Genomic cloning of the gene for an IgE-binding lectin reveals unusual utilization of 5' untranslated regions.

epsilon BP (for epsilon binding protein) is a M(r) 31,000 S-type animal lectin that binds to IgE and has been identified as the homologue of Mac-2, a macrophage cell-surface marker, as well as the lectins RL-29, CBP35, and L-34. The protein is composed of two domains with the amino-terminal portion containing tandem repeats of nine amino acids and the carboxyl-terminal half containing consensus sequences shared by S-type animal lectins. We determined the genomic map in both rat and mouse and isolated overlapping genomic clones that contain the 5' two-thirds of the murine gene. The remaining portion of the gene was obtained by polymerase chain reaction (PCR) amplification of genomic murine DNA followed by subcloning into plasmid vectors. The epsilon BP gene is composed of six exons separated by five introns. The entire amino-terminal repetitive sequence is contained in exon III, and the carboxyl-terminal domain is encoded by the three succeeding exons (IV, V, VI). The latter three exons correspond well in size and share sequence homology with three exons coding for 14-kDa S-type lectins. The sequence in exon I offers an explanation for the generation of two mRNAs differing only in their 5' untranslated sequences, previously reported in Mac-2 cDNA clones. Using cDNA synthesis and PCR amplification, we determined that two alternative splice sites are used in many different types of cells. This alternative splicing results in different 5' untranslated regions of the murine epsilon BP mRNA.

mRNA variants encoding multiple forms of the high-affinity IgE receptor alpha subunit in transformed and nontransformed mast cells.

Multiple mRNA species encoding several predicted forms of the high-affinity IgE receptor alpha subunit (Fc epsilon RI-alpha) have been previously characterized from rat basophilic leukemia cells. Using the polymerase chain reaction procedure, we have extended these findings to show that one Fc epsilon RI-alpha mRNA variant, characterized by a 163-bp deletion within the coding sequence, exists in normal rat connective tissue mast cells as well as in both transformed and non transformed murine mast cell lines. In addition, a partial murine Fc epsilon RI-alpha genomic clone, spanning the internal-deletion sequence, has been identified, and from analysis of this sequence a mechanism of alternative pre-mRNA splicing is proposed. Finally, mRNA variants have been translated in a cell-free system and the protein products partially characterized.

Myasthenia gravis. Identification of skeletal and heart muscle antigens not related to the acetylcholine receptor.

Myasthenia gravis (MG) is a neuromuscular disease thought to have an autoimmune etiology. The acetylcholine receptor has been considered the primary site of antibody binding; however, other muscle components may be involved in the pathogenesis of myasthenia gravis. This study describes a hypertonic sucrose extract of skeletal muscle (Muscle-HSE) that reacts with antibodies in myasthenic sera. The active component in Muscle-HSE is not the acetylcholine receptor as demonstrated by the inability of this extract to bind [125I]alpha-bungarotoxin. Muscle-HSE does, however, contain two distinct antigenic components reactive with MG sera. One antigen reacted with 70% (14/20) of myasthenic sera in the passive hemagglutination assay. This antigen was detected in the HSE of both skeletal muscle and heart, and was unaffected by treatment with Triton X-100. The second antigen reacted with 10% (2/20) of MG sera in the complement fixation assay, was unique to skeletal muscle, and was inactivated by Triton X-100.