Streptococcus Pyogenes Bacterium Human Pathogen Biology Essay

Streptococcus pyogenes bacterium a worldwide known human pathogen is besides known as the Group A Streptococcus, GAS. It is Gram-positive, non-motile, and spherical in form which grows in long ironss or braces ( Figure 1A ) . S.pyogenes is distinguished from the other streptococcus by the presence of the Lance field Group A saccharide found on its cell wall and it produces big zone of beta-hemolysis ( hemoglobin released when the red blood cells are wholly disrupted ) when grown on a home base enriched in blood agar ( Figure 1B ) . Therefore it is known as the Group A ( beta-haemolytic ) Streptococcus ( GAS ) .

Gram-positive bacteriums lack a recollective outer membrane and therefore, they have evolved several mechanisms for grounding proteins within their membrane. Of the several mechanisms that Gram-positive bacteriums have evolved, N-terminal lipidation, a major mechanism which allows proteins to be anchored in the bacteriums membrane and these proteins are known as bacterial lipoproteins ( Lpps ) . These bacterial lipoproteins have their N-terminal modified with N-Acyl Diacyl Glyceryl group ( Sutcliffe and Harrington 2002 ) .

Lipid alteration of bacterial proteins enables them to expeditiously transport out their of import maps between the cell wall and the environment ( Braun et al, 1993 ) . Lpps performs broad scope of critical maps such as substrate binding proteins ( SBPs ) in ABC transporter system ; in antibiotic opposition ; in cell signalling ; in protein export and folding ; in monogenesis and sprouting ; in junction and assorted other maps ( Sutcliffe and Russell 1995 ) .

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Therefore, the maps of lipoproteins of Gram-positive bacteriums are comparable to the surface proteins of Gram- negative bacteriums. For illustration, the substrate binding proteins of the ABC transporter system are typically Lpp in Gram-positive bacterium every bit good as in Gram-negative bacteriums ( Sutcliffe and Russell 1995 ) .

Structure of signal peptides from bacterial lipoproteins

Proteins to be exported across the cell membrane via the two distinguishable export tracts require proteins with N-terminal signal peptides for acknowledgment. There are two types of signal peptides viz. , Type I and Type II signal peptides.

Figure 2: Type I and Type II signal peptides via Sec and Tat Dependent Transport ( a ) General Secretory Pathway, ( B ) Cheapness System

Both Type I and Type II signal peptides is composed of three distinguishable sections: a positively charged amino-terminal N, a cardinal, H- ( hydrophobic ) part and a more polar carboxy terminus C ( cleavage part ) ( von Heijne 1989 ) .

Within type I signal peptide, it sustains a acknowledgment motive ( A-X-A, where Ten can be any aminic acid ) for type I signal protease cleavage activity. In the instance of Type II signal peptide, it contains the acknowledgment motif sequence of ( L-3- [ A/S/T ] -2- [ G/A ] — 1-C+1 ) for type II signal protease and its cleavage site frequently referred as lipoprotein aa‚¬E?lipoboxaa‚¬a„? .

In comparing between the signal peptides recognised by the two export tracts, the Tat system signal peptides has a conserved SRRXFLK sequence between N-region and H-region, within which the twin arginine ( RR ) motive is about perfectly conserved compared to that of Sec pathway signal peptide ( Figure 2 ) .

3. Translocation across cellular membrane

In order to make their site of map, a important proportion of freshly synthesised proteins need to be translocated to outside of cell. This is done chiefly via the general secretory ( Sec ) tract and Tat ( duplicate arginine protein conveyance ) system. The difference between the two systems lies in whether the conformation of the translocated protein is folded or unfolded.

The Sec tract is the prevailing path of transit of proteins across the cytoplasmatic membrane among the two distinguishable tracts ( Driessen and Nouwen 2008 ) . In the Sec tract, proteins with an unfolded conformation have to synthesised with N-terminal signal peptides which will be excised at a ulterior phase during the exportation, via a signal protease situated on the cytoplasmatic membrane. There have been findings in which some of the putative lipoproteins are exported via SecA2 dependent accoutrement tract across the cytoplasmatic membrane in unfolded province found in some of the Gram-positive bacteriums but non all ( Lenz, Mohammadi et Al. 2003 ; Gibbons, Wolschendorf et Al. 2007 ) .

