Zhou M, Ma Z, Sly W S
Edward A. Doisy Department of Biochemistry and Molecular Biology, St. Louis University School of Medicine, MO 63104, USA.
Proc Natl Acad Sci U S A. 1995 Oct 10;92(21):9762-6. doi: 10.1073/pnas.92.21.9762.
We cloned and sequenced the 8767-bp full-length cDNA for the chicken cation-independent mannose-6-phosphate receptor (CI-MPR), of interest because, unlike its mammalian homologs, it does not bind insulin-like growth factor II (IGF-II). The cDNA encodes a protein of 2470 aa that includes a putative signal sequence, an extracytoplasmic domain consisting of 15 homologous repeat sequences, a 23-residue transmembrane sequence, and a 161-residue cytoplasmic sequence. Overall, it shows 60% sequence identity with human and bovine CI-MPR homologs, and all but two of 122 cysteine residues are conserved. However, it shows much less homology in the N-terminal signal sequence, in repeat 11, which is proposed to contain the IGF-II-binding site in mammalian CI-MPR homologs, and in the 14-aa residue segment in the cytoplasmic sequence that has been proposed to mediate G-protein-coupled signal transduction in response to IGF-II binding by the human CI-MPR. Transient expression in COS-7 cells produced a functional CI-MPR which exhibited mannose-6-phosphate-inhibitable binding and mediated endocytosis of recombinant human beta-glucuronidase. Expression of the functional chicken CI-MPR in mice lacking the mammalian CI-MPR should clarify the controversy over the physiological role of the IGF-II-binding site in mammalian CI-MPR homologs.
我们克隆并测序了鸡阳离子非依赖性甘露糖-6-磷酸受体（CI-MPR）的8767碱基对全长cDNA，之所以对其感兴趣，是因为与哺乳动物的同源物不同，它不结合胰岛素样生长因子II（IGF-II）。该cDNA编码一个2470个氨基酸的蛋白质，包括一个推定的信号序列、一个由15个同源重复序列组成的胞外结构域、一个23个残基的跨膜序列和一个161个残基的胞质序列。总体而言，它与人和牛的CI-MPR同源物具有60%的序列同一性，122个半胱氨酸残基中除两个外全部保守。然而，它在N端信号序列、第11个重复序列（在哺乳动物CI-MPR同源物中被认为含有IGF-II结合位点）以及胞质序列中被认为介导人CI-MPR对IGF-II结合的G蛋白偶联信号转导的14个氨基酸残基片段中同源性要低得多。在COS-7细胞中的瞬时表达产生了一种功能性CI-MPR，其表现出甘露糖-6-磷酸抑制性结合并介导重组人β-葡萄糖醛酸酶的内吞作用。在缺乏哺乳动物CI-MPR的小鼠中功能性鸡CI-MPR的表达应该能够澄清关于哺乳动物CI-MPR同源物中IGF-II结合位点生理作用的争议。
