9.A.48 The Unconventional Protein Secretion (UPS) System Family

Mammalian cells export most proteins by the endoplasmic reticulum/Golgi-dependent pathway. However, some proteins are secreted via unconventional mechanisms. The latter include the proinflammatory cytokines interleukin(IL)-1β, IL-18, and IL-33, which require activation by caspase-1 for biological activity. Caspase-1 itself is activated by innate immune complexes, the inflammasomes. Secretion of the leaderless proteins proIL-1α, caspase-1, and fibroblast growth factor (FGF)-2 (see TC# 1.A.108) depends on caspase-1 activity (Keller et al., 2008). Although proIL-1alpha and FGF-2 are not substrates of the protease, they interact physically. Secretome analysis using iTRAQ proteomics revealed caspase-1-mediated secretion of other leaderless proteins with known or unknown extracellular functions. Many of these proteins are involved in inflammation, cytoprotection, or tissue repair. Thus, caspase-1 plays a role in unconventional protein secretion. By this mechanism, stress-induced activation of caspase-1 directly links inflammation to cytoprotection, cell survival, and regenerative processes (Keller et al., 2008). Non-conventional export of cytoplasmic proteins has been reviewed (Ebner and Götz 2018).

A growing number of proteins devoid of signal peptides have been shown to be released through non-classical pathways independent of the endoplasmic reticulum and Golgi. Among them are two potent proangiogenic cytokines, FGF1 (TC# 1.A.108) and IL1α (TC# 1.A.109). Stress-induced transmembrane translocation of these proteins requires the assembly of copper-dependent multiprotein release complexes. It involves the interaction of exported proteins with the acidic phospholipids of the inner leaflet of the cell membrane and membrane destabilization. Not only stress, but also thrombin treatment and inhibition of Notch signaling stimulate the export of FGF1 (Prudovsky et al., 2008).

Stress enhances the expression of S100A13 and induces the actin cytoskeleton-dependent translocation of FGF1, S100A13, and p40 Syt1 to the inner leaflet of the cell membrane where they bind to acidic phospholipids. SK1, located on the inner leaflet of the cell membrane, serves as a source of copper ions (which are provided to SK1 by the transmembrane copper transporters). Copper delivered by SK1 is required for the formation of the release complex. This complex includes the covalent FGF1 dimer, non-covalent S100A13 dimer, p40 Syt1 and SK1. The release complex binds to Anx II, which is also located on the inner leaflet of the cell membrane (Pompa et al. 2017). Transmembrane flipping of acidic phospholipids results in the export of the FGFI release complex and Anx II (Prudovsky et al., 2008) (Giuliani et al., 2011). Galectin-3 may form pores in the plasma membrane that facilitate it's secretion to the outside (Pompa et al. 2017).

There are two non-conventional routes: One sustains the extracellular delivery of cytoplasmic proteins that lack a signal peptide, the other supports the transport of transmembrane proteins to the plasma membrane in a manner that bypasses the Golgi. Some unconventional secretion events are triggered by cellular stress as noted above (Rabouille 2016). A Golgi protein, Golgi Re-Assembly and Stacking Protein (GRASP; Q99JX3), has been shown to be essential to both types of unconventional secretion.

This family belongs to the Synaptotagmin Domain-containing (SynapD) Superfamily.



Ahat, E., S. Bui, J. Zhang, F. da Veiga Leprevost, L. Sharkey, W. Reid, A.I. Nesvizhskii, H.L. Paulson, and Y. Wang. (2022). GRASP55 regulates the unconventional secretion and aggregation of mutant huntingtin. J. Biol. Chem. 298: 102219. [Epub: Ahead of Print]

Ebner, P. and F. Götz. (2018). Bacterial Excretion of Cytoplasmic Proteins (ECP): Occurrence, Mechanism, and Function. Trends Microbiol. [Epub: Ahead of Print]

Giuliani, F., A. Grieve, and C. Rabouille. (2011). Unconventional secretion: a stress on GRASP. Curr. Opin. Cell Biol. 23: 498-504.

