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EMBL Grenoble Outstation, c/o ILL, BP 156, F-38042 Grenoble Cedex 9, France
The activity and stability of influenza virus neuraminidase is known to depend on the presence of calcium ions. The atomic structure of the tetrameric neuraminidase head shows two distinct Ca2+ binding sites, one with low affinity on the molecular fourfold symmetry axis and one with high affinity close to the active site in each of the monomers. Here we show that Ca is essential for the thermostability of the isolated neuraminidase tetramer. Inactivation of Ca-free neuraminidase at high temperatures is accompanied by changes in protein structure leading to protease sensitivity. More than one Ca ion per tetramer is involved in stabilization, suggesting a role for the high affinity Ca binding site and the cooperative stabilization of the subunits. Sites which are located close to the fourfold axis of the neuraminidase tetramer and which are able to bind a variety of different metal ions are also described.
Present address: California Institute of Technology, Division of Biology 156-29, Pasadena, California 91125, U.S.A.
Received 1 July 1993;
accepted 9 September 1993.
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