Novel slc22 transporter homologs in fly, worm, and human clarify the phylogeny of organic anion and cation transporters

Slc22 family organic anion and cation transporters (OATs, OCTs, and OCTNs) are transmembrane proteins expressed predominantly in kidney and liver. These proteins mediate the uptake or excretion of numerous physiologically (and pharmacologically) important compounds, and accordingly have been the focus of intensive study. Here we investigate the molecular phylogeny of the slc22 transporters, identifying homologs in Drosophila and C. elegans, several of which are developmentally regulated, as well as reporting the cloning of a novel human family member, UST6, expressed exclusively in liver in both embryo and adult. The latter helps define a subfamily within the OATs, which appears to have human- and rodent-specific members, raising potential issues with respect to the use of rodents as models for the transport of organic anions (which include many pharmaceuticals) in humans. Although this phylogenetic inference could not be made on the basis of sequence alignment, analysis of intron phasing suggests that the OAT, OCT, and OCTN lineages of the slc22 family formed after the divergence of vertebrates and invertebrates. Subsequently, these lineages expanded through independent tandem duplications to produce multiple gene pairs. After analyzing over 200 other transporter genes, we find such pairing to be relatively specific to vertebrate organic anion and cation transporters, suggesting selection for gene pairing operating within this family in particular. This might reflect a requirement for redundancy or broader substrate specificity in vertebrates (compared to invertebrates), due to their greater physiological complexity and thus potentially broader exposure to organic ions.

Molecular Cloning of a Functional Allatostatin Gut/Brain Receptor and an Allatostatin Preprohormone from the SilkwormBombyx mori

The cockroach-type or A-type allatostatins are inhibitory insect neuropeptides with the C-terminal sequence Tyr/Phe-X-Phe-Gly-Leu-NH2. Here, we have cloned an A-type allatostatin receptor from the silkwormBombyx mori(BAR). BAR is 361 amino acid residues long, has seven transmembrane domains, shows 60% amino acid residue identity with the firstDrosophilaallatostatin receptor (DAR-1), and 48% identity with the secondDrosophilaallatostatin receptor (DAR-2). The BAR gene has two introns and three exons. These two introns coincide with and have the same intron phasing as two introns in the DAR-1 and DAR-2 genes, showing that the three receptors are not only structurally but also evolutionarily related. Furthermore, we have cloned aBombyxallatostatin preprohormone that contains eight different A-type allatostatins. Chinese hamster ovary cells permanently transfected with BAR DNA react on the addition of 4 × 10−9mBombyxA-type allatostatins with a second messenger cascade (measured as bioluminescence), showing that BAR is a functional A-type allatostatin receptor. Southern blots suggest thatBombyxhas at least one other BAR-related gene in addition to the BAR gene described in this paper. Northern blots and quantitative reverse transcriptase-polymerase chain reaction of different larval tissues show that BAR mRNA is mainly expressed in the gut and to a much lesser extent in the brain. To our knowledge, this is the first report on the molecular cloning and functional expression of an insect gut/brain peptide hormone receptor.