Huffaker R C
Plant Growth Laboratory and Department of Agronomy and Range Science, University of California, Davis 95616.
New Phytol. 1990;116:199-231. doi: 10.1111/j.1469-8137.1990.tb04710.x.
Although information has rapidly developed concerning the intracellular localization of plant proteins, relatively few reports concern the intracellular location of endo- and exo-proteolytic activities. Relatively few proteases have been purified, characterized, and associated with a specific cellular location. With the exception of the processing proteases involved in transport of proteins across membranes, little progress has yet been made concerning determination of in vivo products of specific proteases. Information on the turnover of individual proteins and the assessment of rate-limiting steps in pathways as proteins are turned over is steadily appearing. Since chloroplasts are the major site of both protein synthesis and, during senescence, degradation, it was important to show unambiguously that chloroplasts can degrade their own constituents. Another important contribution was to obtain evidence that the chloroplasts contain proteases capable of degrading their constituents. This work has been more tenuous because of the low activities found and the possibility of contamination by vacuolar enzymes during the isolation of organelles. The possible targeting of cytoplasmic proteins for degradation by facilitating their transport into vacuoles is a field which hopefully will develop more rapidly in the future. Information on targeting of proteins for degradation via the ubiquitin (Ub) degradation pathway is developing rapidly. Future research must determine how much unity exists across the different eukaryotic systems. At present, it has important implications for protein turnover in plants, since apparently Ub is involved in the degradation of phytochrome. Little information has been developed regarding what triggers increased proteolysis with the onset of senescence, although it appears to involve protein synthesis. Thus far, the evidence indicates that the complement of proteases prior to senescence is sufficient to carry out the observed protein degradation. This field of study has great practical implications, e.g. maintaining photosynthesis during seed-fill in order to obtain greater crop yields. The current use of stay green' variants in the populations of several crop plants to produce increased yields shows the potential for future development. The near future should see exciting discoveries in these areas of research that will have far reaching effects on the construction of transgenic plants for future research accomplishments and agricultural use.
尽管关于植物蛋白的细胞内定位已有迅速发展的信息，但关于内切和外切蛋白水解活性的细胞内定位的报道相对较少。相对较少的蛋白酶已被纯化、表征并与特定的细胞位置相关联。除了参与蛋白质跨膜转运的加工蛋白酶外，在确定特定蛋白酶的体内产物方面进展甚微。关于单个蛋白质的周转以及在蛋白质周转过程中途径中限速步骤的评估信息正在不断涌现。由于叶绿体是蛋白质合成以及衰老期间降解的主要场所，明确证明叶绿体能够降解其自身成分很重要。另一个重要贡献是获得证据表明叶绿体含有能够降解其成分的蛋白酶。由于发现的活性较低以及在细胞器分离过程中存在被液泡酶污染的可能性，这项工作一直比较薄弱。通过促进细胞质蛋白转运到液泡中从而可能将其靶向降解是一个有望在未来更快发展的领域。关于通过泛素（Ub）降解途径将蛋白质靶向降解的信息正在迅速发展。未来的研究必须确定不同真核系统之间存在多大程度的一致性。目前，这对植物中的蛋白质周转具有重要意义，因为显然Ub参与了光敏色素的降解。关于衰老开始时是什么引发蛋白水解增加的信息很少，尽管这似乎涉及蛋白质合成。迄今为止，证据表明衰老前蛋白酶的补充足以进行观察到的蛋白质降解。这个研究领域具有重大的实际意义，例如在种子灌浆期间维持光合作用以获得更高的作物产量。目前在几种作物群体中使用“保持绿色”变体来提高产量显示了未来发展的潜力。在不久的将来，这些研究领域应该会有令人兴奋的发现，这将对用于未来研究成果和农业用途的转基因植物的构建产生深远影响。
