The alpha C protein (ACP) is found in Streptococcus and acts as an invasin which plays a role in the internalisation and translocation of the organism across human epithelial surfaces. Group B Streptococcus is the leading cause of diseases including bacterial pneumonia, sepsis and meningitis. The N terminal of ACP is associated with virulence and forms a beta sandwich and a three helix bundle [1] [2] [3]. ACP consists of an N-terminal domain (170 amino acids) followed by a variable number of tandem repeats (82 amino acids each) and a C-terminal domain (45 amino acids) containing an LPXTG peptidoglycan-anchoring motif. This entry is the N-terminal domain of ACP (NtACP). NtACP can be further divided into two structurally distinct domains, D1 and D2. D1, the more distal (amino-terminal) portion, consists of a beta sandwich with strong structural homology to fibronectin's integrin-binding region (FnIII10). D2 consists of three antiparallel alpha helix coils containing a portion of the glycosaminoglycan (GAG)-binding domain adjacent to the repeat region. NtACP binds to heparin and GAGs only when it is covalently associated with the adjacent repeat region. NtACP's D1 region contains a K144- T145-D146 (KTD) motif, located within a loop region that is structurally analogous to the loop containing the RGD integrin-binding motif in FnIII10. Single mutation within the KTD motif (D146A), present in the D1 domain, reduces NtACP binding to a1b integrion. The a1b1-integrin is one of four collagen-binding I-domain-containing integrins. Structural analysis of the D1 domain, in particular the region containing the putative integrin-binding loop and KTD motif, shares a strong structural homology with the FnIII10's integrin-binding region. Amino acid sequence alignment of Alps indicates that KTD is highly conserved [4].

1: Crystal structure of the N-terminal domain of the group B streptococcus alpha C protein. Auperin TC, Bolduc GR, Baron MJ, Heroux A, Filman DJ, Madoff LC, Hogle JM; J Biol Chem. 2005;280:18245-18252. PMID:15753100

2: The alpha C protein mediates internalization of group B Streptococcus within human cervical epithelial cells. Bolduc GR, Baron MJ, Gravekamp C, Lachenauer CS, Madoff LC; Cell Microbiol. 2002;4:751-758. PMID:12427097

3: Inactivation of the alpha C protein antigen gene, bca, by a novel shuttle/suicide vector results in attenuation of virulence and immunity in group B Streptococcus. Li J, Kasper DL, Ausubel FM, Rosner B, Michel JL; Proc Natl Acad Sci U S A. 1997;94:13251-13256. PMID:9371832

4: The group B streptococcal alpha C protein binds alpha1beta1-integrin through a novel KTD motif that promotes internalization of GBS within human epithelial cells. Bolduc GR, Madoff LC; Microbiology. 2007;153:4039-4049. PMID:18048918