Expression, purification, crystallization and preliminary X-ray analysis of phosphotransacetylase from Methanosarcina thermophila

Phosphotransacetylase (Pta) from the anaerobic archaeon Methanosarcina thermophila has been heterologously expressed in a soluble form which facilitated crystallization using the hanging-drop vapor-diffusion method with ammonium sulfate as a precipitant. This is the first report of the crystallization of any Pta. While the M. thermophila Pta has high sequence identity to Ptas from other organisms, it has no homology to any previously crystallized proteins. The protein crystallized in space group I4₁, with unit-cell parameters a = b = 114.8, c = 127.8 Å, α = β = γ = 90°. The crystals diffracted to 2.5 Å resolution using Cu Kα radiation. The enzyme had previously been reported to exist as a monomer; however, the self-rotation function showed the presence of a non-crystallographic symmetry axis at ψ = 90, φ = 90, κ = 180°, suggesting oligomerization. Dynamic light-scattering analysis supported a dimeric state for Pta in solution.