FC0085
Immunoglobulin-like B protein (LigB)  -  Fibronectin NTD

Biological function
LigB contributes to binding the Leptospira pathogen to the extracellular matrix elements, such as fibronectin. Lig proteins are categorized as microbial surface components recognizing adhesive matrix molecules due to their ability to bind to eukaryotic cells through their interactions with extracellular matrix components.

Domain organization/sequence features
Lig proteins, including LigA, LigB, and LigC, contain multiple immunoglobulin-like repeat domains (13 in LigA, 12 in LigB and LigC). Interestingly, the first 630 residues, from the N terminus to the first half of the seventh immunoglobulin- like domain, are conserved between LigA and LigB, but the rest of the immunoglobulin-like domains are variable (10–12) between the two proteins.

Structural evidence
In ¹H-¹⁵N HSQC spectrum the part corresponding to the folded region remains unperturbed upon NTD interactions, whereas most sharp and clustered peaks disappear or decrease intensity. The nature and number of peaks that are affected by the addition of NTD suggest that NTD specifically binds to LigBCen2NR and results in chemical exchange in the intermediate rate regime, where the peaks whose chemical environment changes between conformational states are broadened beyond the level of detection. Based on SPR measurements, the line broadening observed for the NMR sample is not due to exchange between free and bound states. One possible explanation is the formation of a ’fuzzy complex’, or a dynamic ensemble of conformational states, upon binding NTD, similarly to SfbI.

Biochemical evidence
The high-affinity binding region for Fn NTD was assigned to the 47 amino acids of the non-repeat region (LigBCen2NR) of LigB. K_D is 379 nM. Adding the folded regions increases affinity by 4-fold (93 nM). The non-repeat region is disordered in solution as it was demonstrated by various techniques: CD, NMR, DSL, Viscosity measurements, gel permeation chromatography. Binding of NTD to LigB decreases the propensity of random coils from 65% to 46%, and concomitantly increases the fraction of β-sheets (28 to 48%). Nevertheless, there is a significant fraction of disordered residues. k_on 2.28x10⁵ k_off 2.47x10^-2s^-1, ITC measurements: ΔH 13.13 kcal/mol, TΔS 22.25 kcal/mol, ΔG 9.12 kcal/mol. The favorable entropy suggests that the complex formation of LigBCen2 and NTD involves significant hydrophobic interactions.

Mechanism category
tethering

Significance
Fuzziness imparts moderate affinity and slow kinetics on the LigBCen2NR interaction with the NTD of Fn, which might aid in Leptospira infection of a host organism by increasing the time it remains adhered to cells while simultaneously facilitating efficient transmission within the host.

Further reading
19581300