In Tat system, proteins with folded conformation, even oligomeric proteins are transported to cytoplasmic membrane ( Sargent, Berks et Al. 2006 ) . Proteins bearing N aa‚¬ ” terminal signal peptide incorporating invariant and indispensable twin arginine motive, are targeted to Tat system for exportation. Lipoproteins precursors exported through Tat system are in a to the full folded conformation were confirmed during an analysis of dimethyl sulphoxide ( Dms ) reductase in Gram-negative bacteriums ( Gralnick, Vali et Al. 2006 ) .

This indicates that Tat system is a mechanism which is basically different from the Sec tract and requires the proteins to be folded before they cross the cellular membrane.

Both translocation pathways require proteins bearing N-terminal signal peptides which are required for the acknowledgment to be exported across the membrane. Proteins destined for lipidation contains a motive in their signal peptides besides known as the lipobox which forwards them to lipoprotein biosynthesis machinery after exportation via either Sec tract or Tat system.

Lipoprotein Biogenesis

To achieve their full fledged map, the freshly synthesised pro-lipoproteins exported across the cytoplasmatic membrane via the Sec tract or Tat system will be channelled to the lipoprotein biosynthesis machinery. This imparting procedure will be guided by the conserved motive ( lipobox ) in the protein signal peptide.

Figure 3: Biosynthesis of Lipoproteins

The lipoprotein biosynthesis tract in Gram-negative bacteriums occurs in three measure pathway in a rigorous order. Once the protein is exported across the cellular membrane with the counsel of the signal peptide II via Sec or Tat dependent system, the conserved cysteine residue within the lipobox of the signal peptide is modified with a diacyglycerol group attached through a thioether linkage. The above whole reaction is catalyzed by the enzyme prolipoprotein diacylglycerol transferase ( Lgt ) , utilizing phospholipid substrates ( Qi, Sankaran et Al. 1995 ; Sankaran, Gupta et Al. 1995 ) .

Once after the conserved residue gets lipdated, the signal peptide is cleaved within the lipobox by a specific lipoprotein signal protease II ( Lsp ) , enzyme to let go of the lipidated cysteine as the N-terminal for the mature bacterial lipoprotein ( Sankaran and Wu 1995 ) . Above mentioned stairss for lipidation of Gram-positive bacteriums, it was confirmed to be critical and sufficient for Gram-positive protein lipidation.

In a 3rd measure alteration of the lipoprotein in the biosynthesis tract would be the lipid alteration of the lipoprotein by a fatty acylation of the amino group of the N-terminal diacylglycerol modified Cys to organize N-acyl diacylglycerol Cysteine by the enzyme, lipoprotein amino acyl transferase. This enzyme is non conserved as its homologues are non available in the genomes of low G-C Gram positive bacteriums ( Tjalsma, Kontinen et Al. 1999 ) . In Gram negative, it is an indispensable measure which frees the lipoproteins from the plasma membrane and transported to the outer membrane via the Lol ( lipoprotein localization of function ) tract.

In Gram-positive bacteriums lipoprotein biogenesis tract, the initial the two phases are similar to that of Gram-negative bacteriums. Intriguingly, in recent surveies, it has been noted the enzyme Lsp has non-specific cleavage activity towards non-lipidated substrates in Listeria monocytogenes every bit good as in Streptococcus agalactiae ( Baumgartner, Karst et Al. 2007 ; Henneke, Dramsi et Al. 2008 ; Bray, Sutcliffe et Al. 2009 ) .

In Listeria monocytogenes, Gram-positive bacteriums, the lgt cistron was deliberately deleted and the non-lipidated lipoproteins were still cleaved by the Lsp enzyme. This proves that lipoprotein lipidation is non indispensable for the Lsp enzyme cleavage activity and Lsp could hold cleavage activity towards non-lipidated substrates in some Gram-positive bacteriums ( Baumgartner, Karst et Al. 2007 ) .

In Streptococcus agalactiae ( GBS ) , both lgt and lsp cistrons were inactivated ; the non-lipidated lipoproteins were still cleaved by the type I signal protease. This indicates that there are alternate proteases to treat the lipoprotein when the biogenesis tract enzymes are inactivated such as type 1 signal protease and Lsp when lgt enzymes inactivated ( Henneke, Dramsi et Al. 2008 ) .

Deriving penetrations into these recent surveies, it can be concluded that the lipoprotein biosynthesis tract in Gram-positive bacterium does non happen in a rigorous mode and there are alternate processing methods when the biosynthetic tract enzymes are inactivated.

Bioinformatic Prediction of Lipoprotein in Gram-positive Bacteria

With the well established characteristics of signal peptide and through the designation of lipobox incorporating invariant cysteine, bioinformatic analysis of these signal peptides is able to place possible lipoproteins in Gram-positive bacteriums.

