Crystallization and preliminary X‐ray diffraction studies of aSFP, a bovine seminal plasma protein with a single CUB domain architecture

Bovine acidic seminal fluid protein (aSFP) is a 12.9 kDa polypeptide of the spermadhesin family built by a single CUB domain architecture. The CUB domain is an extracellular module present in 16 functionally diverse proteins. To determine the three‐dimensional structure of aSFP, the protein was crystallized at 21 °C by vapor diffusion in hanging drops, using ammonium sulfate, pH 4.7, and polyethyleneglycol 4000 as precipitants, containing 10% dioxane to avoid the formation of clustered crystals. Elongated prismatic crystals with maximal size of 0.6 × 0.3 × 0.2 mm3 diffract to beyond 1.9 A resolution and belong to space group P2,2,2, with cell parameters a = 52.4 A, b = 41.5 A, c = 48.2 A. There is one aSFP molecule per asymmetric unit, which corresponds to a crystal volume per unit molecular mass of 2.04 A3/Da, and analytical ultracentrifugation analysis show that aSFP is a monomeric protein.