[[User: Julie Langlois/PsaA]]

NCBI Accession: P42363.1

Uniprot Accesion: POA4G2

PDB ID: 3ZK7

== Overview ==
PsaA (Pneumococcal surface antigen A) is a multi-functional lipoprotein detected on all known serotypes of Streptococcus pneumoniae. This lipoprotein is part of the ABC-type (ATP binding complex) transport protein complex that transports Mn2+ and plays a major role in pneumococcal attachment to the host cell and virulence.

=== PsaA protein ===
PsaA (Pneumococcal surface antigen A) is a multi-functional lipoprotein detected on all known serotypes of ''Streptococcus pneumoniae''. This lipoprotein is part of the ABC-type (ATP binding complex) transport protein complex that transports Mn2+. PsaA is also an adhesion factor that plays a major role in pneumococcal attachment to the host cell and virulence. PsaA is hidden beneath the cell wall. PsaA protein is involved in in colonization of the nasopharyngeal mucosal.
This transporter is composed of the products of three genes, psaB (ATP-binding protein), psaC (integral membrane protein), and psaA (solute-binding lipoprotein), which are organized in an operon with a gene encoding PsaD, a thiol peroxidase <ref>PMID:517531</ref>.

[[Image:operon psa.jpg |500px| thumb| Psa operon <ref>PMID:15255900</ref>]]

[[Image:Linear structure PsaA.jpg |500px| thumb|Linear structure of PsaA]]

Between the first to the 24th amino acid namely the red motif there is the signal peptide and the second one is the Pfam motif<ref>http://string-db.org/cgi/network.pl?taskId=EA2jsQm5Sc5A</ref>.

Zinc in excess has significant toxicity to bacteria because it is an important innate defence mechanism. There are many Zinc in human body. Manganese is important for the virulence, growth and proliferation of ''Streptococcus pneumoniae''. Zinc could compete for Manganese binding. However Manganese has more affinity for PsaA than Zinc but Zinc is not transported by the ABC-transporter. Zinc competition reduces intracellular Manganse resulting in up-regulation of PsaBCA expression. <ref>PMID:22072971</ref>

[[Image:PsaA dans ABC transporteur.jpg |500px| Structure of ABC-transporter <ref>http://www.latrobe.edu.au/biochemistry-and-genetics/research/maher/psabca-manganese-uptake-by-streptococcus-pneumoniae</ref>]]

=== ''Streptococcus pneumoniae''===
''Streptococcus pneumoniae'' is a Gram positive cocci (with a diameter from 0.5 to 1 μm) and a member of the genus ''Streptococcus''. It can live under aerobic or anaerobic conditions. It resides in the nasopharynx of healthy carriers. However, the bacterium may become pathogenic in elderly and immunocompromised adults and children. Then it can spread to other locations and cause disease. The genome of ''S. pneumoniae'' is a closed, circular DNA structure that contains between 2.0 and 2.1 million base pairs.<ref>PMID:4366526</ref>

[[Image:streptococcus pneumoniae.jpg | 350px|thumb| ''Streptococcus pneumoniae'' <ref>http://www.sciencephoto.com/images/download_lo_res.html?id=662360183</ref>]]

===ABC transporter===
Bacterial ABC transporters are an important class of transmembrane transporters. They are involved in the import and export of a wide variety of substrates, including sugars, amino acids, peptides, polyamines, and cations. <ref>https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2715661/</ref>They can be considered as a promising target for antimicrobial strategies. ABC transporter consists of four membrane-associated proteins with two ATP-binding proteins (ATPases) and two permeases. <ref>http://webcache.googleusercontent.com/search?q=cache:3MvkKDHQVzwJ:www.omicsonline.com/open-access/role-of-abc-transporter-proteins-in-stress-responses-of-streptococcus-mutans-2247-2452.1000592.pdf+&cd=1&hl=fr&ct=clnk&gl=fr&client=firefox-b </ref>
== Structure ==
The protein PsaA has a molecular weight of 34.538 kDa with 309 residues<ref>https://www.mybiosource.com/prods/Recombinant-Protein/Manganese-ABC-transporter-substrate-binding-lipoprotein-psaA/psaA</ref>. The overall size of the protein approximated from its crystal structure is 40 by 40 by 70 Å<ref>PMID:28011228</ref>.
As a member of the Lipoprotein receptor-associated antigen I (LraI) family, the PsaA molecule contains four distinct regions. An N-terminal leader sequence of 20 amino acids holds an LxACy consensus sequence that is recognized and cleaved by signal peptidase II <ref>PMID:28011228</ref>. A lipid moiety (diacylglycerol <ref>PMID:PMC99024</ref>) is added to the cysteine residue and mediates the anchorage of the protein to the cytoplasmic membrane. Apart from this leader sequence, the rest of the protein consists of two twofold-pseudosymmetrical (β/α)4 sandwich domains, of which the β-strands of each domain form parallel β-sheets <ref>PMID:PMC99024</ref>. In total the two domains form two lobes connected via an α-helical linker which constitutes the solute-binding site <ref>PMID:28011228</ref>.

