Streptococcus pyogenes FbaB covalent surface-adhesin module (CnaB2 isopeptide domain; SpyTag/SpyCatcher origin)

This module describes the multidomain group A Streptococcus (Streptococcus pyogenes) surface adhesin FbaB (gene fba2; UniProtKB:Q8G9G1) and, in particular, its CnaB2 / SpaA-like prealbumin-fold domain, which autocatalytically forms an intramolecular Lys-Asp isopeptide bond. FbaB is a major virulence factor of invasive M3/M18 GAS strains: it is covalently anchored to the peptidoglycan cell wall via a C-terminal LPXTG sortase motif, binds host fibronectin through C-terminal fibronectin-binding repeats to mediate adhesion to and invasion of host cells, carries an N-terminal thioester (TED) domain of the kind that covalently captures host proteins in related streptococcal adhesins, and uses the CnaB2 isopeptide bond to gain the mechanical, thermal and chemical stability inferred to be needed for force-bearing engagement of host fibronectin during invasion. The CnaB2 domain (residues ~462-541; the region crystallized in PDB 9OJ3 and earlier FbaB-CnaB2 structures) is the natural scaffold that was split and engineered into the widely used SpyTag/SpyCatcher covalent protein-conjugation system (SpyTag = reactive Asp-bearing peptide; SpyCatcher = Lys-bearing protein partner; catalytic Glu), with SpyTag003/SpyCatcher003 being the third-generation variant in PDB 9OJ3. The module is grounded in a single natural gene product, FbaB; SpyTag and SpyCatcher are engineered fragments of this one protein, not separate genes.

MODULE:spytag_spycatcherDRAFTModulemodules/spytag_spycatcher.yaml

fibronectin bindingGO:0001968 adhesion of symbiont to host cellGO:0044650

PMID:12359713

Molecular characterization of a novel fibronectin-binding protein of S. pyogenes (FbaB)

FbaB is a surface-located fibronectin-binding protein with an LPXTG motif and C-terminal Fn-binding repeats; it binds fibronectin strongly, and an FbaB-deficient mutant has ~6-fold lower HEp-2 adhesion/invasion and reduced mouse mortality.

PMID:22366317

Peptide tag forming a rapid covalent bond to a protein, through engineering a bacterial adhesin

The FbaB CnaB2 domain forms a spontaneous Lys-Asp isopeptide bond; splitting it yields SpyTag (reactive peptide) and SpyCatcher (protein partner) that reconstitute the covalent bond. The domain is inferred to bear high force in bridging S. pyogenes to fibronectin during cell invasion.

PMID:40531663

ALS-ENABLE synchrotron resource case study

Reports a case study characterizing the SpyCatcher-SpyTag system; the associated structure is PDB 9OJ3 (SpyTag003-SpyCatcher003, 1.43 A).

PMID:26032562

An internal thioester in a pathogen surface protein mediates covalent host binding

Streptococcal surface protein SfbI uses an internal thioester bond as a covalent "chemical harpoon" to bind host fibrinogen; FbaB carries the same class of thioester (TED) domain.

Scope: one natural gene product (FbaB, UniProtKB:Q8G9G1). The PDB 9OJ3 "complex" is a homo-pairing of two engineered fragments (SpyTag003 + SpyCatcher003) of the FbaB CnaB2 domain, so there is a single companion gene review (genes/STRPY/FbaB). The thioester (TED) covalent host-binding step is asserted on domain-architecture grounds and by analogy to SfbI (PMID:26032562); it is marked optional because direct FbaB TED-fibrinogen data were not located. GO term groundings use stable molecular-function/process/component terms; domain descriptors (LPXTG, TED, CnaB2/SpaA prealbumin fold) are given as preferred_terms with Pfam/InterPro ids in prose rather than as validated ontology terms.

5Nodes

4Parts

0Variant Sets

0Variants

4Annotons

3Connections

Derived QC

Recommended-field compliance

14.3% recommended fields populated

references[0] · findings (0/1)
references[1] · findings (0/1)
references[2] · findings (0/1)
references[3] · findings (0/1)
references[4] · findings (0/1)
references[5] · findings (0/1)

Module deep research

✗ none found

No MODULE:spytag_spycatcher deep-research report alongside the module YAML.

Leaf nodes lacking representative members

✓ every leaf node grounds to a representative protein.

Template conformance

✓ every declared conforms_to bundle matches its template motif.

