Structural insights into small RNA sorting and mRNA target binding by Arabidopsis Argonaute Mid domains

► Crystal structures of AtAgo1 Mid domain and its mutant are determined. ► Structural principle of “5′-U” recognition by AtAgo1 is discovered. ► Structural principle of “5′-A” recognition by AtAgo1 is proposed. ► A putative mRNA target binding groove is identified. ► Structural mechanism of small RNA sorting and mRNA target binding is proposed.