Genomic Organization of Nematode 4TM K+ Channelsa

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Abstract
As many as 50 genes in the C. elegans genome may encode K+ channels belonging to the novel structural class of two-pore (4TM) channels. Many 4TM channels can be grouped into channel subfamilies. We analyzed 4TM channels in C. elegans using methods made possible by having complete genomic sequence. Two genes were chosen for comprehensive analysis, n2P16 and n2P17. By comparing the pattern of conservation in genomic DNA sequences between C. elegans and a closely related species, C. briggsae, we were able to identify all coding regions and predict the gene structure for these two genes. Given the extent of the 4TM channel family, we were surprised to discover that n2P17 produced at least six alternative transcripts encoding a constant central region and variable amino- and carboxyl-termini. Blocks of highly conserved DNA sequences in noncoding regions were also apparent and most likely confer important regulatory functions. The interspecies comparison of the deduced channel proteins revealed that the extracellular loop between M1 and P1 is an apparent hot spot for evolutionary change in both channels. This contrasts with the membrane-spanning domains that are highly conserved. Analysis of intron positions for 36 channels revealed that introns are frequently present at an identical position within the pore region, but very few are located in membrane-spanning domains.