Fig. 2. C. elegans MRP-1. (A) Putative topology of the C. elegans MRP-1 protein (top); structure of the mrp-1 gene and its cDNAs (bottom). Like human MRP1, C. elegans MRP-1 seems to have two nucleotide-binding domains (NBDs) and three membrane-spanning domains (MSDs). The mrp-1 gene has 20 exons, of which the thirteenth exon is variable. The mutation sites of ut151, ut153 and ut155, as well as the region deleted in pk89 are shown in the gene structure. The C. elegans database WormBase listed the mrp-1 cDNA isoforms, a-, b- and c-types, as well as d.1-, d.2- and d.3-types. The latter three cDNAs are not shown in this figure because they encode only short polypeptides. We found a fifth isoform (e-type), but could not confirm the existence of the a-type. (B) Constructs used for transgene experiments.