Function, of MNB/DYRK1A gene cloned from "Down syndrome critical region" on chromosome 21.

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MNB/DYRK1A gene cloned from "Down syndrome critical region" on chromosome 21 is a strong candidate for mental retardation associated with. Down syndrome. This gene encodes a dual-specificity protein kinase whose activity depends on the phoshorylation of tyrosine residues in the activation loop. In this study, we examined the intracellular localization of MNB/DYRK1A protein in HeLa cells. Green fluorescent protein (GFP) fusion protein of MNB/DYRK1A exhibited a speckled pattern inside the nucleus in interphase. After breakdown of the nuclear envelope in mitosis, it gave a diffuse pattern and located in the cytoplasm with relative exclusion from the condensed chromatin region. Subcellular fractionation study revealed that GFP-MNB/DYRK1A was predominantly found in the soluble fraction obtained from the nuclei rather than in the nuclear matrix. GFP-MNB/DYRK1A mutant construct (Y310F/Y312F) also showed the speckled pattern, indicating that the appearance of nuclear dot is independent of phosphorylation of these tyrosine residues. Similar punctate patterns of subnuclear distribution have previously been shown for PML, SC-35, PCNA, and SUMO-1; however, con focal microscopy analysis showed that none of them colocalized with GFP-MNB/DYRK1A. In addition to the cells with the nuclear dots, we observed cells expressing GFP-MNB/DYRK1A all over the nucleus. In these cells, the multinucleation was observed, indicating that the overexpression of MNB/DYRK1A leads to multinucleation in HeLa cells. These results suggest that MNB/DYRKlA play a significant role in coordinating nuclear division (mitosis) with cytoplasmic division (cytokinesis) during cell cycle. Detailed analysis of cytoskeltal proteins such as tubulin and actin, which control cell division, is important.