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8 Enigmatic Proteins from the Surface: the Erp, PE, and PPE Protein Families
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Erp protein has been detected in culture supernatants or cell wall preparations but not in cell extracts. Erp has been detected in protein samples of several mycobacterial species by Western blot. The Erp family is an expanding group of proteins originally identified and characterized from mycobacteria causing tuberculosis (TB) and leprosy. The availability of Mycobacterium tuberculosis genome sequence and its comparison with other mycobacterial genomes has accelerated the study of pathogenesis as never before and has raised a number of questions concerning the roles and functions of a large group of putative unknown proteins, in which Erp is included. Analysis of the Cterminal variable regions of the PE proteins identified the presence of 41 to 43 amino acid-long tandem repeats in two PE proteins, namely Rv0978 and Rv0980. The PE and PPE family of proteins have been shown to be differentially expressed in M. tuberculosis during in vitro growth and, at the same time, exhibit gene expression diversity in different clinical isolates. The possible function of the PE/PPE protein family also became apparent. It was hypothesized that these antigens play a role in evading the host immune response, thereby preventing the establishment of an effective cellular response that is required to contain the disease. Erp, PE, PPE, and PE_PGRS proteins have a highly conserved N-terminal domain that, though different for Erp, PE, and PPE, appears to serve the purpose of exporting these proteins to the surface.
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Schematic representation of the Erp orthologues identified in all mycobacterial genomes in the public domain by January, 2007. The conserved amino- and carboxyterminal domains are shown as gray bars. This figure is adapted from de Mendonça-Lima et al. ( 2001 ).
Photomicrographs of colonies of M. smegmatis. (A) M. smegmatis wild-type; (B) M. smegmatis erp::aph; (C) M. smegmatis erp::aph complemented with a wild-type gene of M. tuberculosis.
Schematic showing the general structure of the PE, PE_PGRS, and PPE multigene families.
Number of genes belonging to the PE and PPE multigene families found in different mycobacterial genomes.
Crystal structure of the M.tb. PE/PPE protein complex. (A) Surface representation of the PE/PPE protein complex. (B) The PE/PPE protein complex viewed down its longitudinal axis. (C) Ribbon diagram of the PE/PPE protein complex. The complex is composed of seven helices. Two helices of the PE protein interact with two helices of the PPE protein to form a four-helix bundle. Regions of high sequence conservation are indicated by arrows and discussed in the text. (D) Interface hydrophobicity of the PPE and PE proteins. Notice the extensive apolar regions that are shielded from solvent as the complex forms. Adapted from Strong et al. ( 2006 ). (See the color insert for the color version of this figure.)
Schematic showing the molecular organization of the different PE_PGRS subgroups.
Major features of the PE_PGRS proteins of M. tuberculosis H37Rv.