SSX2IP promotes centrosome maturation and maintenance at the onset of vertebrate development, preserving centrosome integrity and mitosis during rapid cleavage divisions and in somatic cells.

The conserved Prp19 (pre-RNA processing 19) complex is required for pre-mRNA splicing in eukaryotic nuclei. Recent RNAi screens indicated that knockdown of Prp19 complex subunits strongly delays cell proliferation. Here we show that knockdown of the smallest subunit, BCAS2/Spf27, destabilizes the entire complex and leads to specific mitotic defects in human cells. These could result from splicing failures in interphase or reflect a direct function of the complex in open mitosis. Using Xenopus extracts, in which cell cycle progression and spindle formation can be reconstituted in vitro, we tested Prp19 complex functions during a complete cell cycle and directly in open mitosis. Strikingly, immunodepletion of the complex either before or after interphase significantly reduces the number of intact spindles, and increases the percentage of spindles with lower microtubule density and impaired metaphase alignment of chromosomes. Our data identify the Prp19 complex as the first spliceosome subcomplex that directly contributes to mitosis in vertebrates independently of its function in interphase.

Assembly of the mitotic spindle requires a global change in the activity and constitution of the microtubule-binding-protein array at mitotic onset. An important subset of mitotic microtubule-binding proteins localises to the nucleus in interphase and essentially contributes to spindle formation and function after nuclear envelope breakdown. Here, we used a proteomic approach to selectively identify proteins of this category and revealed 50 poorly characterised human gene products, among them the echinoderm microtubule-associated-protein-like gene product, EML3. Indirect immunofluorescence showed that EML3 colocalises with spindle microtubules throughout all mitotic stages. In interphase, EML3 colocalised with cytoplasmic microtubules and accumulated in interphase nuclei. Using YFP-fusion constructs of EML3, we located a nuclear localisation signal and confirmed the microtubule-binding domain of EML3. Functional analysis of EML3 using time-lapse fluorescence microscopy and detailed end-point analysis of phenotypes after siRNA knockdown demonstrates an important role for EML3 in correct metaphase chromosome alignment. Our proteomic identification screen combined with sensitive phenotypic analysis therefore provides a reliable platform for the identification and characterisation of proteins important for correct cell division