Type II Toxin-Antitoxin Loci: The fic Family

FIC domain containing proteins (Fic proteins) are present in all domains of life but particularly widespread among prokaryotes. FIC domains with a fully conserved HxFx[D/E]GNGRxxR active site motif catalyze adenylylation (also known as AMPylation), the transfer of an adenosine 50-monophosphate moiety onto target proteins. Adenylylation activity is tightly controlled by an inhibitory α-helix (αinh) that can either be part of the Fic protein (intramolecular inhibition) or encoded on a different polypeptide chain (intermolecular inhibition), the latter constituting a novel class of type II toxin-antitoxin (TA) modules represented by VbhT-VbhA of Bartonella schoenbuchensis and FicT-FicA of Escherichia coli. The helix αinh harbors a [S/T]xxxE[G/N] motif with the conserved glutamate partially obstructing the ATP-binding site and forcing ATP to bind in a catalytically incompetent conformation. Release of inhibition by removal of the antitoxin component or by mutation of the conserved glutamate in αinh converts Fic proteins into toxins that severely impair bacterial growth.