'''Secondary structure of PsaA:'''

[[Image:structure secondaire.jpg |1100 px| thumb]] <ref>http://www.uniprot.org/uniprot/P0A4G2</ref>
<Structure load='3zk7' size='300' color='white' frame='true' align='text' caption='3D structure of PsaA in the metal-free, open state' />
<Structure load='3ztt' size='300' color='white' frame='true' align='text' caption='3D structure of PsaA with Manganese ion' />

'''Ramachandran blot'''
The ramachandran blot of PsaA has been obtained by Lovell, Davis, et al. Proteins 50:437.
[[Image:ramachandran.png]]

{|
| [[Image:details psaa avec mn.jpg]]
| [[Image:details psaa avec zn.jpg]]
|}

This image shows the metal binding site in more detail, with the MntC residues and Manganese. Manganse ions are shown as purple spheres. We can see that Manganese interacts with Histine residues, Aspartique acid residues and Glutamique acid residues.
The metal binding site is formed by the sidechains of residues His67, His139, Glu205, and Asp280. This site has tetrahedral coordination geometry. The amino acids His67 andHis139 interact with the metal via Nε2 nitrogen atoms. However, the carboxylate sidechains of Glu205 and Asp280 interact with the metal via their Oε1 and Oδ2 atoms. The atomic distance between His67 Nε2 and the metal is 1.99 Ắ while it is 2,01 Ắ for His 139 Nε2. It is 2,04 Ắ for Glu205 Oε1 and 2,02 Ắ for Asp280 Oδ2. The Oδ1 atom of Asp137 can form a hydrogen bond with Nδ1 of His139. Oδ1 atom of Asp65 can also form a hydrogen bond with His67 N. <ref>PMID:9862808</ref>

This image shows the metal binding site in more detail, with MntC residues and Managanese and Zinc. Manganese ion is shown as a purple sphere and Zinc ion is shown as an orange sphere. Cadmium uptake reduces the millimolar cellular accumulation of manganese and zinc, and thereby increases sensitivity to oxidative stress <ref>http://www.nature.com/articles/ncomms7418</ref>.

[[Image:molec.jpg|left|350px | thumb| "3D structure of PsaA with 2-AMINO-2-HYDROXYMETHYL-PROPANE-1,3-DIOL (tris) and cadmium<ref>http://www.ebi.ac.uk/pdbe/entry/pdb/4UTO</ref>
"]]
[[Image:showNew.jpg| 350px| thumb| "Structure of Tris
PsaA needs to be linked to this molecule to be in an open state"]]

== Function ==
This protein is essential for the existence of ''S. pneumoniae'' as mutations in the ''psaA'' gene cause deficiencies in growth but also in its virulence, adherence and the response to oxidative stress.

===Adhesin and virulence===
It is well hidden beneath the cell wall and the polysaccharide capsule, which is why it does not directly act as an adhesin as initially assumed. This was confirmed by studies on ''psaB'' and ''psaC'' deletion mutants which were not deficient in PsaA but showed impaired adherence, similar to ''psaA'' deletion mutants. As the disruption of either of the members of the Psa operon affects pneumococcal adherence, it seems that this process is coupled to the cellular Mn2+ transport system. With bacterial adhesion being the first step in bacterial pathogenesis, the extinction of PsaA and the associated Psa operon lead to attenuated virulence in affected strains <ref>PMID:PMC517531</ref>.

