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 (nuclear speckles). Here we applied photobleaching techniques to show that a green fluorescent protein (GFP) fusion 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 intranuclear localization and mobility properties of cyclin L1 in different cellular states, and 2) dissect the structural elements required for immobilization of cyclin L1. Transcriptional arrest by actinomycin D caused accumulation of GFP-cyclin L2 in rounded and enlarged nuclear speckles but did not affect the subnuclear pattern of distribution of GFP-cyclin L1. Although 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...