Abstract:
Streptococcus equi (S. equi) is the causative agent of the highly contagious disease ‘strangles’ in equines and zoonotic meningitis in human. Spreading of infection in host tissues is thought to be facilitated by the bacterial gene encoded extracellular hyaluronate lyase (HL) which degrades hyaluronan (HA), chondroitin-6-sulfate (CS-C) and dermatan sulfate (DS) of the extracellular matrix (ECM). The clinical strain S. equi 4047 however, lacks a functional extracellular HL. The prophages of S. equi and other streptococci encode intracellular HLs which are reported to partially degrade HA and do not cleave any other glycosaminoglycans (GAGs). The phage HLs are thus thought to play a role limited to the penetration of streptococcal HA capsules, facilitating bacterial lysogenization and not in the bacterial pathogenesis. Here we systematically looked into the structure-function relationship of S. equi 4047 phage HL. While HA is the preferred substrate, this HL has weak activity towards CS-C and DS and can completely degrade all of them. Even though the catalytic triple-stranded β-helix (TSβH) domain of phage HL is functionally independent, its catalytic efficiency and specificity is influenced by the N-terminal domain. The phage HL also interacts with human transmembrane glycoprotein CD44. The above results suggest that the streptococci can use phage HLs to degrade GAGs of ECM for spreading virulence factors and toxins, while utilizing the disaccharides as a nutrient source for proliferation at the site of infection.
