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Infection and Immunity, August 2008, p. 3550-3560, Vol. 76, No. 8
0019-9567/08/$08.00+0     doi:10.1128/IAI.01613-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Sortase A Utilizes an Ancillary Protein Anchor for Efficient Cell Wall Anchoring of Pili in Streptococcus agalactiae{triangledown}

Angela H. Nobbs, Roberto Rosini, C. Daniela Rinaudo, Domenico Maione, Guido Grandi, and John L. Telford*

Novartis Vaccines, 53100 Siena, Italy

Received 6 December 2007/ Returned for modification 28 April 2008/ Accepted 26 May 2008

Pili are putative virulence factors and promising vaccine candidates in Streptococcus agalactiae (group B Streptococcus [GBS]) infection, a leading cause of neonatal sepsis and meningitis. The genes necessary for pilus synthesis and assembly are clustered in pilus islands (PI). Each gene encodes three structural subunits (a backbone and two ancillary proteins) bearing a C-terminal LPXTG motif and two subfamily C sortases (SrtC) involved in covalent polymerization of the subunits. GBS strains also possess the conserved "housekeeping" sortase A (SrtA), but its role in pilus assembly is unclear. To address this issue, pilus expression and cell wall anchoring were analyzed in srtA deletion mutants. Loss of SrtA did not affect pilus polymerization. However, pilus expression on the cell surface was reduced, and pili accumulated in the culture supernatant. Furthermore, cell-associated pili could be readily released by detergent treatment, indicating that SrtA is involved in covalent anchoring of pili to the cell wall. When each of the genes comprising PI-2a was systematically deleted, only the absence of ancillary subunit GBS150 or the SrtC required for incorporation of GBS150 into pili mimicked the srtA mutant phenotype. Thus, from these data a model for GBS pilus assembly can be proposed in which PI sortases are responsible for polymerization of the pilus structure, while SrtA is required to covalently attach it to the cell wall, utilizing ancillary pilus subunit GBS150 as the anchor protein.

* Corresponding author. Mailing address: Novartis Vaccines, Via Fiorentina 1, 53100 Siena, Italy. Phone: 39 0577 243470. Fax: 39 0577 243564. E-mail: john.telford{at}novartis.com

{triangledown} Published ahead of print on 9 June 2008.

Editor: V. J. DiRita

Infection and Immunity, August 2008, p. 3550-3560, Vol. 76, No. 8
0019-9567/08/$08.00+0     doi:10.1128/IAI.01613-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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