Mann Laboratory, Department of Vegetable Crops, University of California, Davis, California 95616.
Plant Physiol. 1986 Jul;81(3):830-5. doi: 10.1104/pp.81.3.830.
The biosynthesis and processing of cellulase from ripening avocado fruit was studied. The mature protein is a glycoprotein, as judged by concanavalin A binding, with a molecular weight of 54,200. Upon complete deglycosylation by treatment with trifluoromethane sulfonic acid the mature protein has a molecular weight of 52,800 whereas the immunoprecipitated in vitro translation product has a molecular weight of 54,000. This result indicates that cellulase is synthesized as a large molecular weight precursor, which presumably possesses a short-lived signal peptide. A membrane-associated and heavily glycosylated form of the protein was also identified. This putative secretory precursor was enzymically active and the carbohydrate side chains were sensitive to endoglycosidase H cleavage. Results of partial endoglycosidase H digestion suggest that this precursor form of the mature glycoprotein possesses two high-mannose oligosaccharide side chains. The oligosaccharide chains of the mature protein were insensitive to endoglycosidase H cleavage, indicating that transport of the membrane-associated cellulase to the cell wall was accompanied by modification of the oligosaccharide side chains. The presence of a large pool of endoglycosidase H-sensitive membrane-associated cellulase (relative to an endoglycosidase H-insensitive form) suggest that transit of this protein through the Golgi is rapid relative to transit through the endoplasmic reticulum.
本研究旨在探讨鳄梨果实成熟过程中纤维素酶的生物合成与加工。经刀豆球蛋白 A 结合实验判断，成熟蛋白是一种糖蛋白，分子量为 54200。经三氟甲磺酸完全糖基化修饰后，成熟蛋白的分子量为 52800，而体外免疫沉淀翻译产物的分子量为 54000。这一结果表明，纤维素酶是以大分子量前体的形式合成的，该前体可能具有短命的信号肽。还鉴定出一种膜相关的、高度糖基化的蛋白质形式。这种假定的分泌前体具有酶活性，且其碳水化合物侧链对内切糖苷酶 H 的切割敏感。部分内切糖苷酶 H 消化的结果表明，这种成熟糖蛋白的前体形式具有两条高甘露糖寡糖侧链。成熟蛋白的寡糖链对内切糖苷酶 H 的切割不敏感，表明该膜相关纤维素酶向细胞壁的运输伴随着寡糖侧链的修饰。大量的内切糖苷酶 H 敏感的膜相关纤维素酶（相对于内切糖苷酶 H 不敏感的形式）的存在表明，该蛋白通过高尔基体的转运速度相对较快，而通过内质网的转运速度较慢。
