Toyooka K, Okamoto T, Minamikawa T
Department of Biological Sciences, Tokyo Metropolitan University, Tokyo, 192-0397 Japan.
J Cell Biol. 2001 Sep 3;154(5):973-82. doi: 10.1083/jcb.200105096. Epub 2001 Aug 27.
alpha-Amylase is expressed in cotyledons of germinated Vigna mungo seeds and is responsible for the degradation of starch that is stored in the starch granule (SG). Immunocytochemical analysis of the cotyledon cells with anti-alpha-amylase antibody showed that alpha-amylase is transported to protein storage vacuole (PSV) and lytic vacuole (LV), which is converted from PSV by hydrolysis of storage proteins. To observe the insertion/degradation processes of SG into/in the inside of vacuoles, ultrastructural analyses of the cotyledon cells were conducted. The results revealed that SG is inserted into LV through autophagic function of LV and subsequently degraded by vacuolar alpha-amylase. The autophagy for SG was structurally similar to micropexophagy detected in yeast cells. In addition to the autophagic process for SG, autophagosome-mediated autophagy for cytoplasm and mitochondria was detected in the cotyledon cells. When the embryo axes were removed from seeds and the detached cotyledons were incubated, the autophagosome-mediated autophagy was observed, but the autophagic process for the degradation of SG was not detected, suggesting that these two autophagic processes were mediated by different cellular mechanisms. The two distinct autophagic processes were thought to be involved in the breakdown of SG and cell components in the cells of germinated cotyledon.
α-淀粉酶在发芽的绿豆种子子叶中表达，负责降解储存在淀粉粒（SG）中的淀粉。用抗α-淀粉酶抗体对子叶细胞进行免疫细胞化学分析表明，α-淀粉酶被转运到蛋白质储存液泡（PSV）和溶酶体液泡（LV），后者是由PSV通过储存蛋白的水解作用转化而来的。为了观察SG插入/降解到液泡内部的过程，对子叶细胞进行了超微结构分析。结果显示，SG通过LV的自噬功能插入LV，随后被液泡α-淀粉酶降解。SG的自噬在结构上类似于在酵母细胞中检测到的微过氧化物酶体自噬。除了SG的自噬过程外，在子叶细胞中还检测到自噬体介导的细胞质和线粒体自噬。当从种子中去除胚轴并培养分离的子叶时，观察到自噬体介导的自噬，但未检测到SG降解的自噬过程，这表明这两个自噬过程是由不同的细胞机制介导的。这两个不同的自噬过程被认为参与了发芽子叶细胞中SG和细胞成分的分解。
