Litvinov S V, Hilkens J
Division of Tumor Biology, The Netherlands Cancer Institute (Antoni van Leeuwenhoekhuis, Amsterdam.
J Biol Chem. 1993 Oct 5;268(28):21364-71.
Episialin, the sialomucin encoded by the MUC1 gene is abundantly present at the surface of various epithelial cells. It is a transmembrane molecule that matures through several intermediate forms generated by proteolysis and sequential addition and processing of numerous O-linked glycans. Here we demonstrate that one of the biosynthetic intermediates, the premature form, which is incompletely glycosylated appears at the cell surface. By neuraminidase protection assays we show that cell surface-associated forms of episialin are constitutively internalized (0.9% of surface fraction/min) and recycled, with the intracellular residence time and total cycle time being approximately 66 and 143 min, respectively. During recycling, sialic acid residues are added to the premature form of episialin resulting in complete maturation of the molecule. Complete sialylation of episialin requires several rounds of recycling which continues after sialylation is apparently completed. The recycling of episialin may serve to achieve and maintain a high degree of sialylation.
Episialin是由MUC1基因编码的涎粘蛋白，大量存在于各种上皮细胞表面。它是一种跨膜分子，通过蛋白水解以及众多O-连接聚糖的依次添加和加工所产生的几种中间形式而成熟。在这里，我们证明了生物合成中间体之一，即糖基化不完全的早熟形式，出现在细胞表面。通过神经氨酸酶保护试验，我们表明Episialin与细胞表面相关的形式持续内化（每分钟表面部分的0.9%）并循环利用，细胞内停留时间和总循环时间分别约为66分钟和143分钟。在循环利用过程中，唾液酸残基被添加到Episialin的早熟形式中，导致分子完全成熟。Episialin的完全唾液酸化需要几轮循环利用，在唾液酸化明显完成后仍会继续。Episialin的循环利用可能有助于实现和维持高度的唾液酸化。
