X‐ray crystallographic determination of the ­structure of the influenza C virus haemagglutinin‐esterase‐fusion glycoprotein

Zhang, Xiaodong; Rosenthal, Peter B.; Formanowski, Frank; Fitz, Wolfgang; Wong, Chi‐Huey; Meier‐Ewert, H.; Skehel, John J.; Wiley, Don C.

The structure of the haemagglutinin‐esterase‐fusion (HEF) glycoprotein from influenza C virus has been determined to 3.2 Å resolution by X‐ray crystallography. A synthetic mercury‐containing esterase inhibitor and receptor analogue, 9‐acetamidosialic acid α‐thiomethylmercuryglycoside, was designed as the single isomorphous heavy‐atom derivative. The asymmetric unit of one crystal form (form I; P4(3)22, a = b = 155.4, c = 414.4 Å) contained an HEF trimer. Six mercury sites identifying the three haemagglutination and three esterase sites were located by difference Patterson map analysis of a 6.5 Å resolution derivative data set. These positions defined the molecular threefold‐symmetry axis of the HEF trimer. A molecular envelope was defined by averaging a 7.0 Å resolution electron‐density map, phased by single isomorphous replacement (SIR), about the non‐crystallographic threefold‐symmetry axis. Iterative non‐crystallographic symmetry averaging in real space, solvent flattening and histogram matching were used to extend the phases to 3.5 Å resolution. Molecular replacement of the model into a second crystal form (form II; P4(3)2(1)2, a = b = 217.4, c = 421.4 Å) containing two HEF trimers per asymmetric unit permitted iterative ninefold averaging of the electron density. The 3.5 Å electron‐density map allowed an unambiguous tracing of the polypeptide chain and identification of N‐linked carbohydrates. The model has been refined by least squares to 3.2 Å resolution (R (free) = 26.7%).