A molecular function exhibited by a protein that is covalently attached (AKA tagged or conjugated) to another molecule (for example a protein or a lipid) where it acts as a marker, recognized by the cellular apparatus to target the tagged protein for some cellular process such as modification, sequestration, transport or degradation.
Source:PMID:17632063
Comment
Use this term to annotate conjugated tags, not for conjugating enzymes. At the time of writing, all known gene products with this activity are ubiquitin-like, either based on overall sequence similarity or the presence of common motifs and structures.