The substrate specificity switch FlhB assembles onto the export gate to regulate type three secretion.

Protein secretion through type-three secretion systems (T3SS) is critical for motility and virulence of many bacteria. Proteins are transported through an export gate containing three proteins (FliPQR in flagella, SctRST in virulence systems). A fourth essential T3SS protein (FlhB/SctU) functions to "switch" secretion substrate specificity once the growing hook/needle reach their determined length. Here, we present the cryo-electron microscopy structure of an export gate containing the switch protein from a Vibrio flagellar system at 3.2 Å resolution. The structure reveals that FlhB/SctU extends the helical export gate with its four predicted transmembrane helices wrapped around FliPQR/SctRST. The unusual topology of the FlhB/SctU helices creates a loop wrapped around the bottom of the closed export gate. Structure-informed mutagenesis suggests that this loop is critical in gating secretion and we propose that a series of conformational changes in the T3SS trigger opening of the gate through interactions between FlhB/SctU and FliPQR/SctRST.

Interaction network of the mitochondrial outer membrane protein Mcp3.

Mitochondria are organelles containing two membranes that are distinct in composition and function. A role of the mitochondrial outer membrane (MOM) is to mediate contact of the organelle with the rest of the cell. In yeast, the MOM contains about 40 different integral proteins. Recently, we described the MOM protein Mcp3, which can serve as a multicopy suppressor of loss of ERMES complex that mediates mitochondria-endoplasmic reticulum contacts. To shed further light on the role of Mcp3 in the MOM, we analyzed its physical interaction with other proteins. We show that Mcp3 interacts with the MOM protein Om45 and the inner membrane protein Aim19. Our observations hint toward a potential involvement of Mcp3 in a structural and/or functional link between both mitochondrial membranes.