Allostery is a universal phenomenon whereby a perturbation by an effector at one site of a protein leads to a functional change at another site through alteration of shape and/or dynamics. Allosteric perturbations arise from environmental changes (temperature or pH); from noncovalent binding of ions, lipids, cAMP, drugs, proteins, RNA, or DNA, from light absorption or from covalent events, such as phosphorylation, point mutations, or other chemical modifications [1]. Allostery takes place in all dynamic biomolecules that exist in a set of closely related conformational states termed an ensemble [2] and is caused by a perturbation at any site in the structure that leads to a shift in the distribution of the conformational states across the entire population [3]. And consensus is growing that allosteric structural and/or dynamic perturbations do not create new conformational states but only change the relative distributions of the states within the ensembles, referred to as allosteric transitions.

Over the years, we have amassed proteome-scale maps of biological signal transduction pathways [4], but the dynamic properties of the proteins that mediate the allosteric propagation of these signals within the transducing macromolecules and their complexes are largely underappreciated [1]. Also, allostery is still largely perceived as acting on single proteins and their allosteric effectors. However, the fundamental importance of allostery is not in the functional effects on the protein itself, but rather on the cell and on the organism as a whole [5]. And allosteric rather than orthosteric modulation is the ubiquitous strategy employed by nature to regulate receptor function and control cellular processes.

In this review we show that the binding of Nanobodies (Nbs), the variable domains of heavy-chain-only antibodies that naturally occur in camelids [6] can cause allosteric perturbations and perturbs the distribution of conformational ensembles along signal transduction pathways, leading to the allosteric modulation of the signals they transmit. Such single domain antibodies can be made at order [7] to be used as research tools or for developing novel drugs with unprecedented properties [8].