No abstract

The X-ray crystal structure of recombinant human monocyte chemoattractant protein (MCP-1) has been solved in two crystal forms. One crystal form (P), refined to 1.85 A resolution, contains a dimer in the asymmetric unit, while the other (I) contains a monomer and was refined at 2.4 A. Although both crystal forms grow together in the same droplet, the respective quaternary structures of the protein differ dramatically. In addition, both X-ray structures differ to a similar extent from the solution structure of MCP-1. Such extent of variability of quaternary structures is unprecedented. In the crystal structures, the well-ordered N termini of MCP-1 form 3(10)-helices. Comparison of the three MCP-1 structures revealed a direct correlation between the main-chain conformation of the first two cysteine residues and the quaternary arrangements. These data can be used to explain the structural basis for the assignment of residues responsible for biological activity.

Binding of the required metal ions does not lead to significant structural modifications in the active site of the catalytic domain of ASV IN. This indicates that at least one metal-binding site is preformed in the structure, and suggests that the observed constellation of the acidic residues represents a catalytically competent active site. Only a single divalent cation was observed even at extremely high concentrations of the metals. We conclude that either only one metal ion is needed for catalysis, or that a second metal-binding site can only exist in the presence of substrate and/or other domains of the protein. The unexpected differences between the active sites of ASV IN and HIV-1 IN remain unexplained; they may reflect the effects of crystal contacts on the active site of HIV-1 IN, or a tendency for structural polymorphism.