Cloning, expression, purification, crystallization and preliminary crystallographic analysis of the N-terminal domain of serine glutamate repeat A (SgrA) protein from Enterococcus faecium

Serine glutamate repeat A (SgrA) protein is an LPxTG surface adhesin of Enterococcus faecium and is the first bacterial nidogen-binding protein identified to date. It has been suggested that it binds to human nidogen, the extracellular matrix molecule of basal lamina, and plays a key role in the invasion and colonization of eukaryotic host cells. SgrA_28–288, having both a putative ligand-binding A domain and repetitive B domain, was expressed in Escherichia coli and purified using Ni-affinity and hydrophobic interaction chromatography. Further, the putative ligand-binding region, rSgrA_28–153, was subcloned, overexpressed and purified in both native and selenomethionine-derivative forms. The native rSgrA_28–153 protein crystallized in the monoclinic space group P2₁ and diffracted to 3.3 Å resolution using an in-house X-ray source, with unit-cell parameters a = 35.84, b = 56.35, c = 60.20 Å, β = 106.5°.