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Cargo translocation pathways in conventional and unconventional secretion as well as CMA. (A) FGF2 self-translocation across the plasma membrane. With the assistance of ATP1A1, FGF2 binds to PI(4,5)P2 on the plasma membrane and is phosphorylated by the Tec kinase. FGF2 then oligomerizes to form a toroidal pore and translocates across the plasma membrane. The inside-out direction of translocation is achieved by FGF2 higher binding affinity to heparan sulfate proteoglycan (HSPG) which locates on the outside. (B) GSDMD-mediated pore formation and UPE cargo release. In response to stress signals, toxic compounds, or invasive pathogens, GSDMD is activated by proteolytic cleavage by the active caspase, leading to the release of N-terminal (NT) fragment and subsequently formation of large GSDMD pores. Cytokines and other inflammatory mediators are released through GSDMD pores. (C) The TMED10-channelled unconventional protein secretion (THU). The cytoplasmic HSP90A assists the unfolding of leaderless cargoes which bind to TMED10 on the ERGIC and induce the oligomerization of TMED10 to form a protein channel. With the help of HSP90B1, the cargoes are translocated into the lumen of the ERGIC through the TMED10 channel and are ready to be secreted via membrane trafficking. (D) The SEC61-mediated post-translational translocation in conventional secretion. The process begins with the targeting of polypeptide chain to the SEC61 channel, which is mediated by the chaperone HSP70 and its membrane binding partner (SEC62/63 complex). With the aid of ER-luminal BiP ATPase, the polypeptide is translocated unidirectionally from the cytosol into the ER lumen through the SEC61 channel. (E) Protein translocation in CMA. In CMA, the substrate bearing KFERQ motif is recognized by cytosolic chaperone HSC70 and cochaperones, and then translocated into the lysosome for degradation with the help of LAMP2A channel and the lysosomal HSC70 (lys-HSC70). (F) Protein translocation in MAPS. MAPS involves the selective release of misfolded cytosolic proteins delivered to the perinuclear compartment via the aid of ER-located USP19, chaperone HSC70, DNAJC5 and its regulator CD98hc. The detailed mechanism by which misfolded cargoes are translocated into the perinuclear compartment and subsequently release remains unclear.
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