A novel transport pathway for storage proteines

高等植物的种子积累了大量的贮藏蛋白。在种子成熟过程中，粗面内质网上合成的贮藏蛋白前体被分选到蛋白质贮藏泡中，在那里它们被转化为成熟形式并积累。我们发现一种新的囊泡--PAC囊泡介导了成熟南瓜种子中贮藏蛋白向PSV的大量转运。在PAC囊泡的膜上，我们发现PV 72，一种I型整合膜蛋白，由一个大的内腔结构域和一个跨膜结构域组成，随后是一个短的胞质尾部，包含一个潜在的基于酪氨酸的基序。PV 72在成熟的南瓜种子中特异性和瞬时积累，与贮藏蛋白的合成有关，但在营养组织中不表达。PV 72与南瓜2S白蛋白衍生的肽结合，并且在Ca^<2+>存在下也与南瓜2S白蛋白的前体结合。这些观察结果表明，PV 72作为南瓜种子中贮藏蛋白的液泡分选受体发挥功能。我们证明，PV 72的同源物，AtVSR 1/AtELP，拟南芥的功能作为存储蛋白的分选受体。alvsr 1突变体通过将贮藏蛋白从细胞中分泌出来而错误地分类贮藏蛋白，导致种子中细胞外空间扩大且电子密度高。我们的研究结果表明，在高等植物中，种子贮藏蛋白质通过受体介导的方式转运到蛋白质贮藏液泡中。

The seeds of higher plants accumulate large quantities of storage protein. During seed maturation, storage protein precursors synthesized on rough endoplasmic reticulum are sorted to protein storage vacuoles, where they are converted into the mature forms and accumulated. We found that novel vesicles, PAC (precursor-accumulating) vesicles, mediate the mass transport of storage proteins to PSVs in maturing pumpkin seeds, Isolated PAC vesicles contain a large amount of the precursors of major storage proteins. On the membrane of the PAC vesicles, we found PV72, a type I integral membrane protein composed of a large lumenal domain and a transmembrane domain followed by a short cytosolic tail that contains a potential tyrosine-based motif. PV72 is specifically and transiently accumulated in maturing pumpkin seeds in association with the synthesis of storage proteins, but is not expressed in vegetative tissues. PV72 bound to peptides derived from pumpkin 2S albumin and also to the precursor of pumpkin 2S albumin in the presence of Ca^<2+> These observations imply that PV72 functions as a vacuolar sorting receptor for storage proteins in pumpkin seeds. We demonstrated that a PV72 homologue, AtVSR1/AtELP, of Arabidopsis functions as a sorting receptor for storage proteins. The alvsr1 mutant mis-sorts storage proteins by secreting them from cells, resulting in an enlarged and electron-dense extracellular space in the seeds. Our findings demonstrate a receptor-mediated transport of seed storage proteins to protein storage vacuoles in higher plants.