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  • br Conflicts of interest br Acknowledgments The authors ackn


    Conflicts of interest
    Acknowledgments The authors acknowledge National Institute for Medical Research Development (NIMAD) project no. 940943 and National Research Institute for Science Policy (NRISP) no. 1456 for financial support of this work. AM appreciates National Institute for Genetic Engineering and Biotechnology grants. MR appreciates Cancer Biology Research Center grants. Atanas G. Atanasov acknowledges the support by the Polish KNOW (Leading National Research Centre) Scientific Consortium “Healthy Animal—Safe Food,” decision of Ministry of Science and Higher Education No. 05-1/KNOW2/2015. VKG would like to acknowledge European Union's Seventh Framework Programme for Research, Technological Development, and Demonstration under Grant Agreement no. 621364 (TUTIC-Green).
    Introduction The significance and its comprehensive nature of TOR (target of rapamycin) kinase signaling in the regulation of cell growth and proliferation in response to various physiological conditions including stresses and different developmental stages have been well established now in plants too, thanks to rapid progress and expansion in the related researches [[1], [2], [3], [4], [5], [6]]. The plant TOR kinase, which is highly homologous to the animal and yeast counterparts in its structure and the mode of action, also requires binding of Raptor protein for recruiting the substrates for phosphorylation [7,8]. It has been generally considered that, unlike the animals and yeasts, substrates of the plant TOR kinase do not carry a canonical motif called TOS motif, through which the binding with Raptor is mediated [[9], [10], [11]]. From our previous studies, however, we have identified a plant version of the TOS motif that was present in the N-terminal region of the plant ribosomal S6 kinase (S6K) [12]. The motif, in a context of about 12-amino-acid within which a five-amino-acid “core” PS 341 resembling the animal TOS motif sequence is included, was sufficient to provide a specific interaction with the plant Raptor protein when tested by the yeast two-hybrid analysis [13]. In this study, we searched the Arabidopsis genome for proteins containing this core sequence element of the plant TOS motif. Many proteins were identified as having this sequence in the exact PS 341 context of FSDVF as the one in the Arabidopsis S6K (AtS6K) and the orthologues in other plant taxa. Many more proteins having this element with only one or two analogous substitutions were also found. One of those proteins was the plant homologue of the autophagy-related protein 13 (ATG13), which is known to be a key component of the plant autophagy responses [14,15]. As the plant autophagy responses have been also known to be under the control of TOR signaling [16,17], in this study we wanted to examine if the plant autophagy is negatively regulated through phosphorylation of ATG13 by TOR kinase, as the same manner as reported in the animal and the yeast systems [18]. Our results showed that the regulation was dependent upon physical interaction of the ATG13 with Raptor, which would be an indication of the ATG13 being a direct phosphorylation target of TOR kinase. We also verified that the core sequence element of plant TOS motif identified in the Arabidopsis ATG13 played an essential role in mediating the interaction with Raptor, further solidifying our proposition for this sequence element as the central core of the plant TOS motif.
    Materials and methods
    Results and discussions
    Conflicts of interest
    Acknowledgments This work was supported by the Medical Research Center Program (No. 2011-0030074) through National Research Foundation (NRF) grant funded by the Korean government (MSIP).
    Autophagy in synapse formation and pruning Studies in genetic organisms indicate the importance of autophagy and autophagic machinery in early synaptic development. For instance, impairment of autophagy via the knockdown of ATG1, −2, −6, or −18 was found to reduce the number and size of presynaptic terminals at the neuromuscular junction (NMJ) in Drosophila melanogaster [1]. More recently, Stavoe and Colon-Ramos showed that autophagy is required cell-autonomously for presynapse formation in AIY neurons of C. elegans [2]. The authors found that autophagosome biogenesis occurred at developing AIY presynaptic boutons. They then observed that 18 distinct components from every stage of the autophagy pathway − initiation (UNC-15, ATG-13), nucleation (ATG-9), elongation (LGG-1, LGG-3, ATG-3, ATG-4), and retrieval (ATG-2, ATG-9, ATG-18)–were required for the clustering of synaptic vesicle (SV) proteins at nascent presynaptic sites during AIY neuron development [2]. However, these components were not required for SV clustering in other C. elegans neuron types [2], suggesting a cell-specific role for autophagic machinery in presynaptic development. These findings highlight the importance of autophagy for synaptogenesis, and the need for future studies to clarify its roles in different cell types and organisms.