Centre for Organismal Studies, University of Heidelberg, Im Neuenheimer Feld 230, D-69120, Heidelberg, Germany.
Centre for Cell & Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China.
Protoplasma. 2016 Jan;253(1):31-43. doi: 10.1007/s00709-015-0887-1. Epub 2015 Sep 26.
Unconventional protein secretion (UPS) is a collective term for mechanisms by which cytosolic proteins that lack a signal peptide ("leaderless secretory proteins" (LSPs)) can gain access to the cell exterior. Numerous examples of UPS have been well documented in animal and yeast cells. In contrast, our understanding of the mechanism(s) and function of UPS in plants is very limited. This review evaluates the available literature on this subject. The apparent large numbers of LSPs in the plant secretome suggest that UPS also occurs in plants but is not a proof. Although the direct transport of LSPs across the plant plasma membrane (PM) has not yet been described, it is possible that as in other eukaryotes, exosomes may be released from plant cells through fusion of multivesicular bodies (MVBs) with the PM. In this way, LSPs, but also small RNAs (sRNAs), that are passively taken up from the cytosol into the intraluminal vesicles of MVBs, could reach the apoplast. Another possible mechanism is the recently discovered exocyst-positive organelle (EXPO), a double-membrane-bound compartment, distinct from autophagosomes, which appears to sequester LSPs.
非经典蛋白质分泌（UPS）是一个术语，用于描述细胞质蛋白（缺乏信号肽的“无信号肽分泌蛋白”（LSPs））进入细胞外环境的机制。UPS 在动物和酵母细胞中的许多例子已经得到了很好的记录。相比之下，我们对 UPS 在植物中的机制和功能的理解非常有限。这篇综述评估了这个主题的现有文献。植物分泌组中大量的 LSP 表明 UPS 也发生在植物中，但这并不是一个证明。尽管 LSPs 直接穿过植物质膜（PM）的运输尚未被描述，但有可能像在其他真核生物中一样，通过多泡体（MVBs）与 PM 的融合，植物细胞可能会释放出外泌体。通过这种方式，LSPs，以及从小细胞质中被动摄取到 MVBs 腔中的小 RNA（sRNA），可以到达细胞外质。另一种可能的机制是最近发现的具有外泌体阳性细胞器（EXPO），这是一种与自噬体不同的双层膜结合的隔室，似乎可以隔离 LSPs。
