title: Rabin8 suppresses autophagosome formation independently of its guanine nucleotide-exchange activity towards Rab8.
abstract: Macroautophagy is a bulk degradation system conserved from yeast to human. In budding yeast, the guanine nucleotide-exchange factor (GEF) Sec2p is required for autophagy. We examined the role of Rabin8 (a mammalian ortholog of Sec2p) with Rab8-GEF activity in autophagy in mammalian cells. Unexpectedly, depletion of Rabin8 promoted nutrient starvation-induced autophagosome formation, indicating that Rabin8 suppresses autophagosome formation. Depletion of Rab8 did not affect autophagosome formation, and expression of a Rabin8 GEF-domain mutant reverted the Rabin8 depletion-induced increase in autophagosomes, indicating that Rabin8 suppresses autophagosome formation independently of its Rab8-GEF activity. Nuclear Dbf2-related (NDR) kinases phosphorylate Rabin8 at Ser-272. The non-phosphorylatable Rabin8-S272A mutant did not revert the Rabin8 depletion-induced increase in autophagosomes, suggesting that Ser-272 phosphorylation of Rabin8 is involved in its suppressive function in autophagy. Depletion of NDR kinases enhanced autophagosome formation and reduced mammalian/mechanistic target of rapamycin complex 1 (mTORC1) activity, suggesting that NDR kinases suppress autophagosome formation by increasing mTORC1 activity, in addition to phosphorylating Rabin8. Expression of a C-terminal fragment of Rabin8, but not that of Sec2p, suppressed nutrient starvation-induced autophagosome formation. Thus, contrary to the stimulative role of yeast Sec2p, Rabin8 has a suppressive function in autophagy in mammalian cells through its non-conserved C-terminal region.
authors: Laboratory of Molecular Cell Biology, Department of Biomolecular Sciences, Graduate School of Life Sciences, Tohoku University, Sendai, Miyagi 980-8578, Japan; and.
authors: Yuta
authors: Y
authors: Amagai
authors: Laboratory of Membrane Trafficking Mechanisms, Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, Miyagi 980-8578, Japan.
authors: Takashi
authors: T
authors: Itoh
authors: Laboratory of Membrane Trafficking Mechanisms, Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, Miyagi 980-8578, Japan.
authors: Mitsunori
authors: M
authors: Fukuda
authors: Laboratory of Molecular Cell Biology, Department of Biomolecular Sciences, Graduate School of Life Sciences, Tohoku University, Sendai, Miyagi 980-8578, Japan; and kmizuno@biology.tohoku.ac.jp.
authors: Kensaku
authors: K
authors: Mizuno
mesh_terms: D000818:Animals; D001343:Autophagy; D002460:Cell Line; D000081410:Germinal Center Kinases; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D006801:Humans; D000076222:Mechanistic Target of Rapamycin Complex 1; D051379:Mice; D046912:Multiprotein Complexes / Q000378:metabolism; D010588:Phagosomes / Q000378:metabolism*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism; D020691:rab GTP-Binding Proteins / Q000378:metabolism*
chemical_list: D000081410:Germinal Center Kinases; D020662:Guanine Nucleotide Exchange Factors; D046912:Multiprotein Complexes; D034741:RNA, Small Interfering; C064456:SEC2 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D000076222:Mechanistic Target of Rapamycin Complex 1; D017346:Protein Serine-Threonine Kinases; C496014:STK38L protein, human; D058570:TOR Serine-Threonine Kinases; C082377:RAB8A protein, human; D020691:rab GTP-Binding Proteins
keywords: NDR; Rab8; Rabin8; autophagy; mTORC1