MoonProt 4.0: 2026 Update of the Moonlighting Proteins Database.

MoonProt 4.0 (http://moonlightingproteins.org) is an updated open-access database storing manually-curated annotations for moonlighting proteins. Moonlighting proteins exhibit two or more physiologically relevant distinct biochemical or biophysical functions performed by a single polypeptide chain. Here we describe an expansion in the database since our report published in 2021. With the assistance of five undergraduate annotators, we have added approximately 200 protein entries to give a total of over 700 moonlighting proteins. The new entries include more examples from plants, more transmembrane proteins and additional combinations of functions. The MoonProt Database collection of proteins with multiple functions serves as a resource for developing algorithms for predicting protein functions and provides examples of the evolution of new functions on a protein scaffold that can be valuable in developing novel methods for designing proteins with added functions.