Researchers have described aquaporin water channels from diverse eubacterial and eukaryotic species but not from the third division of life, Archaea. Methanothermobacter marburgensis is a methanogenic archaeon that thrives under anaerobic conditions at 65 degrees C. After transfer to hypertonic media, M. marburgensis sustained cytoplasmic shrinkage that could be prevented with HgCl(2). We amplified aqpM by PCR from M. marburgensis DNA. Like known aquaporins, the open reading frame of aqpM encodes two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). Unlike other known homologs, the putative Hg(2+)-sensitive cysteine was found proximal to the first NPA motif in AqpM, rather than the second. Moreover, amino acids distinguishing water-selective homologs from glycerol-transporting homologs were not conserved in AqpM. A fusion protein, 10-His-AqpM, was expressed and purified from Escherichia coli. AqpM reconstituted into proteoliposomes was shown by stopped-flow light scattering assays to have elevated osmotic water permeability (P(f) = 57 microm x s(-1) versus 12 microm x s(-1) of control liposomes) that was reversibly inhibited with HgCl(2). Transient, initial glycerol permeability was also detected. AqpM remained functional after incubations at temperatures above 80 degrees C and formed SDS-stable tetramers. Our studies of archaeal AqpM demonstrate the ubiquity of aquaporins in nature and provide new insight into protein structure and transport selectivity.