Extracytoplasmic proteins of Mycobacterium tuberculosis - mature secreted proteins often start with aspartic acid and proline

A surrogate expression system, based on fusions to the phoA bacterial repor ter gene, was used to identify Mycobacterium tuberculosis genes that encode exported proteins and the promoter regions required for their expression i n the heterologous host Mycobacterium smegmatis. To assess these results in the context of the complete M, tuberculosis genome sequence, the correspon ding genes were identified and computational algorithms were employed to id entify signal peptide (SP), transmembrane domain and membrane lipoprotein a ttachment motifs. This information was used to predict the subset of M. tub erculosis genes that encode exported proteins. Of the 34 genes identified b y the phoA method, 22 were classified to encode potential soluble secreted proteins. Among these, 14 genes may encode novel secreted proteins. Six of the remaining 12 genes were predicted to encode membrane lipoproteins and a n additional six to encode integral membrane proteins, Published observatio ns of proteins proven to be secreted into M. tuberculosis culture filtrates were reviewed to further characterize the mycobacterial SP motif. It was c oncluded that mycobacterial SPs are comparable in size to Gram-positive SPs , but certain features are different. In particular, arginine was the predo minant N-terminally positively charged amino acid in contrast to lysine in the Cram-positives. The hydrophobic transmembrane segment of the SP was dom inated by alanine, in contrast to leucine, At the C-terminal end of the SPs , the (-3, -1) rule (AXA motif) holds, with alanine as the dominant amino a cid in both positions, being most dominant in the (-1) position. A high pro portion of mature sequences start with aspartic acid in the (+1) position a nd proline in the (+2) position - the DP motif. The authors propose that th e DP sequence serves as a sorting signal, following translocation and cleav age by signal peptidase I. Alternatively, the DP motif may be part of the r ecognition site for the signal peptidase.