Mycobacterium tuberculosis molecules by Dr. PI

The antigen 85 complex is composed of three proteins Ag85A, Ag85B and Ag85C, which all belong to the antigen 85 . This complex, also named α-antigen, is a major protein component of the Mycobacterium tuberculosis cell wall. These three abundantly secreted proteins play a key role in the pathogenesis of tuberculosis.

Each protein possesses a mycolyltransferase acitvity required for the biogenesis of trehalose dimycolate in the . This dominant component of cell walls is also known as cord factor and is necessary for maintaining cell wall integrity. The cell wall-linked mycolic acids form the primary barrier for the difffusion of small molecules into the cytoplasma of the bacterial cell.

mycolic acid structure

Two antituberculous drugs target enzymes, involved in cell wall biosynthesis: Isoniazid kills mycobacteria by inhibiting the biosynthesis of mycolic acids by inhA whereas ethambutol inhibits the inhibits the formation of the arabinan that are both major compontents of the mycobacterial cell wall.

In addition to their mycolyltransferase activity, the proteins of the antigen 85 complex are fibronecting binding proteins (FbpA, FbpB, FbpC) that are responsible for the hign affinity of mycobacteria to fibronectin, a eucaryotic cell wall protein. This allows for rapid invasion of alveolar macrophages via direct interactions between the host immune system and the invading bacillus. The active sites of the three antigen 85 proteins are virtually identical, indicating that they share the same substrate. However surface residues disparate from the active site are quite variable. Differential expression of the antigen 85 proteins may be a very important mechanism utilized by M. tuberculosis to evade the human immune system or to persist in a chronic infection.

The structural model displays the conserved amino acid residues of antigen 85C in grey and the residues that vary between antigen 85A, 85B and 85C in white. A single subunit of contains and a made of 8 stretches.Structural analysis placed the antigen 85 proteins in the group of α/β-hydrolases. The amino acid residues required for enzymatic activity form the namely Ser124, Glu228 and His 260. These residues superimpose well with the catalytic triad of 85B and 85C. In contrast to AG85A and 85B the peptide chain of Ag85C does not contain a

The active site can be divided into two discrete sections, a carbohydrate binding and a fatty acid binding site. Helices form the walls of a large hydrophobic pocket lined by additional . This pocket leads to the opening of a long extending through the core of the protein which is well suited to accomadate the shorter branch of a mycolic acid. The longer branch would fit in a long trough on the surface of the protein.

Residues that form the are completely conserved in the M. tuberculosis antigen 85 proteins (see also in Ag85B). Additional form a pocket with a highly negative electrostatic potential. An enzyme inhibitor lide a known inhibitor of α/β-hydrolases covalently ataches to the active site serine.

The site spans residues 56-66. An alternative hypothesis proposes that a large conserved is responsible for the interaction between antigen 85 proteins and human fibronectin that stimulates the uptake of tuberculous bacilli by human macrophages.

spin / | | 3D model needs Jmol

Residues that are not involved in catalysis or fibronectin binding are not conserved between the three antigen 85 proteins and form a highly By differential expression of the three antigen 85 proteins under different environmental conditions M. tubuerculosis can exert its mycolyl-transferase activity in variable disguises and thus evade the hosts immune response.

The structural model was obtained from PDB entry , and the description from . Click here to search for at PDB.

The antigen 85 complex is a multigene family encoding several closely related major secreted proteins. The signal peptide is cleaved in mature Ag85 proteins. is a peptide of with a and a The amino acid composition below shows relatively high glycine and tryptophane levels. The sequence contains additional putative .

The protein contains a fibronectin binding sequence, that is thought to be responsible for the high affinity of mycobacteria to the cell adhesion molecule fibronectin. The central serine residue (number 124 in the mature peptide) together with Glu228 and His260 forms the catalytic triade.

Ag85A and the other members of the antigen 85 complex belong to a of seemingly unrelated proteins, including human esterase D. Intragenome comparison of fbpA identifies 3 genes of high homology in as shown in the table and the aminoacid sequence alignment. The search for homologues in reveals homologues in other pathogenic mycobacteria. More functional links are available from the .

The antigen 85 complex family genes in M. tuberculosis
gene	sanger-orf	antigen		aa	fbpC blastp	Mr	pI	map
fbpA	Rv3804c	antigen 85A	mpt44	338	e-149	35.7 kD	6.5	[11:36]
fbpB	Rv1886c	antigen 85B		325	e-143	34.6 kD	5.6	[05:48]
fbpC, fbpC2	Rv0129c	antigen 85C	mpt45	340	e-157	36.8 kD	5.9	[00:25]
fbpD, fbpC1	Rv3803c	antigen 85C	mpt51	299	e-111	31.1 kD	6.1	[11:36]

amino acid sequence alignment of the antigen 85 complex family proteins

The gene is encoded in a In the M. tuberculosis laboratory strain H37Rv it is a known as which corresponds to gene in the clinical isolate CDC 1551. It can be found on .

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On the circular M. tuberculosis chromosome lies at far away from the related genes fpbA [11:36] and fbpB [05:48].

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blueTB molecules are published by Paul Imboden, Dr. PI Bioconsulting. Authorization to photocopy or reproduce this entry for personal use is granted. Copyright @ 2005 Paul Imboden, Dr. PI Bioconsulting. Last modified May 17, 2008 . Disclaimer: blueTB and the author reserves the right to modify and cancel any statement in these documents and regrets, that he cannot accept any responsibility for the consequences of any such changes. to the best of my knowledge all information is correct, but I cannot accept liability for any errors. References for this blueTB entry are:

Deciphering the biology of Mycobacterium tuberculosis from the complete genome sequence. Cole ST, Brosch R, Parkhill J, Garnier T, Churcher C, Harris D, Gordon SV, Eiglmeier K, Gas S, Barry CE 3rd, Tekaia F, Badcock K, Basham D, Brown D, Chillingworth T, Connor R, Davies R, Devlin K, Feltwell T, Gentles S, Hamlin N, Holroyd S, Hornsby T, Jagels K, Barrell BG, et al. Nature, 393:537-44 (1998) and the Mycobacterium tuberculosis sequencing project from the Sanger Centre.

Whole-genome comparison of Mycobacterium tuberculosis clinical and laboratory strains. Fleischmann RD, Alland D, Eisen JA, Carpenter L, White O, Peterson J, DeBoy R, Dodson R, Gwinn M, Haft D, Hickey E, Kolonay JF, Nelson WC, Umayam LA, Ermolaeva M, Salzberg SL, Delcher A, Utterback T, Weidman J, Khouri H, Gill J, Mikula A, Bishai W, Jacobs Jr WR Jr, Venter JC, Fraser CM. J Bacteriol. 184:5479-90 (2002) and the Mycobacterium tuberculosis sequencing project from TIGR .

3. Dr. PI's Mtbook, The Mycobacterium tuberculosis genome in a book. Paul Imboden, Dr.PI Bioconsulting. mtbook.drpi.ch/ Release 1.8.0 Jan 2004, which itself is based mainly on reference 1.

Crystal structure of the secreted form of antigen 85C reveals potential targets for mycobacterial drugs and vaccines. Ronning DR, Klabunde T, Besra GS, Vissa VD, Belisle JT, Sacchettini JC. Nat Struct Biol. 7:141-146 (2000).

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