Figure 1. The transmembrane protein UNC93B1 (black spring-like shape) associates with various toll-like receptors (horseshoe-like shapes) in the endoplasmic reticulum to help load them into vesicles coated with COPII for transport to the Golgi.

TLR9 (dark blue) follows the secretory pathway (route 1) to the plasma membrane, where AP-2 (yellow oval) binds to the UNC93B1 and transports the TLR9/UNC93B1 complex to the endosome. After being cleaved by proteases inside the endosome, TLR9 becomes active and is able to send signals to induce inflammation, which is part of the immune response. AP-3 (dark pink) can then direct TLR9 to a different type of endosome, from which it can activate the production of proteins called interferons (IFNs), which are part of the antiviral immune response. TLR7 (red) follows a different path (route 2) that does not involve going to the plasma membrane, but does result in the induction of inflammation and the production of interferons. This pathway starts with AP-4 (orange oval) binding to the receptor in the Golgi and transporting it to an endosome. Four other toll-like receptors—TLR3, TLR11, TLR12 and TLR13—also likely follow this path. AP: clathrin adaptor protein; COPII: coat protein complex II; NF-κB: nuclear factor-kappaB.