An endosomal syntaxin and the AP-3 complex are required for formation and maturation of candidate lysosome-related secretory organelles (mucocysts) in Tetrahymena thermophila.
An endosomal syntaxin and the AP-3 complex are required for formation and maturation of candidate lysosome-related secretory organelles (mucocysts) in Tetrahymena thermophila.
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DOI：
10.1091/mbc.e17-01-0018
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发表时间：
2017-06-01
影响因子：
3.3
通讯作者：
Turkewitz AP
中科院分区：
文献类型：
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作者：
Kaur H;Sparvoli D;Osakada H;Iwamoto M;Haraguchi T;Turkewitz AP
溶酶体相关细胞器（LROs）是由分泌途径和内体运输途径汇合形成的分泌细胞器。在四膜虫中，类似于致密核颗粒的分泌囊泡是一类新的LROs，其合成依赖于异型融合所需的保守合成酶和成熟所需的AP-3。纤毛虫嗜热四膜虫合成称为黏液囊的大分泌囊泡。黏液囊的生物合成与动物细胞中的致密核颗粒（DCGs）具有相同的特征，包括在成熟过程中对货物蛋白的蛋白质水解处理。然而，其他分子特征表明与溶酶体相关细胞器（LROs）有关。LROs包括动物体内的多种细胞器，是通过分泌和内吞运输的融合而形成的。在这里，我们分析了四膜虫syntaxin 7-like 1 (Stx7l1p)，这是一个Qa-SNARE，其在其他谱系中的同源物与液泡/LROs有关。Stx7l1p针对未成熟和成熟的黏液囊肿，在黏液囊肿的形成中是必不可少的。在stx7l1敲除细胞中，两类主要的粘液囊货物蛋白独立定位，在大部分不重叠的囊泡中积累。因此，未成熟黏液囊的初始形成涉及异型融合，其中一部分黏液囊蛋白通过内溶酶体隔室传递。此外，我们发现随后的成熟需要AP-3，这是一种广泛参与LRO形成的复合物。敲除µ亚基基因不会阻碍任何已知粘液囊货物的运送，但仍然会阻止粘液囊的成熟。我们的数据表明，纤毛虫的分泌细胞器可能代表了一类新的LROs，并揭示了内体合成蛋白和AP-3在这种复杂隔室组装中的关键作用。
Lysosome-related organelles (LROs) are secretory organelles formed by convergence between secretory and endosomal trafficking pathways. In Tetrahymena, secretory vesicles that resemble dense core granules are a new class of LROs whose synthesis depends on a conserved syntaxin required for heterotypic fusion and AP-3 for maturation. The ciliate Tetrahymena thermophila synthesizes large secretory vesicles called mucocysts. Mucocyst biosynthesis shares features with dense core granules (DCGs) in animal cells, including proteolytic processing of cargo proteins during maturation. However, other molecular features have suggested relatedness to lysosome-related organelles (LROs). LROs, which include diverse organelles in animals, are formed via convergence of secretory and endocytic trafficking. Here we analyzed Tetrahymena syntaxin 7-like 1 (Stx7l1p), a Qa-SNARE whose homologues in other lineages are linked with vacuoles/LROs. Stx7l1p is targeted to both immature and mature mucocysts and is essential in mucocyst formation. In STX7L1-knockout cells, the two major classes of mucocyst cargo proteins localize independently, accumulating in largely nonoverlapping vesicles. Thus initial formation of immature mucocysts involves heterotypic fusion, in which a subset of mucocyst proteins is delivered via an endolysosomal compartment. Further, we show that subsequent maturation requires AP-3, a complex widely implicated in LRO formation. Knockout of the µ-subunit gene does not impede delivery of any known mucocyst cargo but nonetheless arrests mucocyst maturation. Our data argue that secretory organelles in ciliates may represent a new class of LROs and reveal key roles of an endosomal syntaxin and AP-3 in the assembly of this complex compartment.