The surface protein Srr-1 of Streptococcus agalactiae binds human keratin 4 and promotes adherence to epithelial HEp-2 cells

Infect Immun. 2007 Nov;75(11):5405-14. doi: 10.1128/IAI.00717-07. Epub 2007 Aug 20.


Streptococcus agalactiae is frequently the cause of bacterial sepsis and meningitis in neonates. In addition, it is a commensal bacterium that colonizes the mammalian gastrointestinal tract. During its commensal and pathogenic lifestyles, S. agalactiae colonizes and invades a number of host compartments, thereby interacting with different host proteins. In the present study, the serine-rich repeat protein Srr-1 from S. agalactiae was functionally investigated. Immunofluorescence microscopy showed that Srr-1 was localized on the surface of streptococcal cells. The Srr-1 protein was shown to interact with a 62-kDa protein in human saliva, which was identified by matrix-assisted laser desorption ionization-time-of-flight analysis as human keratin 4 (K4). Immunoblot and enzyme-linked immunosorbent assay experiments allowed us to narrow down the K4 binding domain in Srr-1 to a region of 157 amino acids (aa). Furthermore, the Srr-1 binding domain of K4 was identified in the C-terminal 255 aa of human K4. Deletion of the srr-1 gene in the genome of S. agalactiae revealed that this gene plays a role in bacterial binding to human K4 and that it is involved in adherence to epithelial HEp-2 cells. Binding to immobilized K4 and adherence to HEp-2 cells were restored by introducing the srr-1 gene on a shuttle plasmid into the srr-1 mutant. Furthermore, incubation of HEp-2 cells with the K4 binding domain of Srr-1 blocked S. agalactiae adherence to epithelial cells in a dose-dependent fashion. This is the first report describing the interaction of a bacterial protein with human K4.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adhesins, Bacterial / genetics
  • Adhesins, Bacterial / metabolism*
  • Bacterial Adhesion / genetics
  • Bacterial Adhesion / physiology*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Cell Line
  • Enzyme-Linked Immunosorbent Assay
  • Epithelial Cells / microbiology*
  • Gene Deletion
  • Genetic Complementation Test
  • Humans
  • Immunoblotting
  • Keratin-4 / metabolism*
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism
  • Microscopy, Fluorescence
  • Prokaryotic Cells / chemistry
  • Protein Binding
  • Protein Interaction Mapping
  • Saliva / chemistry
  • Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
  • Streptococcus agalactiae / genetics
  • Streptococcus agalactiae / physiology*


  • Adhesins, Bacterial
  • Bacterial Proteins
  • Keratin-4
  • Membrane Proteins
  • Srr-1 protein, Streptococcus agalactiae