Abstract
SummaryAutophagy plays key roles in development, oncogenesis, cardiovascular, metabolic, and neurodegenerative diseases. Hence, understanding how autophagy is regulated can reveal opportunities to modify autophagy in a disease-relevant manner. Ideally, one would want to functionally define autophagy regulators whose enzymatic activity can potentially be modulated. Here, we describe the STK38 protein kinase (also termed NDR1) as a conserved regulator of autophagy. Using STK38 as bait in yeast-two-hybrid screens, we discovered STK38 as a novel binding partner of Beclin1, a key regulator of autophagy. By combining molecular, cell biological, and genetic approaches, we show that STK38 promotes autophagosome formation in human cells and in Drosophila. Upon autophagy induction, STK38-depleted cells display impaired LC3B-II conversion; reduced ATG14L, ATG12, and WIPI-1 puncta formation; and significantly decreased Vps34 activity, as judged by PI3P formation. Furthermore, we observed that STK38 supports the interaction of the exocyst component Exo84 with Beclin1 and RalB, which is required to initiate autophagosome formation. Upon studying the activation of STK38 during autophagy induction, we found that STK38 is stimulated in a MOB1- and exocyst-dependent manner. In contrast, RalB depletion triggers hyperactivation of STK38, resulting in STK38-dependent apoptosis under prolonged autophagy conditions. Together, our data establish STK38 as a conserved regulator of autophagy in human cells and flies. We also provide evidence demonstrating that STK38 and RalB assist the coordination between autophagic and apoptotic events upon autophagy induction, hence further proposing a role for STK38 in determining cellular fate in response to autophagic conditions.
Highlights
Y Current Biology 25, 2479–2492, October 5, 2015 a2015 The Authors 2479 counterpart of human STK38, suggesting that human and fly STK38 can share identical cellular functions
Cell biological, and genetic approaches, we show that STK38 promotes autophagosome formation in human cells and in Drosophila
Upon studying the activation of STK38 during autophagy induction, we found that STK38 is stimulated in a MOB1- and exocyst-dependent manner
Summary
Autophagy plays key roles in development, oncogenesis, cardiovascular, metabolic, and neurodegenerative diseases. Autophagy induction triggers activation of the RalB GTPase and binding to Exo, a subunit of the exocyst complex This in turn leads to the recruitment of the Beclin1/Vps complex to nascent autophagosomes, thereby promoting autophagosome formation [10]. Ser281 auto-phosphorylation on the T-loop of STK38 is stimulated by binding of MOB1A/B to an N-terminal regulatory (NTR) domain of STK38, whereas Thr444 phosphorylation in the hydrophobic motif of STK38 is performed by members of the MST kinase family [12] Phosphorylation of both sites is required for STK38 activation and plays a role in apoptosis and cell-cyclerelated processes [12]. Our study further revealed that Trc (the fly NDR kinase) is required for autophagy in Drosophila, indicating that the autophagic role of STK38 is conserved from flies to humans. Because hyperactivation of STK38 results in STK38-dependent apoptosis under prolonged autophagy conditions, we discovered that STK38 assists the coordination between autophagic and apoptotic events upon autophagy induction
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