Fig 1: P1A4 is a recombinant antibody that specifically recognizes the K-Ras(G12C) inhibitor ARS1620. A. Amino acid sequences of the CDRs of five unique Fabs identified in the phage display selection. B. Biolayer interferometry sensograms of P1A4 Fab binding to the peptide Biotin-KLVVVGAC*GV, where the cysteine residue is modified by ARS1620. Dissociation constant (Kd) was determined by fitting the steady-state response to a 1:1 equilibrium binding model. C. Biolayer interferometry sensograms of P1A4 Fab binding to the K5-ARS A*02:01 MHC-I complex. Fit as described in A. D. Biolayer interferometry sensograms of P1A4 Fab binding to the V7-ARS A*03:01 MHC-I complex. Fit as described in A. E. X-ray crystal structure of ARS1620 bound to Fab P1A4 (PDB: 7KKH). The heavy chain CDRs are shown in blue and the light chain CDRs in purple. Ordered water molecules in the pocket are shown as red spheres. Fo-Fc omit map for ARS1620 is shown in mesh, contoured at 1.0 s. F. P1A4 IgG detects ARS1620-modified K-Ras(G12C) as a recombinant protein or from ARS1620-treated cell lysates. G. Sandwich ELISA of recombinant MHC-I complexes prepared by refolding of the indicated heavy chain in the presence of ß2-microglobulin and the indicated peptide. The complexes were captured by the conformation-specific antibody W6/32 and detected by an ß2-microglobulin-specific antibody (BBM.1) or an ARS1620-specific antibody (P1A4) (One-way ANOVA with Dunnett’s correction for multiple comparisons, ns, not significant, ****, p<0.0001). Individual data points are shown with mean ±SD indicated. H. ARS1620-modified peptide-stabilized MHC complexes on the surface of the TAP-deficient cell line T2 are detectable by the conformational specific antibody W6/32 (left y axis) as well as by P1A4 (right y axis) (One-way ANOVA with Dunnett’s correction for multiple comparisons, ns, not significant, **, p<0.01, ***, p<0.001, ****, p<0.0001). Individual data points are shown with mean ±SD indicated.