Based on the sequence analysis of signal peptides of Gram positive bacteriums and Gram-negative bacteriums, it was noted that lipoprotein signal peptides tend to be shorter that secretory signal peptides which indicate that the c-region is shorter and contains apolar amino acids. It implies that it is a continuance of the hydrophobic sphere which is chiefly based on the sequence preservation predating the invariant lipid-modified cysteine ( von Heijne 1989 ) .

Using the signal peptide sequences incorporating the lipobox, the Prosite consensus form sentence structure depicting the sequence motive finding lipidation was constructed as { DERK } ( 6 ) – [ LIVMFWSTAG ] ( 2 ) – [ LIVMFYSTAGCQ ] – [ AGS ] -C to recognize bacterial lipoprotein sequences. In this form look, the allowed amino acids predating the cysteine are at place -1 to -4 and the losing charged residues in h part are indicated as ( D, E, R or K ) . This form look has certain set of regulations to be adhered that the cysteine must be between places 15 to 35 and at that place has to be a arginine or lysine in the first seven places of the sequence in order to put the form in sync with the N-terminal sequence with n part feature of signal peptide ( Sutcliffe and Harrington 2002 ) .

Large figure of putative Lpps were identified through molecular familial surveies and quite figure of these identified Lpps could be false-positive as they seem to incorporate a cysteine within the signal of exported proteins or proteins targeted for interpolation into the plasma membrane. It was besides noted that there were differences in the stretch of aminic acids predating the invariant cysteine in the signal peptides characteristics of different bacterial taxa.

To deduce to the anticipation of lipoproteins utilizing bioinformatic analysis, Sutcliffe and Harrington ( 2002 ) created a dataset of by experimentation verified Gram-positive lipoproteins. These lipoproteins were identified based on several attacks: ( 1 ) metabolic labelling with radiolabelled fatty acid ( palmitate ) ; ( 2 ) Inhibition of Lsp ( bacterial signal peptide ) utilizing the antibiotic globomycin ; ( 3 ) Biochemical word picture of the purified protein and ( 4 ) Evidence that protein processing is disrupted by mutant in either Lgt or Lsp, or following site directed mutagenesis to replace the lipobox cysteine. Within this set of standards and along with extended reappraisal of scientific diaries, 33 proteins were identified as proved bacterial lipoproteins

To further formalize the above 33 lipoproteins indentified, several other bioinformatic sequence analysis were performed. Bacterial Lpps sequence were obtained from Prosite web site and curtailing the hunts to Bacillus subtilis or S. pyogenes. Using the TMpred plan, membrane spanning spheres ( MSD ) in the above sequences were predicted, with a minimal length of the hydrophobic sphere set 14aa and the signal peptides sequences were analysed utilizing the Signal 2.0 ( refined hidden Markov theoretical account version 2.0 ) . For farther elucidation of the Lpp sequences, TopPred2 ( transmembrane forecaster ) and DAS plans were used.

In the exclusion of the bacterial Lpps that are false positive, Bacterial Lpps N-terminal sequences were analysed separately utilizing TMpred and SignalP. Lpps sequences which clearly denotes the absence of MSD and the extension of the most N-terminal beyond the invariant cysteine were known to be possible false-positives. TMpred was non justifiable as the CatC and the QoxA proven Lpps contained two extra MSD beyond their N-terminal lipid ground tackles. To meet this, SignalP was used to analyze the sequences, bacterial Lpps where signal peptides characteristics were absent and or the lipobox sequence which is internal to an h-region /MSD were confirmed to be false positive. Further elucidation of these sequences were analysed utilizing the TopPred2 and DAS and a general sequence was taken to place of invariant cysteine from the fist predicted MSD.

From the analysis of the signal peptide lipobox characteristics from the above bioinformatic plans, it justified old surveies consequences in which there were high frequence of leucine in -3 place and alanine or serine at -2 place of the lipobox. In comparing with PS0013 form, there were obvious divergences and limitations: alanine and glycine are the lone amino acids indicated at -1 place and two proved Lpps had no arginine or lysine in the first amino acids which is contradictory to the PS0013 form.

Analysis of the lipobox sequences from the 33 by experimentation verified lipoproteins, it was noted that n- parts had average length of 6.7 +- 3.5 within the length of 3-15aa, the h-region length was 12.1+_ 2.3 aa within the length of 6-20aa. These inside informations are in understanding with the findings of h-features indicated for the putative Lpp of B.subtilis. The average invariant cysteine place was 24.0+_3.6 with the scope of 17-33 aa length which proves the bacterial signal peptides are typically shorter compared to the signal peptides involved in directing protein export in Gram-positive. The average length of the combined h-and c-regions to be 17.1 aa was noted as it is sufficient to cross a typical bilayer membrane. From these informations, it is noted the conserved residues are positioned at the outer face of cytoplasmatic membrane in where the Lgt enzyme interacts with the invariant cysteine in the lipobox.

