Sortase-Catalyzed Assembly of Distinct Heteromeric Fimbriae in Actinomyces naeslundii

Department of Molecular, Microbial, and Structural Biology, University of Connecticut Health Center, 263 Farmington Ave., Farmington, CT 06030, USA and Oral Infection and Immunity Branch, National Institute of Dental and Craniofacial Research, National Institutes of Health, 30 Convent Drive, Bethesda, MD 20892

^* To whom correspondence should be addressed. Email: ton-that{at}uchc.edu.

	Abstract

Two types of adhesive fimbriae are expressed by Actinomyces; however, the architecture and the mechanism of assembly of these structures remain poorly understood. We have now characterized two fimbrial gene clusters present in the genome of Actinomyces naeslundii strain MG-1. By immuno-electron microscopy and biochemical analysis, we show that the fimQ-fimP-srtC1-fimR gene cluster encodes a fimbrial structure (designated type 1) that contains a major subunit FimP forming the shaft and a minor subunit FimQ located primarily at the tip. Similarly, the fimB-fimA-srtC2 gene cluster encodes a distinct fimbrial structure (designated type 2) made of a shaft protein FimA and a tip protein FimB. By allelic exchange, we constructed an in frame deletion mutant that is devoid of the SrtC2 sortase. This mutant produces the type 1 fimbriae abundantly and expresses the monomeric FimA and FimB proteins, but it fails to assemble the type 2 fimbriae. Thus, SrtC2 is a fimbria-specific sortase that is essential for assembly of the type 2 fimbriae. Together, these experiments pave the way for several lines of molecular investigation that will be necessary to elucidate the fimbrial assembly pathways in Actinomyces and their function in the pathogenesis of different biofilm-related oral diseases.