Sortase-mediated pilus fiber biogenesis in Streptococcus pneumoniae.

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• *Streptococcus pneumoniae* is a major global pathogen responsible for significant mortality.
• *Pneumococcal* pili are crucial for infection, but their biogenesis and the roles of associated proteins remain poorly understood.
• Understanding pilus formation is vital for developing new therapeutic strategies against antibiotic-resistant strains.

Purpose of the Study:

• *To elucidate the mechanism of pilus biogenesis in Streptococcus pneumoniae.*
• *To characterize the roles of sortases (SrtC-1, SrtC-2, SrtC-3) and pilins (RrgB, RrgA, RrgC) in pilus formation.*
• *To investigate the structural basis of sortase function in pilus assembly.*

Main Methods:

• *In vitro reconstitution of RrgB fibers.*
• *Electron microscopy of reconstituted RrgB fibers.*
• *X-ray crystallography of SrtC-1 and SrtC-3 sortases.*

Main Results:

• *SrtC-1 was identified as the primary pilus-polymerizing transpeptidase.*
• *Reconstituted RrgB fibers structurally mimicked the native pneumococcal pilus backbone.*
• *Crystal structures revealed flexible lids controlling active site access in SrtC-1 and SrtC-3, suggesting substrate specificity is mediated by surface recognition and lid dynamics.*

Conclusions:

• *Pilus-forming sortases possess distinct structural features compared to non-pilus sortases.*
• *A common mechanism for pilus biogenesis is suggested, involving specific sortase-pilin interactions.
• *The unique structures of these sortases present potential targets for novel, broad-spectrum antibacterial agents against *Streptococcus pneumoniae*.