The native structure of the assembled matrix protein 1 of influenza A virus.

Influenza A virus causes millions of severe cases of disease during annual epidemics. The most abundant protein in influenza virions is matrix protein 1 (M1), which mediates virus assembly by forming an endoskeleton beneath the virus membrane1. The structure of full-length M1, and how it oligomerizes to mediate the assembly of virions, is unknown. Here we determine the complete structure of assembled M1 within intact virus particles, as well as the structure of M1 oligomers reconstituted in vitro. We find that the C-terminal domain of M1 is disordered in solution but can fold and bind in trans to the N-terminal domain of another M1 monomer, thus polymerizing M1 into linear strands that coat the interior surface of the membrane of the assembling virion. In the M1 polymer, five histidine residues-contributed by three different monomers of M1-form a cluster that can serve as the pH-sensitive disassembly switch after entry into a target cell. These structures therefore reveal mechanisms of influenza virus assembly and disassembly.

Effect of proteolytic processing at two distinct sites on shape and aggregation of an anchorless fusion protein of human respiratory syncytial virus and fate of the intervening segment.

We have examined the consequences of cleaving the fusion glycoprotein (F) of human respiratory syncytial virus (HRSV) at two distinct furin-recognition sites. Purified anchorless F is a mixture of unaggregated cone-shaped molecules and rosettes of lollipop-shaped spikes. The unaggregated molecules contain a proportion of uncleaved F0 and an intermediate, F(delta1-109), cleaved only at site I, residues 106-109. Inhibition of cleavage at site I, by two amino acid changes (R108N/R109N), reduces the proportion of aggregated molecules with a concomitant increase in the amount of unprocessed F0. Inhibition of cleavage at site II, residues 131-136, by deletion of four amino acids (delta131-134), abrogates aggregation of anchorless F and all molecules are seen as individual cone-shaped rods. In vitro cleavage of anchorless F, or mutant delta131-134, with trypsin at 4, 20, or 37 degrees C, under conditions in which cleavage at site II is complete in all molecules, leads to their aggregation in rosettes of lollipop-shaped spikes. Thus, cleavage at site II is required for the structural changes in anchorless F that lead to changes in shape and to aggregation. The segment between sites I and II, residues 110-136, is not associated with anchorless F in the supernatant of infected cell cultures, indicating that it is released from the processed protein when cleavage at sites I and II is completed.