The N-terminal ELR+ motif of the neutrophil attractant CXCL8 confers susceptibility to degradation by the Group A Streptococcal protease, SpyCEP.

Abstract

Streptococcus pyogenes (Group A Streptococcus or GAS) is a major human pathogen, for which an effective vaccine is highly desirable. Invasive S.pyogenes strains evade the host immune response in part by producing a cell envelope protease, SpyCEP. This neutralises chemokines containing an N-terminal Glu-Leu-Arg motif (ELR⁺ chemokines) by cleavage at a distal C-terminal site within the chemokine. SpyCEP is a component of several S. pyogenes vaccines, yet the molecular determinants underlying substrate selectivity are poorly understood. We hypothesised that chemokine recognition and cleavage is a multi-step process, involving distinct domains of both substrate and enzyme. We generated a panel of recombinant CXCL8 variants where domains of the chemokine were exchanged or mutated. Chemokine degradation by SpyCEP was assessed by SDS-PAGE, Western blot and ELISA. Extension of the CXCL8 N-terminus was found to inhibit chemokine cleavage. Reciprocal exchanges of the N-termini of CXCL8 with that of the ELR^- chemokine CXCL4 resulted in the generation of loss of function and gain of function substrates. This suggested a key role for the ELR motif in substrate recognition, which was supported directly by alanine substitution of the ELR motif of CXCL8, impairing the parameters, K_M, V_max and Kcat in kinetic assays with SpyCEP. Collectively, our findings identify the N-terminal ELR motif as a major determinant for recognition by SpyCEP and expose a vulnerability in the mechanism by which the protease recognises its substrates. This likely presents potential avenues for therapeutic intervention via targeted vaccine design and small molecule inhibition.