Solving the structure of the bubble protein using the anomalous sulfur signal from single-crystal in-house Cu Kα diffraction data only

A small cysteine-rich protein, the function of which remains elusive, was discovered in the exudate of a Penicillium species. Crystal diffraction experiments conducted using in-house Cu Kα radiation and an R-AXIS IV++ imaging-plate detector yielded high-quality data to 1.4 Å, with a distinguishable anomalous signal from sulfur (ΔF/F = 0.031). This was used to phase the data and solve the structure using a single data set; the 64-residue amino-acid sequence was unambiguously determined from the electron density. It revealed a globular all-β protein with a hitherto unknown fold, having a surface electrostatic charge distribution that is similar to that of another small secreted fungal protein, the Williopsis mrakii killer toxin. Aligning the charge distribution superimposed the potential recognition sites of the two proteins, suggesting a similar negatively charged target.