Differential sublocalization of actin variants within the nucleus.

Conventional actin has been implicated in various nuclear processes including chromatin remodeling, transcription, nuclear transport, and overall nuclear structure. Moreover, actin has been identified as a component of several chromatin remodeling complexes present in the nucleus. In animal cells, nuclear actin exists as a dynamic equilibrium of monomers and polymers. Actin-binding proteins (ABPs) such as ADF/cofilin and profilin play a role in actin import and export, respectively. However, very little is known about the localization and roles of nuclear actin in plants. In multicellular plants and animals, actin is comprised of an ancient and divergent family of protein variants. Here, we have investigated the presence and differential localization of two ancient subclasses of actin in isolated Arabidopsis nuclei. Although the subclass 1 variants ACT2 and ACT8 and subclass 2 variant ACT7 were found distributed throughout the nucleoplasm, ACT7 was often found more concentrated in nuclear speckles than subclass 1 variants. The nuclei from the act2-1/act8-2 double null mutant and the act7-5 null mutant lacked their corresponding actin variants. In addition, serial sectioning of several independent nuclei revealed that ACT7 was notably more abundant in the nucleolus than the subclass 1 actins. Profilin and ADF proteins were also found in significant levels in plant nuclei. The possible functions of differentially localized nuclear actin variants are discussed.