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Characterization of cyclin L1 as an immobile component of the splicing factor compartment

Cyclin L1 and cyclin L2 are two closely related members of the cyclin family that contain C‐terminal arginine‐ and serine‐rich (RS) domains and are localized in the splicing factor compartment (nü‐clear speckles). Here we applied photobleaching techniques to show that a green fluorescent protein (GFP) füsion protein of cyclin L1, in contrast to cyclin L2, was not mobile within the nucleus of living COS7 cells. The objectives of this study were to 1) characterize the intranüclear localization and mobility properties of cyclin L1 in different cellular states, and 2) dissect the strüctüral elements required for immobilization of cyclin L1. Transcriptional arrest by actinomycin D caüsed accumulation of GFP‐cyclin L2 in rounded and enlarged nuclear speckles but did not affect the sub‐nuclear pattern of distribution of GFP‐cyclin L1. Al‐though immobile in most phases of the cell cycle, GFP‐cyclin L1 was diffusely distributed and highly mobile in the cytoplasm of metaphase cells. By analysis of a series of chimeras, deletion constrücts, and a point mutant, a segment within the RS domain of cyclin L1 was identified to be necessary for the immobility of the protein in nuclear speckles. This study provides the first characterization of an immobile component of nuclear speckles.—Herrmann, A., Fleischer, K., Czaj‐kowska, H., Müller‐Newen, G., Becker, W. Characterization of cyclin L1 as an immobile component of the splicing factor compartment. FASEB J. 21, 3142–3152 (2007)