Structural Mechanism of Specific Nucleobase Recognition by a Monoclonal Antibody in CoolMPSTM Sequencing

Abstract

Massively parallel sequencing (MPS) has revolutionized genomics, yet traditional methods utilizing fluorescently labeled nucleotides suffer from "scarring" effects that limit read accuracy. The CoolMPS technology overcomes this by using unlabeled reversible terminators (RTs) with a 3-O-azidomethyl blocking group, detected by highly specific fluorescent antibodies. However, the atomic-level mechanism by which these antibodies discriminate between modified and natural nucleotides within a DNA strand remains unclear. Here, we report the crystal structures of the "A-fab" antibody fragment in complex with the monomeric antigen 3-O-azidomethyl-dATP (dATP-N3) at 2.50 [A] resolution, and with an one strand of the duplex carries an azide-modified deoxyadenosine nucleotide at its 3' terminus (dsDNA-dATP-N3) at 1.99 [A] resolution. Isothermal titration calorimetry (ITC) revealed that A-fab binds dATP-N3 with nanomolar affinity (KD{approx}17.14 nM), while binding to natural dATP is negligible (KD{approx}7.51{micro}M), representing a [~]440-fold specificity. Structural analysis reveals a conserved hydrophobic pocket formed by CDR residues (e.g., Trp79, Tyr53, Leu67) that specifically accommodates the 3-azidomethyl group. This interaction is critical for affinity and is maintained in the dsDNA-ATP-N3 complex, validating the antibodys function in a sequencing context. These findings provide the structural rationale for the high fidelity of the CoolMPS platform.