Fig. 4. Mosaic model of protein secretion via the type II pathway. The Xcp-dependent exoproteins exotoxin A (ETA), containing a Sec-dependent signal peptide (SP), or the phospholipases C (Plc’s), bearing a twin-arginine (RR) signal peptide, are exported across the inner membrane through the Sec and the Tat pathway, respectively. After cleavage of the signal peptides, the exoproteins are recognized in the periplasm by the Xcp system, directed to the secretin XcpQ, and released into the external medium. C = cytoplasm; IM = inner membrane; P = periplasm; OM = outer membrane. Different shades in the Sec and Tat translocons correspond to different subunits.