Abstract
Actin and nuclear myosin 1c (NM1) cooperate in RNA polymerase I (pol I) transcription. NM1 is also part of a multiprotein assembly, B-WICH, which is involved in transcription. This assembly contains the chromatin remodeling complex WICH with its subunits WSTF and SNF2h. We report here that NM1 binds SNF2h with enhanced affinity upon impairment of the actin-binding function. ChIP analysis revealed that NM1, SNF2h, and actin gene occupancies are cell cycle-dependent and require intact motor function. At the onset of cell division, when transcription is temporarily blocked, B-WICH is disassembled due to WSTF phosphorylation, to be reassembled on the active gene at exit from mitosis. NM1 gene knockdown and motor function inhibition, or stable expression of NM1 mutants that do not interact with actin or chromatin, overall repressed rRNA synthesis by stalling pol I at the gene promoter, led to chromatin alterations by changing the state of H3K9 acetylation at gene promoter, and delayed cell cycle progression. These results suggest a unique structural role for NM1 in which the interaction with SNF2h stabilizes B-WICH at the gene promoter and facilitates recruitment of the HAT PCAF. This leads to a permissive chromatin structure required for transcription activation.
Highlights
Actin and myosin are involved in many nuclear functions in eukaryotic cells, including chromatin remodelling, transcription by all three RNA polymerases, biogenesis of ribonucleoprotein complexes and the repositioning of active gene loci [1,2,3,4]
Recent evidence indicates that the WSTF component of the B-WICH complex facilitates SNF2h-dependent nucleosomes repositioning and remodels in this way the chromatin at the rRNA gene promoter
We show here that nuclear myosin 1c (NM1) interacts with the WICH complex and that this interaction is required to establish permissive chromatin by promoting H3K9 acetylation
Summary
Actin and myosin are involved in many nuclear functions in eukaryotic cells, including chromatin remodelling, transcription by all three RNA polymerases, biogenesis of ribonucleoprotein complexes and the repositioning of active gene loci [1,2,3,4]. NM1 is recruited in this way at the rRNA gene promoter before transcription initiation These observations have led to the idea that actin and NM1 cooperate to assemble pol I at the gene promoter, and this leads to transcription initiation [5,6,7]. Several further observations have led to the hypothesis that the actomyosin complex facilitates the postinitiation phase of pol I transcription. These observations are that polymeric actin interacts with pol I, that controlled actin polymerization is required for transcription and that the NM1 ATPase cycle regulates association with the transcription machinery [6,7,8,9]. NM1, but not actin, is part of the multiprotein assembly
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