cellular protein modification process protein tagging activity biological_process GO:0006464 protein modification process process resulting in protein modification The covalent alteration of one or more amino acids occurring in proteins, peptides and nascent polypeptides (co-translational, post-translational modifications) occurring at the level of an individual cell. Includes the modification of charged tRNAs that are destined to occur in a protein (pre-translation modification). protein modification process cellular protein metabolic process