Crustins are distinctive members of the WAP-containing protein superfamily: An improved classification approach

Crustins are considered effector molecules of innate immunity in arthropods, and classification schemes have been proposed over the last 10 years. However, classification problems have emerged: for example, proteins that have been well identified as members of a particular category have also been classified as crustins. Therefore, the objective of this manuscript was to analyze and, based on solid arguments, improve the original proposed nomenclature to make crustins a distinctive group of antibacterial proteins. The presence of WAP or 4DSC domain has been considered a distinctive feature of crustins; however, several antibacterial proteins containing WAP domains have been detected in diverse taxonomic groups (including mammals). Here, we present evidence supporting the idea that the Cys-rich region and the 4DSC domain can be considered a signature of crustins and, together with some distance arrangements occurring within this 12-Cys region, yield enough information for the classification of these proteins. Herein, the core characteristics to be considered for classification purposes are the length of the Gly-rich region and the repetitive tetrapeptides occurring within this region; these characteristics are then hierarchically followed by the F and A distances located within the 4DSC domain. Finally, the proposed system considers the crustin signature as the common structure in all members, which is a differentiator from other proteins containing WAP domains, separating crustins as a well-distinguished member of the superfamily of WAP-domain containing proteins.