Robert Alan Reid

Variants of human L1 cell adhesion molecule arise through alternate splicing of RNA

The L1 cell adhesion molecule was initially identified and characterized in mouse as a cell-surface glycoprotein that mediates neuron-neuron and neuron-Schwann cell adhesion. We have characterized L1 in humans using cDNA structural and mRNA expression analyses. We present the entire coding sequence for human L1, which predicts a 1253-amino acid protein displaying a signal sequence, transmembrane segment, RGD sequence, and potential glycosylation and phosphorylation sites. Nucleotide and deduced amino acid sequence identities between human and mouse L1 are 85% and 87%, respectively. In contrast, the amino acid identity between human L1 and the L1-related molecule chicken Ng-CAM is only 45%. Using Northern blot analyses, a single L1 transcript of 5.5 kb is detected in human fetal brain and in neuroblastoma (IMR-32) and retinoblastoma (Y-79) cell lines. L1 is also expressed in the rhabdomyosarcoma cell lines RD and A-204, which display several muscle characteristics. Two forms of L1, which differ by the presence or absence of a 12-bp cytoplasmic segment, are expressed in both human and mouse. This segment is encoded by a single exon that can be alternately spliced to give rise to the two forms, which appear to be expressed in tissue-specific patterns.

Identification and characterization of the human cell adhesion molecule contactin

We have prepared a monoclonal antibody, Neuro-1, that recognizes the human homolog of the chicken contactin/F11 and mouse F3 cell adhesion molecules. The Neuro-1 antigen, structurally characterized as a 135 kDa glycosylphosphatidylinositol-linked glycoprotein, was immunoaffinity purified and partially sequenced. Comparison of an internal peptide sequence to that predicted from the chicken contactin/F11, mouse F3 and human contactin (reported herein) cDNA sequence identifies the Neuro-1 antigen as human contactin. Moreover, a polyclonal antisera generated against the purified Neuro-1 antigen was immunoreactive with a fragment of human contactin expressed in bacteria. The complete coding and deduced amino acid sequences of human contactin were determined and are 86% and 95% identical to the respective mouse F3 sequences. Structural features shared with contactin/F11/F3 include six immunoglobulin type C2 and four fibronectin type III-like domains, multiple sites for asn-linked glycosylation and a COOH-terminal signal peptide presumably removed during the generation of a phosphatidylinositol cell surface linkage. The potential for glycosylation and GPI-linkage is also consistent with protein chemical studies of human contactin. Contactin mRNA expression was characterized using Northern blot analyses of human tissues and cell lines. High level expression of a single contactin transcript in adult brain, and low level expression of multiple transcripts in lung, pancreas, kidney and skeletal muscle are observed. Highly expressed multiple transcripts, similar in pattern to that of pancreas, lung, kidney and skeletal muscle, are also observed in human neuroblastoma and retinoblastoma cell lines.

Characterization of cDNA clones defining variant forms of human neural cell adhesion molecule N-CAM.

The neural cell adhesion molecule N-CAM has been identified in a number of species and comprises at least three major cell surface polypeptides of different molecular structures and tissue distributions. We report here the isolation and characterization of cDNA clones encoding two of the three major forms of N-CAM from a human neuroblastoma cDNA libary. One of the clones, NII-6, provides the first complete sequence of a small cytoplasmic domain (140 kDa) form of the molecule in humans and differs in a number of respects from cDNA clones derived from human muscle. These differences include the presence of a 30-bp insert in the fourth immunoglobulin-like domain of N-CAM, a 3-bp insert in the extracellular portion of the molecule, and an additional 6 pb in the middle of the membrane-spanning segment. Based on the analysis of a genomic DNA clone spanning these regions of N-CAM, the first two differences arise by alternate splicing of RNA and occur in some, but not all clones; the additional 6 bp may reflect a genetic polymorphism. A second cDNA clone, NI-10, encodes the complete sequence of a segment that is specific to the large cytoplasmic domain (180 kDa) polypeptide of human N-CAM and is very similar to corresponding segments of mouse, chicken, and rat N-CAM. This sequence also arises by alternative splicing of RNA. In addition, we have identified a genomic DNA segment encoding sequences specific to the third, small surface domain (120 kDa) polypeptide of N-CAM. The data presented here and previously define the DNA sequences of the membrane-bound forms and known variants of human N-CAM. From these sequences, a wide variety of probes can be generated for investigating the expression of particular N-CAM polypeptides in normal and pathological tissues.