FIGURE 1.

Schematic representation of unconventional protein secretion. Transmembrane proteins and ER luminal cytosolic proteins in general pass through the ER and the Golgi apparatus in their route to their final destinations (conventional secretion). Some leaderless cytosolic proteins can be secreted to extracellular space from cytosol via the membrane pore (Type I UPS), ABC-transporter (Type II UPS), or vesicles (Type III UPS). Some transmembrane proteins may alternatively be transported to the plasma membrane via an unconventional secretory pathway that bypasses the Golgi (Type IV UPS). Diverse vesicular systems, including autophagosomes, multivesicular bodies (MVBs) and CUPS, are involved in the type III and IV UPS pathways. The UPS cargos utilizing autophagy machineries are marked in cyan (see text for details).