A number of genes have been identified as members of the Argonaute family in various nonhuman organisms and these genes are considered to play important roles in the development and maintenance of germ-line stem cells. In this study, we identified the human Argonaute family, consisting of eight members. Proteins to be produced from these family members retain a common architecture with the PAZ motif in the middle and Piwi motif in the C-terminal region. Based on the sequence comparison, eight members of the Argonaute family were classified into two subfamilies: the PIWI subfamily (PIWIL1/HIWI, PIWIL2/HILI, PIWIL3, and PIWIL4/HIWI2) and the eIF2C/AGO subfamily (EIF2C1/hAGO1, EIF2C2/hAGO2, EIF2C3/hAGO3, and EIF2C4/hAGO4). PCR analysis using human multitissue cDNA panels indicated that all four members of the PIWI subfamily are expressed mainly in the testis, whereas all four members of the eIF2C/AGO subfamily are expressed in a variety of adult tissues. Immunoprecipitation and affinity binding experiments using human HEK293 cells cotransfected with cDNAs for FLAG-tagged DICER, a member of the ribonuclease III family, and the His-tagged members of the Argonaute family suggested that the proteins from members of both subfamilies are associated with DICER. We postulate that at least some members of the human Argonaute family may be involved in the development and maintenance of stem cells through the RNA-mediated gene-quelling mechanisms associated with DICER.
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