Cutinases are extracellular
enzymes that are able to degrade
cutin, a
polyester protecting plant leaves and many kinds of
lipids. Although cutinases are mainly found in phytopathogenic fungi or bacteria, 7 genes related to the
cutinase family have been predicted in the genome of Mycobacterium tuberculosis. These genes may encode
proteins that are involved in the complex lipid metabolism of the bacterium. Here, we report on the biochemical characterization of two secreted
proteins of M.
tuberculosis, Rv1984c and Rv3452, belonging to the
cutinase family. Although their amino acid sequence shows 50% identity with that of the well-characterized
cutinase from Fusarium solani pisi, and a high level of homology has been found to exist between these two
enzymes, they show distinct substrate specificities. Rv1984c preferentially hydrolyzes medium-chain carboxylic
esters and
monoacylglycerols, whereas Rv3452 behaves like a
phospholipase A(2), and it is able to induce macrophage lysis. The
tetrahydrolipstatin inhibitor, a specific
lipase inhibitor, abolishes the activity of both
enzymes. Site-directed mutagenesis was performed to identify the catalytic triad of Rv1984c. Structural models for Rv1984c and Rv3452 were built, based on the crystal structure of F. solani
cutinase, with a view to investigating the contribution of specific residues to the substrate specificity. Our findings open new prospects for investigating the physiological roles of
cutinase-like
proteins in the lipid metabolism and virulence of M.
tuberculosis.