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The signal peptidase complex (SPC) is an essential membrane complex in the endoplasmic reticulum (ER), where it removes signal peptides (SPs) from a large variety of secretory pre-proteins with exquisite specificity. Although the determinants of this process have been established empirically, the molecular details of SP recognition and removal remain elusive. Here, we show that the human SPC exists in two functional paralogs with distinct proteolytic subunits. We determined the atomic structures of both paralogs using electron cryo-microscopy and structural proteomics. The active site is formed by a catalytic triad and abuts the ER membrane, where a transmembrane window collectively formed by all subunits locally thins the bilayer. Molecular dynamics simulations indicate that this unique architecture generates specificity for SPs based on the length of their hydrophobic segments.

Extracellular vesicles (EV) are membrane encapsulated nanoparticles that can function in intercellular communication, and their presence in biofluids can be indicative for (patho)physiological conditions. Studies aiming to resolve functionalities of EV or to discover EV-associated biomarkers for disease in liquid biopsies are hampered by limitations of current protocols to isolate EV from biofluids or cell culture medium. EV isolation is complicated by the >105-fold numerical excess of other types of particles, including lipoproteins and protein complexes. In addition to persisting contaminants, currently available EV isolation methods may suffer from inefficient EV recovery, bias for EV subtypes, interference with the integrity of EV membranes, and loss of EV functionality. In this study, we established a novel three-step non-selective method to isolate EV from blood or cell culture media with both high yield and purity, resulting in 71% recovery and near to complete elimination of unrelated (lipo)proteins. This EV isolation procedure is independent of ill-defined commercial kits, and apart from an ultracentrifuge, does not require specialised expensive equipment.

Several hundred million years of co-evolution of vertebrates and invading pathogens have shaped the adaptive immune system to fight back the unwanted invaders through highly sophisticated defense mechanisms. Herpesviruses manage to dodge this immune response by hampering one of the central hinges of human adaptive immunity, the major histocompatibility complex (MHC) class I antigen presentation pathway. One of the bottlenecks of this pathway is the loading of pathogen-derived peptides onto MHC-I molecules in the endoplasmic reticulum (ER). This task is accomplished by the MHC class I peptide-loading complex (PLC), of which the transporter associated with antigen-processing (TAP) is a central component. In this review, we summarize recent structural and functional insights into the molecular architecture of the PLC, how TAP accomplishes the transport of peptides across the ER membrane, and how herpes- and poxviruses inhibit TAP-mediated peptide translocation and subsequent antigen presentation.

Adenovirus type 37 (Ad37) is one of the principal agents responsible for epidemic keratoconjunctivitis (EKC), a severe ocular infection that remains without any available treatment. Recently, a trivalent sialic acid derivative (ME0322, Angew. Chem. Int. Ed., 2011, 50, 6519) was shown to function as a highly potent inhibitor of Ad37, efficiently preventing the attachment of the virion to the host cells and subsequent infection. Here, new trivalent sialic acid derivatives were designed, synthesized and their inhibitory properties against Ad37 infection of the human corneal epithelial cells were investigated. In comparison to ME0322, the best compound (17a) was found to be over three orders of magnitude more potent in a cell-attachment assay (IC50 = 1.4 nM) and about 140 times more potent in a cell-infection assay (IC50 = 2.9nM). X-ray crystallographic analysis demonstrated a trivalent binding mode of all compounds to the Ad37 fiber knob. For the most potent compound ophthalmic toxicity in rabbits was investigated and it was concluded that repeated eye administration did not cause any adverse effects.

Most adenoviruses attach to host cells by means of the protruding fiber protein that binds to host cells via the coxsackievirus and adenovirus receptor (CAR) protein. Human adenovirus type 52 (HAdV-52) is one of only three gastroenteritis-causing HAdVs that are equipped with two different fiber proteins, one long and one short. Here we show, by means of virion-cell binding and infection experiments, that HAdV-52 can also attach to host cells via CAR, but most of the binding depends on sialylated glycoproteins. Glycan microarray, flow cytometry, surface plasmon resonance and ELISA analyses reveal that the terminal knob domain of the long fiber (52LFK) binds to CAR, and the knob domain of the short fiber (52SFK) binds to sialylated glycoproteins. X-ray crystallographic analysis of 52SFK in complex with 2-O-methylated sialic acid combined with functional studies of knob mutants revealed a new sialic acid binding site compared to other, known adenovirus: glycan interactions. Our findings shed light on adenovirus biology and may help to improve targeting of adenovirus-based vectors for gene therapy.