal signal sequences that, in cotranslational targeting, are recognized by the signal recognition particle protein SRP54 without sequence specificity. The most abundant membrane proteins on Earth are the light-harvesting chlorophyll a/b binding proteins (LHCPs). They are synthesized in the cytoplasm, imported into the chloroplast, and posttranslationally targeted to the thylakoid membrane by cpSRP, a heterodimer formed by cpSRP54 and cpSRP43. We present the 1.5 angstrom crystal structure of cpSRP43 characterized by a unique arrangement of chromodomains and ankyrin repeats. The overall shape and charge distribution of cpSRP43 resembles the SRP RNA, which is absent in chloroplasts. The complex with the internal signal sequence of LHCPs reveals that cpSRP43 specifically recognizes a DPLG peptide motif. We describe how cpSPR43 adapts the universally conserved SRP system to posttranslational targeting and insertion of the LHCP family of membrane proteins.

Cytomegalovirus (CMV) is a widespread opportunistic pathogen that causes birth defects when transmitted transplacentally and severe systemic illness in immunocompromised individuals. MSL-109, a human monoclonal IgG isolated from a CMV seropositive individual, binds to the essential CMV entry glycoprotein H (gH) and prevents infection of cells. Here, we suggest a mechanism for neutralization activity by MSL-109. We define a genetic basis for resistance to MSL-109 and have generated a structural model of gH that reveals the epitope of this neutralizing antibody. Using surface-based, time-resolved FRET, we demonstrate that gH/gL interacts with glycoprotein B (gB). Additionally, we detect homodimers of soluble gH/gL heterodimers and confirm this novel oligomeric assembly on full-length gH/gL expressed on the cell surface. We show that MSL-109 perturbs the dimerization of gH/gL:gH/gL, suggesting that dimerization of gH/gL may be required for infectivity. gH/gL homodimerization may be conserved between alpha-and betaherpesviruses, because both CMV and HSV gH/gL demonstrate self-association in the FRET system. This study provides evidence for a novel mechanism of action for MSL-109 and reveals a previously undescribed aspect of viral entry that may be susceptible to therapeutic intervention.H uman CMV is a β-group herpesvirus that causes severe complications in immunocompromised individuals. CMV infects between 60% and 80% of the adult population worldwide (1). As with other herpesviruses, CMV establishes a lifelong latency in the host but is largely asymptomatic among infected immunocompetent individuals (2). However, during severe immunosuppression (e.g., in the setting of hematopoietic stem cell transplantation and solid organ transplantation, or advanced HIV/AIDS), CMV reactivation or primary infection can result in life-threatening disease. In addition, the acquisition of primary CMV infection during pregnancy, although of little consequence to the mother, can have severe clinical consequences in the developing fetus (3, 4). The current therapy for CMV disease is treatment with either ganciclovir or valganciclovir, which are associated with significant toxicity and not approved for use in pregnant women or for congenitally damaged infants (5). CMV hyperimmunoglobulin (CMV-HIG; pooled human IgG from CMV-positive individuals) has demonstrated efficacy in certain solid organ transplant recipients and more recently found to show limited success in protecting infants from congenital CMV disease (1, 6, 7). These findings suggest that more potent or differently targeted antibody therapy may prove to be an effective and safe alternative to the current forms of CMV therapy.Like other herpesviruses, CMV uses multiprotein entry complexes to initiate infection of host cells. Three glycoproteins, gB, gH, and gL, known as the "core fusion machinery," are conserved in all herpesviruses and are required for entry (8,9). gB, the most conserved of these glycoproteins, exists as a homotrimer and catalyzes membrane fusion during vira...