Hemperly J J, Edelman G M, Cunningham B A
Proc Natl Acad Sci U S A. 1986 Dec;83(24):9822-6. doi: 10.1073/pnas.83.24.9822.
In embryonic chicken brains, the neural cell adhesion molecule N-CAM is expressed mainly as two polypeptides, the large intracellular-domain polypeptide (ld) (Mr = 160,000) and the small intracellular-domain polypeptide (sd) (Mr = 130,000) chains, that differ in their cytoplasmic domains and that arise by alternative splicing of RNA transcribed from a single gene. There is evidence for a minor N-CAM polypeptide of Mr = 120,000 that is similar to the ld and sd chains for most of its amino-terminal sequence, but which lacks a cytoplasmic domain. We report here the isolation and characterization of a cDNA clone, lambda N151, that appears to encode this third N-CAM polypeptide, which we designate the ssd (small surface-domain) polypeptide chain. The cDNA insert of lambda N151 consists of 2437 base pairs (bp). DNA hybridization and sequencing indicate that the first 1721 bp are nearly identical to the corresponding sequences of clone lambda N208, which encodes the ld chain. Following in the same reading frame, lambda N151 encodes 25 amino acids not present in lambda N208. The rest of lambda N151 consists of a 637-bp noncoding region containing an AATACA polyadenylylation sequence and a 55-bp poly(A) tract. Messenger RNAs complementary to lambda N151 appear later in development than those complementary to the ld and sd chains, and their appearance is correlated with the appearance of the ssd polypeptide. Although the polypeptide encoded by lambda N151 lacks a membrane region that would define a cytoplasmic domain, it does contain at its carboxyl end a relatively hydrophobic stretch of amino acids similar to those seen in precursors of membrane proteins that are attached to membranes via the lipid phosphatidylinositol. We show here that the ssd chain of chicken N-CAM can be released from brain vesicles by treatment with phospholipase C, suggesting that it too may have a phosphatidylinositol anchor. These results define two additional modes by which N-CAM expression can be modulated: by RNA splicing at a new site and by differential membrane attachment of the resulting polypeptide through a lipid intermediate.
在鸡胚脑中，神经细胞黏附分子N-CAM主要以两种多肽形式表达，即大细胞内结构域多肽（ld）（分子量=160,000）和小细胞内结构域多肽（sd）（分子量=130,000）链，它们的胞质结构域不同，由单个基因转录的RNA经可变剪接产生。有证据表明存在一种分子量为120,000的次要N-CAM多肽，其大部分氨基末端序列与ld和sd链相似，但缺少胞质结构域。我们在此报告一个cDNA克隆λN151的分离和鉴定，它似乎编码这种第三种N-CAM多肽，我们将其命名为ssd（小表面结构域）多肽链。λN151的cDNA插入片段由2437个碱基对（bp）组成。DNA杂交和测序表明，前1721 bp与编码ld链的克隆λN208的相应序列几乎相同。在同一阅读框中，λN151编码25个λN208中不存在的氨基酸。λN151的其余部分由一个637 bp的非编码区组成，其中包含一个AATACA聚腺苷酸化序列和一个55 bp的poly(A)尾。与λN151互补的信使RNA在发育后期出现，比与ld和sd链互补的信使RNA出现得晚，它们的出现与ssd多肽的出现相关。虽然由λN151编码的多肽缺少定义胞质结构域的膜区域，但它在羧基末端确实含有一段相对疏水的氨基酸序列，类似于通过脂质磷脂酰肌醇附着于膜上的膜蛋白前体中的序列。我们在此表明鸡N-CAM的ssd链可通过用磷脂酶C处理从脑泡中释放出来，这表明它也可能有一个磷脂酰肌醇锚定。这些结果确定了N-CAM表达可被调节的另外两种方式：通过在新位点的RNA剪接以及通过脂质中间体对所得多肽进行差异膜附着。
