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contributed equally to this work During in¯uenza virus infection, viral ribonucleo proteins (vRNPs) are replicated in the nucleus and must be exported to the cytoplasm before assembling into mature viral particles. Nuclear export is mediated by the cellular protein Crm1 and putatively by the viral protein NEP/NS2. Proteolytic cleavage of NEP de®nes an N-terminal domain which mediates RanGTP-dependent binding to Crm1 and a Cterminal domain which binds to the viral matrix protein M1. The 2.6 A Ê crystal structure of the C-terminal domain reveals an amphipathic helical hairpin which dimerizes as a four-helix bundle. The NEP±M1 interaction involves two critical epitopes: an exposed tryptophan (Trp78) surrounded by a cluster of glutamate residues on NEP, and the basic nuclear localization signal (NLS) of M1. Implications for vRNP export are discussed.

Spontaneous proteolysis of influenza virus M1 protein during crystallisation has defined an N-terminal domain of amino acids 1--164. Full-length M1, the N-terminal domain, and the C-terminal part of M1 (residues 165--252) were produced in Escherichia coli. In vitro tests showed that only full-length M1 and its N-terminal domain bind to negatively charged liposomes and that only full-length M1 and its C-terminal part bind to RNP. However, only full-length M1 had transcription inhibition activity. Several independent experimental approaches indicate that in vitro transcription inhibition occurs through polymerisation/aggregation of M1 onto RNP, or of M1 onto M1 already bound to RNP, rather than by binding to a specific active site on the nucleoprotein or the polymerase. The structure/function of influenza virus M1 will be compared with that of the Ebola virus matrix protein, VP40.

Streptococcus pneumoniae is the main causal agent of pathologies that are increasingly resistant to antibiotic treatment. Clinical resistance of S. pneumoniae to ␤-lactam antibiotics is linked to multiple mutations of high molecular mass penicillin-binding proteins (H-PBPs), essential enzymes involved in the final steps of bacterial cell wall synthesis. H-PBPs from resistant bacteria have a reduced affinity for ␤-lactam and a decreased hydrolytic activity on substrate analogues. In S. pneumoniae, the gene coding for one of these H-PBPs, PBP2x, is located in the cell division cluster (DCW). We present here structural evidence linking multiple ␤-lactam resistance to amino acid substitutions in PBP2x within a buried cavity near the catalytic site that contains a structural water molecule. Site-directed mutation of amino acids in contact with this water molecule in the ''sensitive'' form of PBP2x produces mutants similar, in terms of ␤-lactam affinity and substrate hydrolysis, to altered PBP2x produced in resistant clinical isolates. A reverse mutation in a PBP2x variant from a clinically important resistant clone increases the acylation efficiency for ␤-lactams and substrate analogues. Furthermore, amino acid residues in contact with the structural water molecule are conserved in the equivalent H-PBPs of pathogenic Gram-positive cocci. We suggest that, probably via a local structural modification, the partial or complete loss of this water molecule reduces the acylation efficiency of PBP2x substrates to a point at which cell wall synthesis still occurs, but the sensitivity to therapeutic concentrations of ␤-lactam antibiotics is lost.

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