Combined results from solution studies on intact influenza virus M1 protein and from a new crystal form of its N-terminal domain show that M1 is an elongated monomer

The amino-terminal domain of influenza A virus matrix protein (residues 1-1 64) was crystallized at pH 7 into a new crystal form in space group P1. Thi s packing of the protein implies that M1(1-164) was monomeric in solution w hen it crystallized. Otherwise, the structure of the M1 fragment in the pH 7 crystals was the same as the monomers in crystals formed at pH 4 where cr ystal packing resulted in dimer formation [B. sha and M. Luo, 1997, Nature Struct Biol. 4, 239-244]. Analysis of intact M1 protein, the N-terminal dom ain, and the remaining C-terminal fragment (residues 165-252) in solution a lso showed that the N-terminal domain was monomeric with the same dimension s as determined from the crystal structure. Intact M1 protein was also mono meric but with an elongated shape due to the presence of the C-terminal par t. Circular dichroism showed that the C-terminal part of M1 contained helic al structure. A model for soluble M1 is presented, based on the assumption that the C-terminal domain is spherical, in which the N- and C-terminal dom ains are connected by a linker sequence which is available for proteolytic attack, (C) 2001 Academic Press.