Erp, an extracellular protein family specific to mycobacteria

Erp ((e) under bar xported (r) under bar epeated (p) under bar rotein) was originally characterized as a virulence factor in Mycobacterium tuberculosi s and was thought to be present only in Mycobacterium leprae and members of the TB complex. Here it is shown that Erp is a ubiquitous extracellular pr otein found in all of the mycobacterial species tested. Erp proteins have a modular organization and contain three domains: a highly conserved amino-t erminal domain which includes a signal sequence, a central variable region containing repeats based on the motif PGLTS, and a conserved carboxy-termin al domain rich in proline and alanine. The number and fidelity of PGLTS rep eats of the central region differ considerably between mycobacterial specie s. This region is, however, identical in all of the clinical M. tuberculosi s strains tested. In addition, it is shown here that a Mycobacterium smegma tis erp::aph mutant displays altered colony morphology which is complemente d by all the Erp orthologues tested. The genome sequence flanking the erp g ene includes cell-wall-related ORFs and displays extensive conservation bet ween saprophytic and pathogenic mycobacteria.