Activation mechanisms of Class F receptors

Abstract

Our genetic makeup is composed of thousands of genes that code for the proteins involved in all aspects of cellular function. Proteins are molecular machines that dynamically and structurally respond to stimuli. Cells are able to sense their environment and react accordingly through the help of receptors – a type of protein – embedded in the cellular membrane that binds extracellular molecules known as ligands. The innate ability of receptors to specifically interact with and influence structural changes in partner proteins forms the basis for signalling cascades that regulate cellular processes. In diseased individuals, genetic mutations can result in changes in the structure of proteins that will affect the tertiary and quaternary structures of a protein or proteins in complex. This can have drastic effects on protein function – ultimately compromising cellular fate.

The use of animal and plant-based products in modern medicine led to the development of pharmacology – the study of drug effects in living organisms. Receptor-labelling techniques and molecular cloning enabled us to establish that many of these molecules were acting on a family of proteins called G protein-coupled receptors (GPCRs). Our knowledge of the structural changes exerted on the receptor upon ligand binding has led to the development of new drugs with increased subtype specificity and fewer side effects. While our knowledge related to prototypical GPCRs like adrenoreceptors and opioid receptors has flourished in recent years, their distantly related cousins – known as Frizzleds (FZDs) and Smoothened (SMO) – have suffered from an inability to assess their GPCR nature. FZDs interact with the WNT family of lipoglycoproteins and SMO is indirectly regulated by Hedgehog (Hh) to orchestrate important processes in embryonic development and adult homeostasis. Misregulation of WNT and Hh signalling leads to the pathogenesis of numerous diseases for which treatments are limited. With respect to WNT signalling, the lack of effective drugs targeting FZDs is in large part due to a knowledge gap resulting from current dogma that places emphasis on protein complex formation rather than the structural changes involved in protein activation.

The findings compiled in papers I to III provide structural insight into the mechanisms underlying FZD/SMO activation. A combination of genomic, biophysical and biochemical approaches was used to unravel the details surrounding FZD/SMO complex formation and the structural dynamics upon ligand binding that lead to full receptor activation and downstream signalling. This has implications for the development of effective treatments for diseases where FZDs and SMO can be targeted. Characterizing the conformational landscape of these receptors will enable us to drive drug discovery efforts forward.