Gerasimaitė Rūta, Sharma Shruti, Desfougères Yann, Schmidt Andrea, Mayer Andreas
Department of Biochemistry, University of Lausanne, Ch. des Boveresses 155, 1066 Epalinges, Switzerland.
Department of Biochemistry, University of Lausanne, Ch. des Boveresses 155, 1066 Epalinges, Switzerland
J Cell Sci. 2014 Dec 1;127(Pt 23):5093-104. doi: 10.1242/jcs.159772. Epub 2014 Oct 14.
Eukaryotes contain inorganic polyphosphate (polyP) and acidocalcisomes, which sequester polyP and store amino acids and divalent cations. Why polyP is sequestered in dedicated organelles is not known. We show that polyP produced in the cytosol of yeast becomes toxic. Reconstitution of polyP translocation with purified vacuoles, the acidocalcisomes of yeast, shows that cytosolic polyP cannot be imported, whereas polyP produced by the vacuolar transporter chaperone (VTC) complex, an endogenous vacuolar polyP polymerase, is efficiently imported and does not interfere with growth. PolyP synthesis and import require an electrochemical gradient, probably as a driving force for polyP translocation. VTC exposes its catalytic domain to the cytosol and carries nine vacuolar transmembrane domains. Mutations in the VTC transmembrane regions, which are likely to constitute the translocation channel, block not only polyP translocation but also synthesis. Given that they are far from the cytosolic catalytic domain of VTC, this suggests that the VTC complex obligatorily couples synthesis of polyP to its import in order to avoid toxic intermediates in the cytosol. Sequestration of otherwise toxic polyP might be one reason for the existence of acidocalcisomes in eukaryotes.
真核生物含有无机多聚磷酸盐（多聚P）和酸性钙小体，后者可隔离多聚P并储存氨基酸和二价阳离子。多聚P为何被隔离在特定细胞器中尚不清楚。我们发现，酵母细胞质中产生的多聚P具有毒性。用纯化的液泡（酵母的酸性钙小体）重建多聚P转运过程，结果表明细胞质中的多聚P无法被导入，而由液泡转运伴侣（VTC）复合物（一种内源性液泡多聚P聚合酶）产生的多聚P则能被有效导入且不影响生长。多聚P的合成和导入需要电化学梯度，这可能是多聚P转运的驱动力。VTC将其催化结构域暴露于细胞质中，并带有九个液泡跨膜结构域。VTC跨膜区域的突变（这些区域可能构成转运通道）不仅会阻断多聚P的转运，还会阻断其合成。鉴于这些区域距离VTC的细胞质催化结构域较远，这表明VTC复合物必须将多聚P的合成与其导入过程耦合起来，以避免细胞质中产生有毒中间体。隔离原本有毒的多聚P可能是真核生物中存在酸性钙小体的一个原因。
