The family-3 glycoside hydrolases: from housekeeping functions to host-microbe interactions.

Glucose, xylose, and N-acetylglucosamine are elementary building blocks for disaccharides, oligomers, and polymers, as well as key molecules allowing fine modifications of many kinds of biological compounds. The highly diverse molecules containing these three sugars are ubiquitous components of the architecture of eukaryotic and prokaryotic cells and may also be involved in signaling between cells or organisms (all of these roles are illustrated in this minireview). Any enzymes modifying the structure of these molecules, such as glycosylhydrolases (GHs) or glycosyltransferases, are essential to modulate the biological properties of these glycosides and therefore are likely to modify their functions in vivo. Based on the comparison of their amino acid sequences, GHs are presently classified in more than eighty families (20). Among them, family-3 encompasses more than one hundred enzymes, including eubacterial and eukaryotic -glucosidases, -xylosidases, and -N-acetylhexosaminidases. Most of the cloned and sequenced genes encoding family-3 GHs, as well as the purified family-3 GHs, were investigated as encoding -glucosidases and therefore characterized as such in cellulolytic and xylanolytic microorganisms. This bias led scientists to think, originally, that these enzymes were involved mainly in the degradation of macromolecules by microbes. Recent data on their functions in vivo (Table 1) and their distribution among sequenced genomes (Fig. 1) suggest that family-3 GHs play roles in addition to the assimilation of plant polymer products. This prompted me to select several wellstudied family-3 GHs to investigate their functions in vivo. These functions range from the assimilation of exogenous saccharides by bacteria and fungi to the turnover of cell architecture components such as cell wall polymers and from the modification of biologically active molecules such as antibiotics and antifungal compounds to interactions between pathogens and the immune systems of their hosts.