Fig. 1: Working principle of the LITESEC systems—light-controlled activation and deactivation of protein translocation by the type III secretion system.

a Schematic representation of the active T3SS injectisome (modified from ref. ⁶). Left side, main substructures; right side, dynamic cytosolic T3SS components. Effector translocation by the T3SS is licensed by the functional interaction of the unbound bait-SctQ fusion with the T3SS. b Different states of the bait and anchor proteins in dark and light conditions. In the LITESEC-supp system (top), the bait protein, a fusion of the smaller interaction switch domain SspB_Nano and the essential T3SS component SctQ, is tethered to the inner membrane (IM) by a membrane anchor, a fusion of a transmembrane helix (TMH) and the larger interaction switch domain, iLID, in the light, and gets released in the dark. Conversely, in the LITESEC-act system (bottom), the bait protein, a fusion of the smaller interaction switch domain, Zdk1, and the essential T3SS component SctQ, is tethered to the membrane anchor, a TMH fusion of the larger interaction switch domain, LOV2, in the dark, and gets released by illumination. c Sequestration of the bait-SctQ fusion protein to the membrane prevents effector secretion. HM host membrane, OM bacterial outer membrane, IM bacterial inner membrane.