The nucleus is highly organized and contains a cytoskeletal-like matrix or nucleoskeleton that provides overall structural integrity, as well as the internal infrastructure that allows the movement of components, complexes, and machineries within the nucleus. This dynamic structural organization spatially and temporally regulates assembly and disassembly of large multi-protein complexes required for transcription or DNA replication, and the movement of chromosomes and/or chromosomal domains/territories between different subdomains (compartments) within the nucleus. The nuclear envelope is a key component in the organization and structure of the nucleus, and like its plasma membrane counterpart, is the site for signal sensing, molecule trafficking, and inter-/intra-nuclear attachments.
Wiskott-Aldrich Syndrome (WAS) family members participate in cytoskeleton reorganization and signal transduction by acting as effectors of Rho family GTPases and promoting actin polymerization through the Arp2/3 complex. Rho family GTPases, subunits of the Arp2/3 complex, and the WAS family proteins N-Wasp and JMY have also been observed in the nucleus and implicated in transcriptional regulation. We recently identified Wash, a new subfamily of WAS proteins, that functions downstream of the Rho1 GTPase, exhibits Arp2/3 dependent branched actin nucleation activity, exhibits actin and microtubule bundling and crosslinking activities, and interacts with the actin nucleation factors Capu (formin) and Spire. In addition to its cytoplasmic localization and roles, we find that Wash is expressed in the nucleus.
Wash knockdown by RNAi or loss of function mutation leads to altered nuclear morphology and organization. Importantly, these nuclear phenotypes are recapitulated by a Wash transgene harboring a mutation that disrupts Wash’s nuclear localization signal.
We find that Wash is recruited to chromatin overlapping with Lamin-associated domains. Lamins are exclusively nuclear intermediate filaments that form a meshwork lining the inner-nuclear membrane and have been implicated in nuclear shape and positioning, chromatin organization, DNA replication, transcription and signal transduction. Consistent with this, we find that Wash interacts physically and functionally with B-type Lamin. Both Wash and Lamin knockdown disrupt the chromatin accessibility of repressive compartments in agreement with an observed global redistribution of repressive histone modifications and subsequent changes in gene expression in these mutants. Interestingly, this is the first time that the brown-based and white-based PEV models have yielded different results when assaying a given mutation, suggesting that more than one type of repression domain exists near the nuclear periphery.
Thus, our results reveal a novel role for Wash in modulating nucleus morphology and in the organization of both chromatin and non-chromatin nuclear sub-structures.