===Virulence===
Next to its involvement in bacterial adhesion, ''S. pneumoniae'' actively produces H2O2 in large amounts to augment its own virulence on the one hand, and, on the other hand, to utilize it as a weapon in the competition against other pathogens<ref>PMID:PMC517531</ref>.

===Oxidative stress===
In ''S. pneumoniae'' H2O2 and superoxides are generated as toxic derivatives of molecular oxygen in several metabolic processes and during growth by pyruvate oxidase. Subsequently, H2O2 reacts with Fe2+ during the Fenton reaction. This results in the formation of the hydroxylyl radical that causes severe damages in the cell. As a protective response against this oxidative stress, ''S. pneumoniae'' produces an Mn2+-dependent superoxide dismutase (SodA) and the thiol peroxidase encoded by ''psaD'' in the Psa operon. Due to the use of H2O2 during pneumococcal virulence these two proteins and the dependence on Mn2+ are of major importance. When Mn2+ transport is disrupted, bacterial cells display hypersensitivity towards oxidative stress<ref>PMID:PMC517531</ref>.

Mn2+ enables ''Streptococcus Pneunomiae'' to have a prompt and rapid growth. Superoxides are a key player in oxidative stress generated in the presence of iron. Therefore, they inhibit growth in the absence of Mn2+.

Mn2+ is also required for the activity of CpsB, a tyrosine phosphatase involved in the regulation of capsule production. And in some streptococcal species, lectin-mediated adherence requires Mn2+. Finally, mutations in the psa operon result in an almost complete attenuation of virulence for all tested models of animal infection.

==Interactions with other proteins==

- piaA, pneumococcal iron acquisition A which is an iron-compound ABC transporter permease. It is required for bacterial growth and full virulence in pulmonary infection.

-pavA, Pneumococcal adherence and virulence factor A is an adherence and virulence protein A. It binds fibronectin.

-Wzg, which regulates the capsule production and is an integral membrane regulatory protein Cps2A

-Tpx, a thiol peroxidase with a Has antioxidant activity. This protein can remove peroxides or H2O2 by similarity.

-psaC, a manganese ABC transporter permease

-psaB, a manganese ABC transporter ATP-binding protein. Deleting psaB results in decreased expression of PsaA. PsaC and psaC require Mn2+ supplementation for growth and transformation.

-SPD_1450, an iron-dependent transcriptional regulator

-adcC, antibody-dependent cell-mediated cytotoxicity is a zinc ABC transporter ATP-binding protein.

-adcB, a zinc ABC transporter permease

== Disease ==
=== Meningitis ===
=== Otitis ===
PsaA has been recognized to be involved in the adherence and virulence mechanisms of otitis media. Streptococcus pneumoniae is one of the main agents causing bacterial acute otitis media, directly or as complication of a viral upper respiratory tract infection. This disease is a highly prevalent pediatric disease worldwide. Hearing loss is a common problem associated with this disease. Streptococcus pneumoniae in middle ear can be resistant to penicillin and cephalosporin<ref>https://www.google.com/patents/US20130078254</ref>
=== Pneumonia ===
PsaA protein is an adhesin which is involved in colonization of the nasopharyngeal mucosal. Moreover, alveolar pneumonia is caused by the spread of ''Streptococcus pneumoniae'' from nasopharynx. Therefore PsaA protein is involved in infection of pulmonary parenchyma by ''Streptococcus pneumoniae''.
Sometimes, ''Streptococcus pneumoniae'' pass into the blood and causes a bacteremia besides pneumonia.
== Application in Biotechnology ==
PsaA is being actively evaluated as a component of a vaccin in formulations composed of pneumococcal common proteins. PsaA is a component of a vaccin because this protein is immunogenic and stimulates an increase in antibody production when the nasopharynx is naturally colonized. PsaA has been expressed as an ''E.coli'' recombinant protein, purified, and evaluated in a phase one clinical trial.
Progress in vaccine development is most advanced for Streptococcus pneumoniae. Indeed, there is a seven-valent capsular-conjugate vaccine, PREVNAR® but it is rather non efficient for otitis media.<ref>PMID:11176564</ref> PsaA has been shown to be an effective vaccine in animal models at preventing Streptococcus pneumoniae infection.
==References==
<references/>