Since the PS0013 form is contradictory to certain proved Lpps in Gram-Positive bacterium every bit good as extra favoritisms is likely to ensue due to the differences in signal peptide characteristics of different bacterial taxa, a modified form, G+LPP was constructed for placing the 33 proved bacterial Lpps. G+LPP form, is described as & lt ; [ MV ] -X ( 0,13 ) – [ RK ] – { DERKQ } ( 6,20 ) – [ LIVMFESTAG ] – [ LVIAM ] – [ IVMSTAFG ] – [ AG ] -C ( utilizing Prosite sentence structure ) .

In comparing of the G+LPP form tightness to that of PS0013 form in placing putative bacterial Lpps, it provided a greater favoritism against the false-positive bacterial Lpps sequences when tested in B.subtilis genome. PS0013 pattern hunt identified 103 putative Lpps while G+LPP pattern identified 61 likely Lpps together with 6 proved Lpps in the above mentioned being. Therefore, the use of G+LPP form to foretell bacterial Lpps with a great assurance.

Both the Prosite form every bit good the G+ LPP form were applied to the S.pyogenes genome, retrieved from SWISS-PROT/TrEMBL database. The Prosite form hunt identified 36 sequences, out of which 9 were excluded as improbable Lpps while the G+LPP form hunt identified 26 Lpps, out of which merely one was known to be improbable Lpps. Therefore with these informations, 8 out of 9 Lpps identified by PS00013 were excluded.

Both the hunt patterns identified antecedently identified and proven LppC Lpp every bit good several other Lpps that were identified and proven. A sum of 24 Lpps identified in the S.pyogenes genome utilizing the form hunt represents 1.5 % of the S.pyogenes proteome which is comparable to the 36 Lpps identified in the S. pneumoniae genome.

Apart from placing common antecedently identified Lpps by the both forms, there were sequences which were picked up as possible putative Lpps particular to each form but non to both. In PS00013 form hunt, three putative Lpps sequences viz. , Spy1972, Spy1361, Spy2066 were identified but non with the G+LPP form. Spy1972 n-region signal sequence is unusual in length and contains a LPXTG motive in the C-terminal. Spy1361 contains glutamine residue within the h-region and Spy 2006 signal sequence are non clear. Due to above differences in signal sequences, they were non picked by the G+LPP form which warrants grounds to turn out that they are so putative bacterial Lpps. Likewise, G+LPP pattern hunt identified a signal sequence, Spy0903 but non with PS0013 form due to its drawn-out signal characteristics in the n-region.

Bacterial Lpps signal sequences that were missed by the both pattern hunts were farther analysed by utilizing a combination of schemes viz. , analysis of the S.pyogenes genome note, homologues hunts of pneumococcal Lpps, PEDANT hunt and blast hunts with low tightness. With the above hunts, six possible false-positive bacterial Lpps were identified ; Spy0163, Spy1592, Spy0778, Spy1306, Spy0457 and Spy2033.

Among these possible bacterial Lpps, four of them are substrate binding proteins ( SBP ) . Spy0163 is a paralogue of Spy1228 but after polishing the signal sequence of n part with alternate start methionine and lysine of this Lpp, it was accepted by G+LPP form and their motives were proven by Rosati et Al. Spy1592 was excluded by the form hunt as it contains asparagine at -4 place in the signal sequence. However in the ORF of the S.pyogenes genome contains serine at this place which indicates that it is so a Lpp in some strains. Likewise the Spy0457 Lpp, peptidyl-prolyl isomerase of the cyclophilin household, contains an asparagine in the -4 place but it is extremely homologous to the pneumococcal Sp0771 which indicates it may help in the folding of exported proteins.

Both Spy0778 and Spy1306 were excluded in the form hunt as they contain proline in the -4 place which warrants further grounds. In the instance of Spy2033, it has unnatural signal sequence of 64aa but intriguingly, its h-region terminals within the invariant lipobox cysteine which tallies with the general consensus of a typical Lpp signal peptide. Its alternate start at M41 is consistent with sequence alliance every bit good as its homologue, the Streptococcus cristatus putative Lpp TptA which warrant farther confirmation of this sequence.