Integrin α3β1 Binding to Fibronectin Is Dependent on the Ninth Type III Repeat

Area of Science:

• Cell Biology
• Biochemistry
• Biophysics

Background:

• Fibronectin (Fn) is a key extracellular matrix protein involved in cell adhesion.
• Integrin receptors mediate cell adhesion to Fn, primarily through the Arg-Gly-Asp (RGD) motif.
• Some integrins, like αIIbβ3 and α5β1, also utilize a synergy site (PHSRN) for binding, which is sensitive to Fn conformation.

Purpose of the Study:

• To investigate whether the synergy site modulates integrin α3β1 binding to fibronectin.
• To explore the impact of specific mutations and insertions in the Fn binding domain on α3β1 interactions.

Main Methods:

• Surface plasmon resonance (SPR) was used to quantify binding kinetics between integrin α3β1 and various recombinant Fn fragments.
• Engineered Fn fragments included a stabilizing mutant (FnIII9'10), a synergy site mutation (FnIII9(R)→(A)10), and glycine insertions (FnIII9(2G)10, FNIII9(4G)10).
• Cell attachment assays were performed to assess α3β1-mediated epithelial cell adhesion to the Fn fragments.

Main Results:

• Integrin α3β1 exhibited the highest binding affinity for FnIII9'10 and FnIII9(2G)10 fragments.
• Mutation of the synergy site reduced α3β1 binding affinity by 17-fold compared to FnIII9'10.
• A four-glycine insertion in the linker region decreased binding affinity by 39-fold, and cell attachment studies corroborated these findings.

Conclusions:

• The synergy site and the spacing of binding motifs within fibronectin significantly influence integrin α3β1 affinity.
• These findings highlight the conformational sensitivity of integrin-Fn interactions and their implications for cell adhesion.