Tissue-specific expression and dynamic organization of SR splicing factors in Arabidopsis thaliana

Yuda Fang and David L. Spector

Cold Spring Harbor Laboratory, Cold Spring Harbor, NY

The organization of the pre-mRNA splicing machinery has been extensively studied in cultured mammalian and yeast cells. However, far less is known about its organization and dynamics in living plant cells and in different cell types of an intact living organism. Here we report on the expression, organization and dynamics of pre-mRNA splicing factors (SR33, SR1/atSRp34 and atSRp30) in Arabidopsis thaliana. Distinct tissue-specific expression patterns were observed, and in addition, differences in the distribution of the respective proteins within the nuclei of different cell types were identified. These factors localized in a cell type-dependent speckled pattern as well as being diffusely distributed throughout the nucleoplasm. Upon examination by immunoelectron microscopy, these speckles correspond to interchromatin granule clusters (IGCs). Time-lapse microscopy revealed that speckles move within a constrained nuclear space and their organization is regulated during the cell cycle. Fluorescence recovery after photobleaching (FRAP) analysis revealed a rapid exchange rate of splicing factors in nuclear speckles. The dynamic organization of plant speckles is closely related to the transcriptional activity of the cells and can also be modulated by the phosphorylation state of the splicing factors. The organization and dynamic behavior of speckles in Arabidopsis cell nuclei provides significant insight into understanding the functional compartmentalization of the nucleus and its relationship to chromatin organization within various cell types of a single organism.