Oligopeptidase B is a clan SC, family S9 serine peptidase found in gram positive bacteria, plants and try-panosomatids. Evidence suggests it is a virulence factor and thus therapeutic target in both Trypanosome cruzi and T. brucei, but little is known about its function in Leishmania. In this study L major OPB-deficient mutants (Delta opb) were created. These grew normally as promastigotes, had a small deficiency in their ability to undergo differentiation to metacyclic promastigotes, were significantly less able to infect and survive within macrophages in vitro, but were virulent to mice. These data suggest that L major OPB itself is not an important virulence factor, indicating functional differences between trypanosomes and Leishmania in their interaction with the mammalian host. The possibility that an OPB-like enzyme (designated OPB2) in L major might compensate for the loss of OPB in Delta opb was investigated via by mapping its sequence onto the 1.6 angstrom structure of L. major OPB. This suggested that the residues involved in the S1 and S2 subsites of OPB2 are identical to OPB and hence the substrate specificity would be similar. Consequently there may be redundancy between the two enzymes. (C) 2010 Elsevier B.V. All rights reserved.