Regulation of starvation-induced autophagy through the interaction between Metadherin and RAF kinase inhibitory protein
Trang Lai, Sahib Zada, Huynh Nguyen, Mahmoud Ahmed, Jin Seok Hwang and Deok Ryong Kim
Department of Biochemistry and Convergence Medical Sciences, School of Medicine, Gyeongsang National University, Jinju, Republic of Korea 527-27
Autophagy, a self-degradation process, mediates cell survival or cell death in response to cellular stresses through the lysosome-dependent degradation. It can be activated by the low cellular ATP level sensed by AMP-activating protein kinase (AMPK) under starvation. Recently, we suggested that RAF kinase inhibitory protein (RKIP) could act as a negative regulator of autophagy induced by starvation. Metadherin (MTDH), originally identified as astrocyte elevated gene (AEG1), plays an important role in regulation of several signal pathways. Furthermore, it has been suggested that it could promote autophagy under starvation by activating the AMPK pathway. In this study, we identified MTDH as a RKIP-binding protein by yeast two-hybrid system. We further confirmed that it bound specifically to RKIP using bimolecular fluorescence complementation and co-immunoprecipitation analyses. MTDH was also co-localized with RKIP in the cytoplasm, and this co-localization was highly elevated at the starvation condition. MTDH significantly up-regulated several autophagy markers: LC3-II production, p62 degradation, AMPK activation, and etc, through modulation of RKIP binding. We also identified that the interaction between RKIP and MTDH increased LC3-lipidation with PE necessary for formation of autophagosomes in cellular stresses. In summary, this study indicates that MTDH functions as an autophagy activator by sequestering the inhibitory protein of autophagy such as RKIP. Furthermore, the RKIP-MTDH complex could provide an important platform for the further understanding of autophagy linked to other signaling pathways.
Keywords: Autophagy, RKIP, Metadherin, LC3-lipidation, AMPK