Ubiquitin binds to other cellular proteins through a series of reactions (mediated by the E1, E2 and E3 enzymes), modifying their stability or activity. Prokaryotes lack ubiquitin, but pathogenic bacteria have several enzymes that can affect protein ubiquitination. For instance, during Legionella infection, the pathogen produces an effector, SdeA, that ubiquitinates substrate proteins single handedly and independent of the host’s ubiquitination machinery. Instead, it is thought to have two catalytic platforms—a mono-ADP-ribosyltransferase (mART) domain that is used first to catalyse the ADP-ribosylation of ubiquitin, and a phosphodiesterase (PDE) domain that catalyses the subsequent conjugation of phophoribosyl-bridged ubiquitin to the substrate protein. Three teams of researchers independently provide insights into the molecular mechanism by which SdeA carries out ubiquitination. Their findings could pave the way for future studies of the role this unusual type of ubiquitination has in bacterial infection and possibly beyond prokaryotes.
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