Signal peptides (SPs) are short N-terminal sequences that direct proteins to the endoplasmic reticulum (ER). After cleavage of the SP, these proteins are mostly trafficked to the Golgi apparatus for s Show more
Signal peptides (SPs) are short N-terminal sequences that direct proteins to the endoplasmic reticulum (ER). After cleavage of the SP, these proteins are mostly trafficked to the Golgi apparatus for secretion. Lipocalin-2 (LCN2), a neurotoxic secretory protein, was recently identified as a target of autophagy. The presence of an SP is a prerequisite for secretion and autophagic degradation. Based on these observations, we investigated whether the SP of LCN2 is sufficient to enable proteins to be secreted or degraded via autophagy. We fused the SP of LCN2 to a non-secretory green fluorescent protein (GFP) and found that this ER-generated GFP was either secreted or degraded via autophagy. These results indicate that the LCN2-derived SP alone is sufficient to direct proteins to the ER and subsequent secretion or autophagic degradation. This dual regulation was abolished when the SP was deleted from LCN2. Notably, the effect was preserved even when the LCN2 SP was replaced with the SP from brain-derived neurotrophic factor, another secretory protein. These results suggest that SPs with different sequences can similarly direct proteins to the ER and subsequent secretion or autophagic degradation. Furthermore, we found that even when LCN2 reached the Golgi apparatus for secretion, it could also be degraded via autophagy. Thus, we propose that SP-directed and ER-generated secretory proteins can undergo autophagic degradation during ER-Golgi transport, including at the ER, the ER-Golgi intermediate compartment, or the Golgi apparatus. Taken together, degradation of secretory proteins via autophagy suggests implications for the potential control of secretory protein homeostasis. Show less
PINK1-dependent activation of PRKN/parkin on depolarized mitochondria causes mitophagy. The deficiency of NME3, a nucleoside diphosphate kinase/NDPK on the outer mitochondria membrane (OMM), is associ Show more
PINK1-dependent activation of PRKN/parkin on depolarized mitochondria causes mitophagy. The deficiency of NME3, a nucleoside diphosphate kinase/NDPK on the outer mitochondria membrane (OMM), is associated with a fatal neurodegenerative disorder. Here, we report that NME3 deficiency impairs p-S65-ubiquitin (Ub)-dependent PRKN binding on depolarized mitochondria without involving the loss of Ub phosphorylation by PINK1. Our mechanistic investigation revealed that NME3 interacts with PLD6/MitoPLD to generate phosphatidic acid (PA) from cardiolipin on the OMM of damaged mitochondria after depolarization. This lipid signal is essential for positioning MFN2 nearby PINK1 for phosphorylation of Ub conjugates on MFN2, thus enabling the subsequent amplification of PRKN binding to mitochondria. We provide further evidence that mitochondria-endoplasmic reticulum (Mito-ER) tethering prohibits the proximity of MFN2 with PINK1 and PRKN amplification on mitochondria. Importantly, the loss of NME3-regulated PA signal causes Mito-ER tethering. Overall, our findings suggest that NME3 cooperates with PLD6 to generate PA as a critical step in Mito-ER untethering, allowing MFN2 access to PINK1 for p-S65-poly-Ub-dependent feedforward activation of PRKN. Show less