The ExPortal: An Organelle Dedicated to the Biogenesis of Secreted Proteins in Streptococcus Pyogenes

Mol Microbiol. 2005 Nov;58(4):959-68. doi: 10.1111/j.1365-2958.2005.04887.x.


The Gram-positive pathogen Streptococcus pyogenes secretes proteins through the ExPortal, a unique single microdomain of the cellular membrane specialized to contain the Sec translocons. It has been proposed that the ExPortal functions as an organelle to promote the biogenesis of secreted proteins by coordinating interactions between nascent unfolded secretory proteins and membrane-associated chaperones. In this study we provide evidence to support this model. It was found that HtrA (DegP), a surface anchored accessory factor required for maturation of the secreted SpeB cysteine protease, was localized exclusively to the ExPortal. Furthermore, the ATP synthase beta subunit was not localized to the ExPortal, suggesting that retention is likely restricted to a specific subset of exported proteins. Mutations that disrupted the anchoring, but not the protease activity, of HtrA, also altered the maturation kinetics of SpeB demonstrating that localization to the ExPortal was important for HtrA function. These data indicate that the ExPortal provides a mechanism by which Gram-positive bacteria can coordinate protein secretion and subsequent biogenesis in the absence of a specialized protein-folding compartment.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Bacterial Proteins / analysis
  • Bacterial Proteins / metabolism*
  • Cell Membrane / chemistry
  • Exotoxins / analysis
  • Exotoxins / metabolism
  • Heat-Shock Proteins / analysis
  • Heat-Shock Proteins / genetics
  • Microscopy, Fluorescence
  • Microscopy, Immunoelectron
  • Mutation
  • Organelles / physiology*
  • Periplasmic Proteins / analysis
  • Periplasmic Proteins / genetics
  • Protein Processing, Post-Translational*
  • Protein Transport
  • Serine Endopeptidases / analysis
  • Serine Endopeptidases / genetics
  • Streptococcus pyogenes / metabolism*
  • Streptococcus pyogenes / ultrastructure*


  • Bacterial Proteins
  • Exotoxins
  • Heat-Shock Proteins
  • Periplasmic Proteins
  • erythrogenic toxin
  • DegP protease
  • Serine Endopeptidases