The N-terminal extension domain of the C. elegans half-molecule ABC transporter, HMT-1, is required for protein-protein interactions and function.
BACKGROUND
Members of the HMT-1 (heavy metal tolerance factor 1) subfamily of the ATP-binding cassette (ABC) transporter superfamily detoxify heavy metals and have unique topology: they are half-molecule ABC transporters that, in addition to a single transmembrane domain (TMD1) and a single nucleotide-binding domain (NBD1), possess a hydrophobic NH2-terminal extension (NTE). These structural features distinguish HMTs from other ABC transporters in different species including Drosophila and humans. Functional ABC transporters, however, are comprised of at least four-domains (two TMDs and two NDBs) formed from either a single polypeptide or by the association of two or four separate subunits. Whether HMTs act as oligomers and what role the NTE domain plays in their function have not been determined.


RESULTS
In this study, we examined the oligomeric status of Caenorhabditis elegans HMT-1 and the functional significance of its NTE using gel-filtration chromatography in combination with the mating-based split-ubiquitin yeast two-hybrid system (mbSUS) and functional in vivo assays. We found that HMT-1 exists in a protein complex in C. elegans. Studies in S. cerevisiae showed that HMT-1 at a minimum homodimerizes and that oligomerization is essential for HMT-1 to confer cadmium tolerance. We also established that the NTE domain plays an important structural and functional role: it is essential for HMT-1 oligomerization and Cd-detoxification function. However, the NTE itself was not sufficient for oligomerization suggesting that multiple structural features of HMT-1 must associate to form a functional transporter.


CONCLUSIONS
The prominence of heavy metals as environmental toxins and the remarkable conservation of HMT-1 structural architecture and function in different species reinforce the value of continued studies of HMT-1 in model systems for identifying functional domains in HMT1 of humans.
Kim
Sungjin
S
Department of Crop and Soil Sciences, Cornell University, Ithaca, New York, United States of America.
Selote
Devarshi S
DS
Vatamaniuk
Olena K
OK
D018528:ATP-Binding Cassette Transporters / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D002104:Cadmium / Q000378:metabolism; D017173:Caenorhabditis elegans / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D029742:Caenorhabditis elegans Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D055503:Protein Multimerization; D017434:Protein Structure, Tertiary; D004718:Saccharomycetales / Q000235:genetics / Q000378:metabolism; D020798:Two-Hybrid System Techniques
D018528:ATP-Binding Cassette Transporters; D029742:Caenorhabditis elegans Proteins; C504067:HMT-1 protein, C elegans; D002104:Cadmium