Accessory proteins are vital for the functional expression of certain G protein-coupled receptors.

Certain G protein-coupled receptors (GPCRs) fail to be expressed in a functional form at the cell surface. This may be due to the improper folding and maturation of GPCRs which are highly intricate events that need to take place before these integral membrane proteins can be transported from the endoplasmic reticulum (ER), where they are synthesised, to the plasma membrane which is their site of action. Once at the plasma membrane they act as the recognition elements for a vast range of endogenous ligands including biogenic amines, peptides, glycoproteins, lipids, nucleotides, ions and proteases. The assistance of molecular chaperones has been widely implicated in the trafficking and function of these proteins. Characterisation of certain GPCRs has identified a novel group of membrane proteins collectively named 'accessory proteins' as being important for the expression and function of GPCRs. In this review we will summarise the importance of these accessory proteins for the function of their respective GPCRs. Understanding their roles in GPCR expression would not only give us an insight into these receptors from a cell biological point of view but may also potentially lead to the development of novel therapeutics.