The type VII protein translocation/secretion system, unique to Gram-positive bacteria is a key virulence determinant in Staphylococcus aureus. We aim to characterise the architecture of this secretion machinery and now describe a study of S. aureus EssB, a 52-kDa bitopic membrane protein essential for secretion of the ESAT-6-family-proteins, the prototypic substrate of Type VII secretion. Full-length EssB was heterologously expressed in Escherichia coli, solubilised from the bacterial membrane, purified to homogeneity and shown to be dimeric. A C-terminal truncation, EssB?C, and two soluble fragments termed EssB-N and EssB-C, predicted to occur on either side of the cytoplasmic membrane, have been successfully purified in recombinant form, characterised and together with the full-length protein used in crystallisation trials. EssB-N, the 25 kDa N-terminal cytoplasmic fragment, gave well-ordered crystals and we report the structure, determined by single-wavelength anomalous diffraction (SAD) targeting an SeMet derivative, refined to atomic (1.05 Å) resolution. EssB-N is dimeric in solution but crystallises as a monomer and displays a fold composed of two globular domains separated by a cleft. The structure is related to that of Ser/Thr protein kinases and our study identifies that the type VII secretion system exploits and re-uses a stable modular entity and fold that has evolved to participate in protein-protein interactions in a similar fashion to the catalytically inert pseudokinases.