Phytochelatins (PCs), a family of cysteine-rich heavy metal binding peptides are required for heavy metal detoxification in plants, Schizosaccharomyces pombe and Caenorhabditis elegans. PCs are derived from glutathione and related thiols by the action of phytochelatin synthases (PC synthases), chelate heavy metals with high affinity, and facilitate their sequestration into the vacuoles (a lysosomal-like compartments) of plant and S. pombe cells. We established previously that PC synthase (CePCS-1) plays a critical role in heavy metal detoxification in worms. Weve more recently identified a half-molecule ATP-binding cassette transporter, CeHMT-1, orthologous to the putative PC transporter from S. pombe (SpHMT1). Functional analysis of ce-
hmt-1 established that its heterologous expression in S. pombe
hmt1- mutants alleviates their cadmium (Cd2+)-hypersensitivity concomitant with the localization of CeHMT-1 to the vacuolar membrane. Importantly, functional CeHMT-1 is required for Cd2+ tolerance in intact worms since CeHMT1-deficient RNAi worms are acutely sensitive to Cd2+. Unexpectedly, the Cd2+ hypersensitive phenotype of CeHMT-1-deficient RNAi worms is more pronounced and is morphologically different from that exhibited by CePCS-1 RNAi worms. The results of the analysis of genetic interactions between ce-
pcs-1 and ce-
hmt-1 suggest an alternate or auxiliary role for CeHMT-1 not suspected from the earlier investigations of SpHMT1. In agreement with this suggestion are results of the analysis of the expression pattern of ce-
pcs-1 and ce-
hmt-1 RNA. Our preliminary studies established that although the fluorescence pattern of GFP expressed under the control of ce-
hmt-1 promoter partially overlaps with the fluorescence of GFP expressed from ce-
pcs-1 promoter, both have clearly distinct and unique distribution. ce-
hmt-1 is widely expressed in intestinal cells while ce-
pcs-1 manifests distribution in pharyngeal grinder, pharyngeal-intestinal valve, somatic and vulval muscles. Interestingly, ce-
hmt-1 and ce-
pcs-1 are expressed in coelomocytes, raising an intriguing possibility that coelomocytes might be involved in heavy metal detoxification. This model will be tested by analyzing Cd2+-hypersensitivity of coelomocyte-deficient mutants (from Dr. H. Faress laboratory).