Characterization of MdpS: An In-depth Analysis of a MUC5B-Degrading Protease from Streptococcus oralis

Oral biofilms, comprising hundreds of bacteria and other microorganisms on oral mucosal and dental surfaces, play a central role in oral health and disease dynamics. Streptococcus oralis, a key constituent of these biofilms, contribute significantly to their formation, serving as an early colonizer and microcolony scaffold. The interaction between S. oralis and the orally predominant mucin, MUC5B, is pivotal in biofilm development, yet the mechanism underlying MUC5B degradation remains poorly understood. This study introduces MdpS (Mucin Degrading Protease from Streptococcus oralis), a protease that extensively hydrolyses MUC5B and offers an insight into its sequence homology, physicochemical properties, and substrate- and amino acid specificity. MdpS exhibits high sequence conservation within the species and also explicitly among early biofilm colonizing streptococci. It is characterized as a calcium or magnesium dependent serine protease with strict physicochemical preferences, including narrow pH and temperature tolerance, and high sensitivity to increased sodium chloride and reducing agent concentrations. Furthermore, MdpS primarily hydrolyze proteins with O-glycans, but also show activity towards immunoglobulins IgA1/2 and IgM, suggesting potential immunomodulatory effects. Significantly, MdpS extensively degrades MUC5B in the N- and C-terminal domains, emphasizing its role in mucin degradation with implications in carbon and nitrogen sequestration for S. oralis with a potential function by cross-feeding the oral biofilm. Moreover, the enzyme displays amino acid preferences of serine, threonine or cysteine depending on substrate glycosylation. Understanding the interplay between S. oralis and MUC5B, facilitated by MdpS, has significant implications for the management of a healthy eubiotic oral microenvironment, offering potential targets for interventions aimed at modulating oral biofilm composition and succession. Additionally, the MdpS data challenges the presently acknowledged model of MUC5B degradation, because contrarily MdpS does not necessitate O-glycan removal prior to extensive peptide backbone hydrolysis. These findings emphasize the necessity for further research in this field.