A novel transport pathway for storage proteines

高等植物的种子积累了大量的贮藏蛋白质。在种子成熟过程中，粗面内质网合成的贮藏蛋白前体被分拣到蛋白质储藏液泡中，在那里转化为成熟的形式并积累。我们发现，在成熟的南瓜种子中，新的小泡PAC(前体积累)小泡介导了贮藏蛋白向PSV的质量运输，分离的PAC小泡含有大量主要贮藏蛋白的前体。在PAC囊泡的膜上，我们发现了PV72，这是一种I型完整的膜蛋白，由一个大的管腔结构域和一个跨膜结构域组成，紧随其后的是一条短的胞浆尾巴，其中包含一个潜在的酪氨酸基序。PV72在成熟的南瓜种子中特异地瞬时积累，与贮藏蛋白的合成有关，但在营养组织中不表达。PV72可与南瓜2S白蛋白的多肽结合，也可与南瓜2S白蛋白前体结合。这些结果表明，PV72是南瓜种子中贮藏蛋白的液泡分选受体。我们证明了拟南芥的PV72同源物AtVSR1/AtELP作为贮藏蛋白的分选受体发挥作用。Alvsr1突变体通过从细胞中分泌储存蛋白来错误地对它们进行分类，导致种子中细胞外空间扩大和电子密度高。我们的研究结果表明，在高等植物中，种子贮藏蛋白通过受体传递到蛋白质贮藏液泡。

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.