Gene-review completeness (1/1 grounded genes reviewed)

1 complete review(s) · 0 with deep research · 0 missing review · 1 reviewed but lacking deep research

Gene	Review	Complete	Deep research
FbaB Q8G9G1	✓	✓	✗

Tree

FbaB covalent surface-adhesin moduleModulespytag_spycatcher

Parts
1. 1. covalent display of FbaB on the cell wall
2. LPXTG/sortase anchoring of FbaB to the cell wallReactionfbab_cell_wall_anchoring
  
  Annotons
  - FbaB as LPXTG sortase substrate Gene Product: Fibronectin binding protein FbaB (fba2)
3. 2. fibronectin-mediated host adhesion and invasion
4. FbaB fibronectin binding and host-cell adhesion/invasionBiological Processfbab_fibronectin_adhesion
  
  Annotons
  - FbaB fibronectin-binding adhesin activity Gene Product: Fibronectin binding protein FbaB (fba2)
5. 3. mechanical stabilization by the CnaB2 intramolecular isopeptide bond
6. CnaB2 intramolecular Lys-Asp isopeptide bond (SpyTag/SpyCatcher scaffold)Molecular Functionfbab_cnab2_isopeptide
  
  Annotons
  - FbaB CnaB2 autocatalytic isopeptide cross-link Gene Product: FbaB CnaB2 / SpaA-like prealbumin-fold domain
7. 4. covalent host capture via the N-terminal thioester domain optional
8. FbaB N-terminal thioester (TED) domain covalent host bindingMolecular Functionfbab_thioester_host_capture
  
  Annotons
  - FbaB TED-domain covalent host-protein capture (inferred) Gene Product: FbaB N-terminal thioester (TED) domain

Details

Context

Streptococcus pyogenes (group A Streptococcus)NCBITaxon:1314

bacterial cell surfaceGO:0009986 peptidoglycan cell wallGO:0005618

FbaB covalent surface-adhesin moduleModulespytag_spycatcher

The FbaB adhesin program: cell-wall display, fibronectin-mediated host adhesion and invasion, mechanical stabilization by the CnaB2 intramolecular isopeptide bond (the SpyTag/SpyCatcher scaffold), and an N-terminal thioester domain for covalent host capture.

fibronectin bindingGO:0001968 adhesion of symbiont to host cellGO:0044650

Context

Streptococcus pyogenes (group A Streptococcus)NCBITaxon:1314

bacterial cell surfaceGO:0009986 peptidoglycan cell wallGO:0005618

Connections

fbab_cell_wall_anchoring -> fbab_fibronectin_adhesion Provides Input For

Cell-wall anchoring displays FbaB at the surface so it can engage host fibronectin.

fbab_cnab2_isopeptide -> fbab_fibronectin_adhesion Provides Input For

The CnaB2 isopeptide bond mechanically stabilizes the adhesin for force-bearing fibronectin engagement during invasion.

fbab_thioester_host_capture -> fbab_fibronectin_adhesion Positively Regulates

Covalent TED-mediated host capture is inferred to reinforce non-covalent fibronectin-mediated adhesion.

Part 1: covalent display of FbaB on the cell wall

LPXTG/sortase anchoring of FbaB to the cell wallReactionfbab_cell_wall_anchoring

FbaB carries a C-terminal LPXTG sortase-recognition motif (NCBIfam TIGR01167) and is therefore covalently anchored to the peptidoglycan cell wall by sortase, displaying its N-terminal adhesive domains at the bacterial surface.

Annotons

FbaB as LPXTG sortase substrate

fbab_lpxtg_substrate

Participant: Gene Product: Fibronectin binding protein FbaB (fba2)

Gene Product:

Fibronectin binding protein FbaB (fba2)UniProtKB:Q8G9G1 Group A Streptococcus surface adhesin; 733 aa; LPXTG-anchored.

Locations

peptidoglycan cell wallGO:0005618 bacterial cell surfaceGO:0009986

Substrate of the housekeeping sortase; its LPXTG motif is cleaved and the protein is covalently linked to cell-wall peptidoglycan.

PMID:12359713

FbaB possesses an LPXTG motif and Fn-binding repeat domains in the C-terminal region and is expressed on the cell surface of TSLS strains.

