Fig 2.

Binding kinetics between Fc-tagged nanobodies and prototypic SARS-CoV-2 RBD as measured by surface plasmon resonance (SPR). (A)–(C) Each of the Fc-tagged nanobodies (Nanosota-2A-Fc, Nanosota-3A-Fc, and Nanosota-4A-Fc) was immobilized to a protein A sensor chip. Prototypic SARS-CoV-2 RBD (His-tagged) was injected at different concentrations. The resulting data were analyzed using Biacore Evaluation Software. (D) Competition SPR analysis. His-tagged RBD was immobilized onto four CM5 sensor chips. Each of the Fc-tagged nanobodies was injected to one of the first three chips. In contrast, the fourth chip received only the running buffer. Following this, after the nanobodies had bound to the RBD, a mixture of recombinant human ACE2 and the corresponding nanobody was added to the first three chips. The fourth chip received only ACE2. Sensorgrams from all chips were then overlaid for comparison. No change in the resonance signal from nanobody-bound RBD indicated that ACE2 could not displace nanobody from binding to the RBD, as in the case of Nanosota-2A-Fc and Nanosota-3A-Fc. A slight increase in the resonance signal from nanobody-bound RBD suggested ACE2 largely could not displace nanobody from binding to the RBD, as in the case of Nanosota-4A-Fc.