Fig. 2. Genomic structure of human Sec31B. The gene is located on chromosome 10p24, spans 33,187 bp and incorporates 29 exons. (A) Constitutively expressed exons are depicted as green boxes; alternatively utilized exons as red boxes. The size of each exon is drawn to scale. Arrows indicate the start-codon in exon 2 and the stop-codon in exon 29; these sites are utilized in Sec31B-F. Exon 19 has two putative splice-donor sites and is therefore represented as 19A (closed) and 19B (open). (B) Each intron-exon junction of human Sec31B is displayed with 20 base pairs of flanking sequence. The first exon begins with the adapter primer used for 5' RACE and the last exon ends with a polyA tail. Lower- and upper-case letters denote intron and exon sequences, respectively (colored as above). (C) Exon utilization of the Sec31B-F isoform as represented by GenBank (AF274863). This is the longest transcript observed experimentally, predicted to encode a 129 kDa protein. Also depicted are the exons that had been translated to generate the antibodies used in this study. Mab1D4 (shown) and Mab1G10 were both raised against the same exons.