STRUCTURE, BIOSYNTHESIS AND FUNCTION OF GLYCOPROTEINS

我们最近的努力集中在研究生物合成和 将新合成的溶酶体酶靶向溶酶体。 的 溶酶体酶的天冬酰胺连接的寡糖单元经历 作为新合成的酶， 从粗面内质网开始， 糖基化，最终到达溶酶体。 最重要的 修饰是磷酸甘露糖基识别标记的产生， 这分两步进行。 首先，N-乙酰葡糖胺1-磷酸是 通过UDP-N-乙酰葡糖胺转移至受体甘露糖：溶酶体 酶N-乙酰葡糖胺1-磷酸转移酶，产生磷酸盐 外部N-乙酰葡糖胺和内部N-乙酰葡糖胺之间的二酯键中的基团 甘露糖 接下来，磷酸二酯糖苷酶除去 N-乙酰葡糖胺，留下磷酸单酯键连接到 潜在的甘露糖残基。 该暴露的磷酸甘露糖基残基用作 识别标志物的一种基本成分，其导致与 高亲和力受体和随后的易位到溶酶体。 的 转移酶特异性作用于溶酶体酶，从而催化 最初的决定性步骤，通过这些步骤，新合成的溶酶体酶被 与其他新合成的糖蛋白不同， 最终靶向溶酶体。 我们发现去糖基化 溶酶体酶是转移酶的有效抑制剂，表明 这些酶含有共同的蛋白质序列或构象， 被转移酶识别。 我们目前的实验是针对 这个识别信号的识别。 我们的初步结果 表明溶酶体酶的构象对于 识别信号的表达。 我们还发现了一些 缺乏215 Kda Man-6-P受体但具有 高水平的细胞内酸水解酶，其包含在致密的 溶酶体特有的颗粒。 我们最近发现， 在这些细胞中分离出第二种Man-6-P受体。 这个受体，一个46 Kda 糖蛋白广泛分布于细胞类型中。 我们正在努力 确定每个受体的生理功能。 （一）

We have focused our recent efforts on studying the biosynthesis and targeting of newly synthesized lysosomal enzymes to lysosomes. The asparagine-linked oligosaccharide units of lysosomal enzymes undergo an extensive series of processing reactions as the newly synthesized enzymes move from the rough endoplasmic reticulum, where they are initially glycosylated, to their final destination in lysosomes. The most important modification is the generation of the phosphomannosyl recognition marker, which occurs in two steps. First, N-acetylglucosamine 1-phosphate is transferred to an acceptor mannose by UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine 1-phosphotransferase, resulting in a phosphate group in diester linkage between outer N-acetylglucosamine and an inner mannose. Next, a phosphodiester glycosidase removes the N-acetylglucosamine, leaving the phosphate in monoester linkage to the underlying mannose residue. This exposed phosphomannosyl residue serves as an essential component of a recognition marker which leads to binding to high-affinity receptors and subsequent translocation to lysosomes. The transferase acts specifically on lysosomal enzymes, thereby catalyzing the initial, determining step by which newly synthesized lysosomal enzymes are distinguished from other newly synthesized glycoproteins and marked for eventual targeting to lysosomes. We have found that deglycosylated lysosomal enzymes are potent inhibitors of the transferase, indicating that these enzymes contain a common protein sequence or conformation that is recognized by the transferase. Our current experiments are directed toward the identification of this recognition signal. Our preliminary results indicate that the conformation of the lysosomal enzymes is important for the expression of the recognition signal. We have also identified a number of murine cell lines that lack the 215 Kda Man-6-P receptor and yet possess high levels of intracellular acid hydrolases which are contained in dense granules characteristic of lysosomes. We have recently identified and isolated a second Man-6-P receptor in these cells. This receptor, a 46 Kda glycoprotein, is widely distributed among cell types. We are trying to determine the physiologic function of each receptor. (A)