Global analysis of VHHs framework regions with a structural alphabet

TL;DR

This study systematically analyzes the structural diversity of VHH framework regions using a structural alphabet, revealing multiple conformations and complex influences beyond amino acid composition, which impacts structural modeling.

Contribution

First comprehensive structural analysis of VHH framework regions showing their conformational variability and complex determinants beyond amino acid sequence.

Findings

Each FR exhibits multiple structural variants.

No direct link between amino acid composition and conformation.

Long-range interactions influence local conformations.

Abstract

The VHHs are antigen-binding region/domain of camelid heavy chain antibodies (HCAb). They have many interesting biotechnological and biomedical properties due to their small size, high solubility and stability, and high affinity and specificity for their antigens. HCAb and classical IgGs are evolutionary related and share a common fold. VHHs are composed of regions considered as constant, called the frameworks (FRs) connected by Complementarity Determining Regions (CDRs), a highly variable region that provide interaction with the epitope. Actually, no systematic structural analyses had been performed on VHH structures despite a significant number of structures. This work is the first study to analyse the structural diversity of FRs of VHHs. Using a structural alphabet that allows approximating the local conformation, we show that each of the four FRs do not have a unique structure but exhibit many structural variant patterns. Moreover, no direct simple link between the local conformational change and amino acid composition can be detected. These results indicate that long-range interactions affect the local conformation of FRs and impact the building of structural models.