The secreted proteins ESAT-6 (early secreted antigen target 6) and CFP10 (culture filtrate protein 10) from Mycobacterium tuberculosis
play a key role in virulence and elicit a strong antigenic response (forming the basis of a new diagnostic test), although their precise biological function has yet to be defined. NMR (Renshaw et al.
; PDB code 1wa8
) and crystal (Poulsen et al.
; PDB code 3fav
) structures have confirmed that the two proteins are homologous to one another and form a tight 1:1 heterodimeric complex composed of antiparallel helical hairpins arranged in a head-to-tail fashion and held together by a predominantly hydrophobic interface. Pallen (2002
) used sequence analyses to show that ESAT-6 and CFP10 are archetypes of a large family of WXG100 proteins (so-called because of their central WX
G amino-acid motif and a length commonly of ~100 amino acids), members of which are widely distributed among Gram-positive bacteria. Drawing on these observations, Burts et al.
) showed that WXG100 proteins contribute to virulence in Staphylococcus aureus
. Subsequently, crystal structures of WXG100 proteins from S. aureus
(Sundaramoorthy et al.
; PDB codes 2vs0
) and Helicobacter pylori
(Jang et al.
; PDB codes 3fx7
) have shown these proteins to form homodimeric head-to-tail structures. Most recently, the M. tuberculosis
Rv3019c–Rv3020c ESX complex has been shown to form both heterodimers and heterotetramers in solution in a relative ratio of approximately 15:1, but only the heterotetrameric complex could be crystallized (Arbing et al.
; PDB code 3h6p
). The EsxR protein (the ESAT-6 homologue) folds as an antiparallel helical hairpin as expected, but strikingly the EsxS protein (the CFP10 homologue) folds as one long extended helix, with the C-terminal half of the helix domain-swapping with that of a second equivalent EsxRS complex to form a heterotetramer composed of two four-helix bundles (see Fig. 2 of Arbing et al.
(group B streptococcus; GBS) is a leading cause of neonatal sepsis and infections in pregnant women (Larsen & Sever, 2008
) and can also cause invasive infection in other settings (Sendi et al.
). Given the contribution of WXG100 proteins to virulence in M. tuberculosis
and S. aureus
, we searched for additional members of this protein family in the predicted proteome of S. agalactiae
strain NEM316 (Herbert et al.
). We solved the crystal structure of one such protein, GBS1074, to 2.0 Å resolution. As with related WXG100 proteins from S. aureus
and H. pylori
, GBS1074 adopts a homodimeric structure, but intriguingly the crystal packing suggests the potential to form higher order polymers, with a second intermolecular interface being formed between the extended C-terminal helical regions.