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.

They help to maintain the highly hydrophobic cell wall of M.tuberculosis which is important for its survival in the host environment. Ag85A (as well as Ag85B and AG85C) contributes to this in two ways: Firstly, it catalyses the transfer of the long, hydrophobic mycolic acids to the cell wall arabinogalactan. This mycolyltransferase activity is essential for the biosynthesis of the very hydrophobic mycobacterial cell wall and for the survival of the mycobacteria.

alpha-mycolid acid

Secondly, antigen 85 proteins catalyse the synthesis 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.

In addition, the proteins of the antigen 85 complex are fibronectin 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 85A in grey and the residues that vary between antigen 85A, 85B and 85C in white. identical protein molecules ( ) are found in this antigen 85A model. 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. This indicates that they bind the same substrate and catalyze the same mycolyl transfer reaction. The peptide chain contains one .

The active site can be divided into two discrete sections, a carbohydrate binding and a fatty acid binding site. Helices form most of the hydrophobic contacts between the fatty acid part of the substrate and the protein. form the "floor" of the acyl-binding site. Residues that form the are completely conserved in the M. tuberculosis antigen 85 proteins.

The sequence that was originally identified as a fibronectin binding protein. The site spans residues 58-68. An alternative hypothesis proposes that a large conserved (approx. a third of the total surface area) is responsible for the interaction between antigen 85 proteins and human fibronectin.

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.

spin / | | 3D model needs Jmol

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 It is also called extracellular α-antigen. The amino acid composition below shows relatively high glycine and tryptophane levels.

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. Other pathogens like Streptococcus pyogenes and Neisseria gonorrhoeae use this strategy to ensure cellular attachment and promote the initiation of infection. 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 in the amino acid sequence alignement. 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	fbpA blastp	binding
fbpA	Rv3804c	antigen 85A	mpt44	338	e-159	fibronectin binding protein A
fbpB	Rv1886c	antigen 85B		325	e-143	fibronectin binding protein B
fbpC, fbpC2	Rv0129c	antigen 85C'	mpt45	340	e-143	fibronectin binding protein C'
fbpD, fbpC1	Rv3803c	antigen 85C	mpt51	299	e-107	-

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 . One of the immediate neighbouring genes is fbpC1, a member of the antigen 85 complex family that has no mycolyl transferase activity.

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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.

Mycobacterium tuberculosis antigen 85A and 85C structures confirm binding orientation and conserved substrate specificity. Ronning DR, Vissa V, Besra GS, Belisle JT, Sacchettini JC. J Biol Chem. 279:36771-36777 (2004).

The M. tuberculosis antigen 85 complex and mycolyltransferase activity. Kremer L, Maughan WN, Wilson RA, Dover LG, Besra GS. Lett Appl Microbiol. 34:233-237 (2002).

Mycobacterial lipids: a historical perspective. Asselineau J, Laneelle G. Front Biosci. 1998 Oct 1;3:e164-74. (Web-Site)

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