A ubiquitin-like protein controls assembly of a bacterial type VIIb secretion system

Type VII secretion systems (T7SS) are protein translocation machines crucial for virulence and bacterial competition in Gram-positive bacteria. Despite their importance, the structural basis for assembly of type VIIb secretion systems (T7SSb), a widely distributed variant in Firmicutes, remains poorly understood. We present the cryo-electron microscopy structure of the T7SSb core complex from <i>Bacillus subtilis</i>, revealing how the ubiquitin-like protein YukD, coordinates assembly of the secretion machinery. YukD interacts extensively with the central channel component YukB and facilitates its association with the pseudokinase YukC, forming a stable building block for channel assembly. Time-lapse microscopy and competition assays demonstrate that YukD is essential for proper T7SSb complex formation and contact-dependent bacterial killing. Our findings reveal how bacteria have adapted a ubiquitin-like protein as a structural regulator for assembling a large secretion complex.