ABSTRACT Streptococcus mutans is considered the major etiological agent in dental caries. S. mutans is also an important agent of infective endocarditis. The organism colonizes the oral cavity by forming diverse, multispecies biofilms on the tooth surface, known as dental plaque. To survive in the hostile environment of the oral cavity, S. mutans has developed a robust stress tolerance response that depends on regulated proteolysis of damaged or misfolded proteins. Regulated proteolysis in bacteria is primarily controlled by the Clp (caseinolytic protease)-family of proteins that are composed of a small cytoplasmic serine protease, called ClpP, and various ATPases. ClpP associates with a partner ATPase to form a functional complex that specifically targets proteins for degradation or translocation. While ClpP degrades the damaged/misfolded protein, it is the ATPase component that determines the substrate specificity. In S. mutans, the Clp-system, in addition to stress tolerance responses, is also required for biofilm formation, bacteriocin production, competence development, cell-cell communications, and bacteriophage resistance. The Clp-system is well studied in Escherichia coli in which it encodes two major ATPases: ClpA and ClpX. While streptococci, including S. mutans, do not encode ClpA, they do encode ClpX. Furthermore, two other ATPases, ClpC and ClpE, which are absent in E. coli, are uniquely present in streptococci and other Fermicutes. Very little is known about the substrate specificity or the molecular mechanisms of protein degradation in streptococci in general. The first major goal of this project is to understand at the molecular level how ClpX recognizes and degrades substrate proteins and regulates various virulence traits. The second major aim is to understand the role of ClpE in stress response and pathophysiology in S. mutans since our studies indicate that ClpE is an important ATPase. We anticipate that completion of this project will lead to the identification of novel regulatory pathways and new players for protein quality control in S. mutans. Moreover, knowledge acquired from this project can also be extended to other important pathogens such as S. pneumoniae and S. pyogenes, and may lead to the identification of a novel regulatory pathway.