Magrané J, Casaroli-Marano R P, Reina M, Gåfvels M, Vilaró S
Department of Cellular Biology, Faculty of Biology, University of Barcelona, Spain.
FEBS Lett. 1999 May 14;451(1):56-62. doi: 10.1016/s0014-5793(99)00494-9.
The very low density lipoprotein receptor is a member of the low density lipoprotein receptor supergene family for which two isoforms have been reported, one lacking and the other containing an O-linked sugar domain. In order to gain insight into their functionality, transient and stable transformants separately overexpressing previously cloned bovine variants were analyzed. We report evidence that the variant lacking the O-linked sugar domain presented a rapid cleavage from the cell and that a large amino-terminal very low density lipoprotein receptor fragment was released into the culture medium. As only minor proteolysis was involved in the other very low density lipoprotein receptor variant, the clustered O-linked sugar domain may be responsible for blocking the access to the protease-sensitive site(s). To test this hypothesis, a mutant Chinese hamster ovary cell line, ldlD, with a reversible defect in the protein O-glycosylation, was used. The instability of the O-linked sugar-deficient very low density lipoprotein receptor on the cell surface was comparable to that induced by the proteolysis of the variant lacking the O-linked sugar domain. Moreover, our data suggest that the O-linked sugar domain may also protect the very low density lipoprotein receptor against unspecific proteolysis. Taken together, these results indicate that the presence of the O-linked sugar domain may be required for the stable expression of the very low density lipoprotein receptor on the cell surface and its absence may be required for release of the receptor to the extracellular space. The exclusive expression of the variant lacking the O-linked sugar domain in the bovine aortic endothelium opens new perspectives in the physiological significance of the very low density lipoprotein receptor.
极低密度脂蛋白受体是低密度脂蛋白受体超基因家族的成员，据报道该家族有两种亚型，一种缺乏O-连接糖结构域，另一种含有该结构域。为了深入了解它们的功能，对分别过表达先前克隆的牛变体的瞬时和稳定转化体进行了分析。我们报告的证据表明，缺乏O-连接糖结构域的变体从细胞上快速裂解，并且一个大的氨基末端极低密度脂蛋白受体片段被释放到培养基中。由于另一种极低密度脂蛋白受体变体仅涉及少量蛋白水解，成簇的O-连接糖结构域可能负责阻断蛋白酶敏感位点的暴露。为了验证这一假设，使用了一种在蛋白质O-糖基化方面存在可逆缺陷的突变中国仓鼠卵巢细胞系ldlD。细胞表面缺乏O-连接糖的极低密度脂蛋白受体的不稳定性与缺乏O-连接糖结构域的变体经蛋白水解诱导的不稳定性相当。此外，我们的数据表明，O-连接糖结构域还可能保护极低密度脂蛋白受体免受非特异性蛋白水解。综上所述，这些结果表明，O-连接糖结构域的存在可能是极低密度脂蛋白受体在细胞表面稳定表达所必需的，而其缺失可能是受体释放到细胞外空间所必需的。在牛主动脉内皮中缺乏O-连接糖结构域的变体的特异性表达为极低密度脂蛋白受体的生理意义开辟了新的前景。