Part 2: fibronectin-mediated host adhesion and invasion

FbaB fibronectin binding and host-cell adhesion/invasionBiological Processfbab_fibronectin_adhesion

The surface-displayed FbaB binds host fibronectin through its C-terminal fibronectin-binding repeats, bridging the bacterium to host extracellular matrix/cell-surface fibronectin and promoting adhesion to and invasion of host epithelial cells. An FbaB-deficient mutant shows ~6-fold reduced HEp-2 adhesion/invasion and decreased virulence.

adhesion of symbiont to host cellGO:0044650

Annotons

FbaB fibronectin-binding adhesin activity

fbab_fibronectin_binding

Participant: Gene Product: Fibronectin binding protein FbaB (fba2)

Gene Product:

Fibronectin binding protein FbaB (fba2)UniProtKB:Q8G9G1

Function

fibronectin bindingGO:0001968

Targets: host fibronectin

Processes

adhesion of symbiont to host cellGO:0044650

Locations

bacterial cell surfaceGO:0009986

PMID:12359713

Recombinant FbaB exhibits strong fibronectin-binding ability.

PMID:12359713

An FbaB-deficient mutant has ~6-fold lower HEp-2 adhesion and invasion.

Part 3: mechanical stabilization by the CnaB2 intramolecular isopeptide bond

CnaB2 intramolecular Lys-Asp isopeptide bond (SpyTag/SpyCatcher scaffold)Molecular Functionfbab_cnab2_isopeptide

The CnaB2 / SpaA-like prealbumin-fold domain of FbaB (Pfam PF17802; InterPro IPR041033; SUPFAM SSF49478 Cna protein B-type; residues ~462-541) autocatalytically forms an intramolecular isopeptide (amide) bond between a Lys and an Asp, assisted by a catalytic Glu. This covalent cross-link gives the domain extreme mechanical, thermal and chemical stability and, biologically, is inferred to let FbaB bear high force while bridging the bacterium to fibronectin during invasion. Splitting this domain produced the SpyTag peptide (reactive Asp) and SpyCatcher protein (reactive Lys), whose reconstituted covalent bond underlies the SpyTag/SpyCatcher conjugation toolkit; SpyTag003/SpyCatcher003 (PDB 9OJ3) is the optimized third generation.

Annotons

FbaB CnaB2 autocatalytic isopeptide cross-link

fbab_isopeptide_crosslink

Participant: Gene Product: FbaB CnaB2 / SpaA-like prealbumin-fold domain

Gene Product:

FbaB CnaB2 / SpaA-like prealbumin-fold domainUniProtKB:Q8G9G1 CnaB2 domain of FbaB (residues ~462-541); the isopeptide-bond-forming domain split into SpyTag/SpyCatcher.

Function

autocatalytic intramolecular Lys-Asp isopeptide (amide) bond formationGO:0018149 Spontaneous, Glu-assisted intramolecular isopeptide bond between Lys and Asp side chains; irreversible and resistant to >1 nN force.

Provides covalent mechanical reinforcement of the adhesin; engineered split yields the SpyTag/SpyCatcher covalent conjugation system.

PMID:22366317

The FbaB CnaB2 domain contains a spontaneous Lys-Asp isopeptide bond; the domain may bear high force in bridging S. pyogenes to fibronectin during cell invasion.

PMID:24161952

Crystal structures (PDB 4MLI/4MLS) of the CnaB2-derived SpyCatcher:SpyTag covalent complex.

Part 4: covalent host capture via the N-terminal thioester domain (optional)

FbaB N-terminal thioester (TED) domain covalent host bindingMolecular Functionfbab_thioester_host_capture

FbaB carries an N-terminal thioester domain (Pfam PF08341/TED; InterPro IPR013552; residues ~92-212) with an internal Cys-Gln thioester bond. In related streptococcal adhesins (e.g. SfbI, Cpa) such domains act as a covalent "chemical harpoon" that cross-links the bacterium to host proteins such as fibrinogen. This step is inferred for FbaB from its domain architecture and by analogy; direct FbaB TED-host data were not located.

Annotons

FbaB TED-domain covalent host-protein capture (inferred)

fbab_ted_covalent_binding

Participant: Gene Product: FbaB N-terminal thioester (TED) domain

Gene Product:

FbaB N-terminal thioester (TED) domainUniProtKB:Q8G9G1

Function

thioester-mediated covalent binding of a host protein (e.g. fibrinogen)GO:0070051

Targets: host fibrinogen (candidate covalent target)

Inferred covalent host-capture function of the TED domain.

PMID:26032562

The streptococcal surface protein SfbI uses an internal thioester bond as a covalent chemical harpoon to bind host fibrinogen; FbaB carries the same class of TED domain.