siRNA analysis in cells expressing mutant AGO1 proteins. A, small RNAs isolated from each cell line as indicated was analyzed by Northern blot hybridization with an Ingi radiolabeled probe. Hybridization to tRNAMet served as a loading control (con). B, association of wild-type (wt), ASUB, and KSUB AGO1 with Ingi siRNAs. A cytoplasmic extract from cells expressing N-terminal BB2-tagged AGO1 proteins was subjected to immunoprecipitation with anti-BB2 antibodies, and supernatant (S) and pellet (P) fractions were processed for Western blot analysis for AGO1 (upper panel), Northern blot analysis for Ingi siRNAs (middle panel), and tRNAMet (bottom panel), which served as a control for immunoprecipitation specificity. C, siRNAs in cells expressing wild-type (wt), R735, ΔRGG, ASUB, and KSUB AGO1 are single-stranded. RNA from an S100 extract without (−) or with incubation at 95 °C (+) prior to electrophoresis was electrophoresed on a 15% native polyacrylamide gel and hybridized to an oligonucleotide probe complementary to the most abundant Ingi siRNA. A synthetic 32P-labeled siRNA duplex (con) served as a marker for migration of single-stranded (ss) and double-stranded (ds) siRNAs.