Sauer M K, Kelly R B
Hormone Research Institute, University of California, San Francisco 94143, USA.
J Eukaryot Microbiol. 1995 Mar-Apr;42(2):173-83. doi: 10.1111/j.1550-7408.1995.tb01559.x.
Tetrahymena thermophila possesses a regulated secretory pathway in which mucin proteins are stored in dense-core granules, called mucocysts. Exocytosis-defective mutants exist that fail to secrete mucin in response to secretagogues. Four of the mutants (SB281, SB283, SB285 and SB715) appear to be blocked at different steps of the regulated secretory pathway. SB281 and SB285 accumulate mucin proteins in heterogeneous cytoplasmic organelles which have not yet been identified; SB283 makes mucocyst-like structures but they contain no immunologically identifiable 80-kDa or 50-kDa mucin proteins; and SB715 has more than normal amounts of immature and undocked mucocysts. The organelles that accumulate in exocytosis-defective mutants could be either normal intermediates in the biosynthetic pathway or aberrant structures that form as a result of the mutations. We have used conjugation rescue to analyze steps in the biogenesis of exocytosis-competent mucocysts and to identify functional intermediates. The cytoplasmic organelles that accumulate in SB281 appear to be unidentified biosynthetic intermediates, and the defect is in a cytosolic protein essential for mucocyst maturation. The organelles which accumulate in the other mutants are likely biosynthetic, but their mutations are in proteins which are labile or not free to diffuse into the mutant conjugant.
嗜热四膜虫拥有一条受调控的分泌途径，其中黏蛋白被储存在称为黏液囊泡的致密核心颗粒中。存在胞吐作用缺陷型突变体，它们无法响应促分泌剂而分泌黏蛋白。其中四个突变体（SB281、SB283、SB285和SB715）似乎在受调控的分泌途径的不同步骤被阻断。SB281和SB285在尚未鉴定的异质性细胞质细胞器中积累黏蛋白；SB283形成类似黏液囊泡的结构，但其中不含免疫学上可识别的80 kDa或50 kDa黏蛋白；并且SB715具有比正常数量更多的未成熟和未对接的黏液囊泡。在胞吐作用缺陷型突变体中积累的细胞器可能是生物合成途径中的正常中间体，也可能是由突变形成的异常结构。我们利用接合拯救来分析具有胞吐能力的黏液囊泡生物发生过程中的步骤，并鉴定功能中间体。在SB281中积累的细胞质细胞器似乎是未鉴定的生物合成中间体，缺陷在于黏液囊泡成熟所必需的一种胞质蛋白。在其他突变体中积累的细胞器可能是生物合成的，但它们的突变存在于不稳定或无法自由扩散到突变接合体中的蛋白质中。