Hirai, Y., C.M. Nelson, K. Yamazaki, K. Takebe, J. Przybylo, B. Madden, and D.C. Radisky. (2007). Non-classical export of epimorphin and its adhesion to alphav-integrin in regulation of epithelial morphogenesis. J Cell Sci 120: 2032-2043.

Keller, M., A. Rüegg, S. Werner, and H.D. Beer. (2008). Active caspase-1 is a regulator of unconventional protein secretion. Cell 132: 818-831.

Lai, Y., X. Lou, Y. Jho, T.Y. Yoon, and Y.K. Shin. (2013). The synaptotagmin 1 linker may function as an electrostatic zipper that opens for docking but closes for fusion pore opening. Biochem. J. 456: 25-33.

Pompa, A., F. De Marchis, M.T. Pallotta, Y. Benitez-Alfonso, A. Jones, K. Schipper, K. Moreau, V. Žárský, G.P. Di Sansebastiano, and M. Bellucci. (2017). Unconventional Transport Routes of Soluble and Membrane Proteins and Their Role in Developmental Biology. Int J Mol Sci 18:.

Prudovsky, I., F. Tarantini, M. Landriscina, D. Neivandt, R. Soldi, A. Kirov, D. Small, K.M. Kathir, D. Rajalingam, and T.K. Kumar. (2008). Secretion without Golgi. J. Cell. Biochem. 103: 1327-1343.

Rabouille, C. (2016). Pathways of Unconventional Protein Secretion. Trends Cell Biol. [Epub: Ahead of Print]


TC#NameOrganismal TypeExample

The Unconventional Protein Secretion System, UPSS (Giuliani et al., 2011).  IT secretes FGF1 (an annexin; TC#1.A.31) and Epimorphin (syntaxin 2; 8.A.91) (Hirai et al. 2007). GRASP55 regulates this unconventional secretion and aggregation of mutant huntingtin (Ahat et al. 2022). Golgi reassembly stacking proteins (GRASPs) regulate Golgi-independent unconventional secretion of certain cytosolic and transmembrane protein cargoes.  Ahat et al. 2022 surveyed several neurodegenerative disease-related proteins, including mutant huntingtin (Htt-Q74), superoxide dismutase 1 (SOD1), tau, and TAR DNA-binding protein 43 (TDP-43), for unconventional secretion; Htt-Q74 was most robustly secreted in a GRASP55-dependent manner. Unconventional secretion of Htt is GRASP55 and autophagy dependent and is enhanced under stress conditions such as starvation and ER stress. GRASP55 facilitates Htt secretion by tethering autophagosomes to lysosomes to promote autophagosome maturation and subsequent lysosome secretion and by stabilizing p23/TMED10, a channel for translocation of cytoplasmic proteins into the lumen of the ER-Golgi intermediate compartment. Novel cytosolic cargoes secreted by the same unconventional pathway, include transgelin (TAGLN), multifunctional protein ADE2 (PAICS), and peroxiredoxin-1 (PRDX1) (Ahat et al. 2022). Golgi reassembly-stacking protein 2, (GRS2; GRASP55; Golgi phosphoprotein 6, GOLPH6) regulates this unconventional protein secretion (Ahat et al. 2022).



UPSS of Mus musculus
1) S100A13 Ca2+ binding protein (98 aas)(P97352)
2) p40Syt1 synaptogamin1 (homologous to 8.A.30.1.1) (421 aas)(P46096)
3) Small conductance Ca2+-activated K+ channel protein 1 (very similar to SK2 (1.A.1.16.1) (537 aas) (Q9EQR3)
4) Annexin II (A2) (339 aas) (P07356)
5) FGF1 Fibroblast Growth Factor; Heparin-binding growth factor (155 aas) (P61148)
6) Golgi reassembly - stacking protein 2, Gorasp2 or GRASP (451 aas) (Q99JX3)