Conformational triggers associated with influenza matrix protein 1 polymerization.

A central role for the influenza matrix protein 1 (M1) is to form a polymeric coat on the inner leaflet of the host membrane that ultimately provides shape and stability to the virion. M1 polymerizes upon binding membranes, but triggers for conversion of M1 from a water-soluble component of the nucleus and cytosol into an oligomer at the membrane surface are unknown. While full-length M1 is required for virus viability, the N-terminal domain (M1NT) retains membrane binding and pH-dependent oligomerization. We studied the structural plasticity and oligomerization of M1NT in solution using NMR spectroscopy. We show that the isolated domain can be induced by sterol-containing compounds to undergo a conformational change and self-associate in a pH-dependent manner consistent with the stacked dimer oligomeric interface. Surface-exposed residues at one of the stacked dimer interfaces are most sensitive to sterols. Several perturbed residues are at the interface between the N-terminal subdomains and are also perturbed by changes in pH. The effects of sterols appear to be indirect and most likely mediated by reduction in water activity. The local changes are centered on strictly conserved residues and consistent with a priming of the N-terminal domain for polymerization. We hypothesize that M1NT is sensitive to changes in the aqueous environment and that this sensitivity is part of a mechanism for restricting polymerization to the membrane surface. Structural models combined with information from chemical shift perturbations indicate mechanisms by which conformational changes can be transmitted from one polymerization interface to the other.