Live cell imaging of Argonaute proteins in mammalian cells

Hsp90 cochaperones p23 and FKBP4 physically interact with hAgo2 and activate RNA interference-mediated silencing in mammalian cells

Argonaute proteins rely on the activity of Hsp90 to mediate their interaction with small RNAs. The activity of Hsp90 is modulated by proteins known as cochaperones. This study identifies p23 and FKBP4 as cochaperones that interact with hAgo2 and shows that they, along with Cdc37 and Aha1, are required for efficient RNAi.

Argonaute proteins and small RNAs together form the RNA-induced silencing complex (RISC), the central effector of RNA interference (RNAi). The molecular chaperone Hsp90 is required for the critical step of loading small RNAs onto Argonaute proteins. Here we show that the Hsp90 cochaperones Cdc37, Aha1, FKBP4, and p23 are required for efficient RNAi. Whereas FKBP4 and p23 form a stable complex with hAgo2, the function of Cdc37 in RNAi appears to be indirect and may indicate that two or more Hsp90 complexes are involved. Our data also suggest that p23 and FKBP4 interact with hAgo2 before small RNA loading and that RISC loading takes place in the cytoplasm rather than in association with RNA granules. Given the requirement for p23 and FKBP4 for efficient RNAi and that these cochaperones bind to hAgo2, we predict that loading of hAgo2 is analogous to Hsp90-mediated steroid hormone receptor activation. To this end, we outline a model in which FKBP4, p23, and Aha1 cooperatively regulate the progression of hAgo2 through the chaperone cycle. Finally, we propose that hAgo2 and RNAi can serve as a robust model system for continued investigation into the Hsp90 chaperone cycle.

3'-UTR-located inverted Alu repeats facilitate mRNA translational repression and stress granule accumulation

Alu repeats within human genes may potentially alter gene expression. Here, we show that 3'-UTR-located inverted Alu repeats significantly reduce expression of an AcGFP reporter gene. Mutational analysis demonstrates that the secondary structure, but not the primary nucleotide sequence, of the inverted Alu repeats is critical for repression. The expression levels and nucleocytoplasmic distribution of reporter mRNAs with or without 3'-UTR inverted Alu repeats are similar; suggesting that reporter gene repression is not due to changes in mRNA levels or mRNA nuclear sequestration. Instead, reporter gene mRNAs harboring 3'-UTR inverted Alu repeats accumulate in cytoplasmic stress granules. These findings may suggest a novel mechanism whereby 3'-UTR-located inverted Alu repeats regulate human gene expression through sequestration of mRNAs within stress granules.