The Upf1 protein in yeast has been implicated in the modulation of efficient translation termination as well as in the accelerated turnover of mRNAs containing premature stop codons, a phenomenon called nonsense-mediated mRNA decay (NMD). A human homolog of the yeast UPF1, termed HUpf1/RENT1, has also been identified. The HUpf1 has also been shown to play a role in NMD in mammalian cells. Comparison of the yeast and human UPF1 proteins demonstrated that the amino terminal cysteine/histidine-rich region and the region comprising the domains that define this protein as a superfamily group I helicase have been conserved. The yeast Upf1p demonstrates RNA-dependent ATPase and 5' --> 3' helicase activities. In this paper, we report the expression, purification, and characterization of the activities of the human Upf1 protein. We demonstrate that human Upf1 protein displays a nucleic-acid-dependent ATPase activity and a 5'--> 3' helicase activity. Furthermore, human Upf1 is an RNA-binding protein whose RNA-binding activity is modulated by ATP. Taken together, these results indicate that the activities of the Upf1 protein are conserved across species, reflecting the conservation of function of this protein throughout evolution.