Expansins and expansin-like proteins revisited: Untangling the 'Gordian' knots

Expansins and expansin-like proteins, which together comprise the expansin superfamily, are small cell wall (CW)-loosening proteins present in diverse organisms, including plants, fungi, bacteria, protists, molluscs and nematodes. Proteins belonging to the expansin superfamily (ESPs) have diverse biological roles, including enabling CW growth, serving as virulence factor for plant pathogens, and assisting plant CW decomposition by microbial plant CW-active enzymes. In this review, we offer a biomechanical perspective of the role of ESPs, by providing an overview of the CW properties they affect, examining their reported substrate-binding affinities and assessing how these relate to the functions of the respective CW components. Moreover, we refine the current nomenclature of ESPs. Finally, within the framework of our recent discovery of catalytic activity in an expansin-like protein, we itemise biomechanical and biochemical assays that are commonly employed to evaluate activity of ESPs, and we discuss challenges associated with determining ESP activity. We hope that this review will promote research and open new research avenues in the field of ESPs.