@misc{oai:ir.soken.ac.jp:00001373,
 author = {濱崎, 万穂 and ハマサキ, マホ and HAMASAKI, Maho},
 month = {2016-02-17},
 note = {Yeast cells are constantly challenged by the changes in their external environments thus they must be well-equipped to rapidly adapt their internal system for their survival.  Most frequently, yeast cells face the nutritionally challenging conditions;  they overcome such stress by altering several of their cellular mechanisms, including those leading to cell growth arrest, which are strongly influenced by the secretory pathway.  One aspect of this cellular adaptation is achieved by the reorganization of genomic expression that has been studied by many groups;  expression of genes required in protein synthesis and/or uptake of specific amino acids are reduced whereas genes that encode hydrolases are induced.  Another aspect of adaptation to such stress is carried through the dynamic changes in membrane trafficking, which has not been given much attention.  An example of such change is that an amino acid transporter already expressed at the plasma membrane is endocytosed and transported to the vacuole for degradation under starvation to reduce an amino acid uptake.  In recent years, autophagy is discovered as one of the starvation responses in yeast and delivers cytoplasmic components to the vacuole for degradation mediated by a double membrane structure called autophagosome.  Degrading proteins in the vacuole release free amino acids, which are used to rebuild the internal system to fit into a new environment, thus degradation plays an important role in adapting to starvation stress.  She studied from the two points of view to understand the membrane dynamics during starvation:  one is by observing the changes induced by such stress in the secretory pathway and another by investigating the involvement of factors engaged in membrane trafficking in autophagy.<br />  First part of this study shows the involvement of an early secretory flow by the studies of COPII factors which function in the formation of C0PII vesicles that bud from the ER in autophagy.  The defect in autophagosome formation was found in sec12, sec16, sec23 and sec24 mutants but not in the sec13 and sec31 mutant cells.  This unanticipated result puzzled me to understand how only the particular COPII components were involved in autophagy.  She coped to find their relations by converging my interest on an active flow created by COPII vesicles and not on the specific role of each COPII components.  The autophagic defect in sec24 deleted mutant cells was suppressed upon the recovery of its secretory flow by the overexpression of its homologue, Sfb2p.  She also found that the starvation stress suppressed the secretory defect of sec13 and sec31 mutant cells.  These observations reveal that an active flow in the carly secrctory pathway plays an important role in autophagy;  autophagy proceeds in the presence but not in the absence of the early' secretory flow.  Both autophagy and its closely related the cytoplasm to vacuole-targeting (Cvt) pathway occur through the pre-autophagosomal structure (PAS), and since the PAS and the functional Cvt pathway exist in all sec mutants, the early secretory pathway must be involved specifically in autophagy, subsequent to the PAS formation.<br />  The second part of this study shows the effects of starvation on the secretory pathway by following the dynamics of several organelle markers.  Three major changes were found to occur at the site of ER, Golgi and the Golgi to the plasma membrane transport pathway.  First, starvation induced the degradation of ER fragments in the vacuole.  Autophagosomes engulfed both lumenal and membrane ER marker proteins (HDEL fusion protein and Sec71p) with high frequencies, indicating that autophagy, a bulk degradation of cytoplasmic constituents, mediated this degradation.  Second, starvation provoked the transfer of a significant amount of cis-Golgi proteins to the vacuole.  The well-regulated localization of membrane proteins (Rer1p and Wbp1p) was distorted in autophagy independent but the vacuolar protein sorting pathway dependent manner.  Third, starvation shifted the default destination of soluble proteins of the secretory pathway (carboxypeptidase Y in Δvps10 cells) from extracellular to the vacuole. These data demonstrate that proteins of the secretory pathway are redirected to the degradative pathway by adaptation to a stressful environment.<br />  Last part of this study shows the possible involvement of the vacuole membrane in autophagosome formation.  FM4-64, a lipophilic styryl dye that is used to stain the vacuole membrane, also stained autophagic bodies accumulated in wild-type and Δpep4 mutant cells but not in Δfab1 and Δvac14 mutant cells that have defect in the backflow from the vacuole.  It also stained autophagosome accumulated in sec18 ts mutant cells that have a defect at the fusion step of autophagosome and the vacuole, but not in Δvam5 mutant cells that have a defect in the vacuole morphology.  These results indicate that the backflow from the vacuole is involved in autophagosome formation.  This is another membrane flow found to function in membrane biogenesis of autophagy.<br />  In this study, she examined and discussed cellular responses to starvation stress from the view of membrane trafficking.  It is quite a dynamic phenomenon;  starvation induces not only autophagy but also many changes in the secretory pathway.  The topic on the membrane biogenesis of autophagy found throughout this study is also discussed., 総研大甲第691号},
 title = {Studies on membrane dynamics under starvation in yeast},
 year = {}
}