Cytoplasmic assembly and selective nuclear import of Arabidopsis Argonaute4/siRNA complexes

Mol Cell. 2012 Jun 29;46(6):859-70. doi: 10.1016/j.molcel.2012.04.013. Epub 2012 May 17.


In plants, DNA methylation can be mediated by a class of Argonaute4 (AGO4)-associated heterochromatic siRNAs (hc-siRNAs), through a pathway termed RNA-directed DNA methylation (RdDM). It has been thought that RdDM is solely a nuclear process, as both the biogenesis and functioning of hc-siRNAs take place in the nucleus. In this study, we unexpectedly found that hc-siRNAs are predominantly present in the cytoplasm. We demonstrated that AGO4 is loaded with hc-siRNAs in the cytoplasm and the formation of mature AGO4/siRNA complexes requires HSP90 and the cleavage activity of AGO4. Intriguingly, siRNA binding facilitates the redistribution of AGO4 into the nucleus, likely through inducing conformational change that leads to the exposure of the nuclear localization signal (NLS). Our findings reveal an unsuspected cytoplasmic step in the RdDM pathway. We propose that selective nuclear import of mature AGO4/siRNA complexes is a key regulatory point prior to the effector stage of RdDM.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Active Transport, Cell Nucleus*
  • Arabidopsis / genetics
  • Arabidopsis / metabolism
  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / metabolism*
  • Argonaute Proteins / genetics
  • Argonaute Proteins / metabolism*
  • Binding Sites
  • Cell Nucleus / metabolism
  • Cytoplasm / metabolism*
  • DNA Methylation
  • DNA, Plant / genetics
  • DNA, Plant / metabolism
  • Gene Silencing
  • HSP90 Heat-Shock Proteins / genetics
  • HSP90 Heat-Shock Proteins / metabolism
  • Models, Biological
  • Nuclear Localization Signals
  • RNA, Plant / genetics
  • RNA, Plant / metabolism
  • RNA, Small Interfering / metabolism*


  • AGO4 protein, Arabidopsis
  • Arabidopsis Proteins
  • Argonaute Proteins
  • DNA, Plant
  • HSP90 Heat-Shock Proteins
  • Nuclear Localization Signals
  • RNA, Plant
  • RNA, Small Interfering