Abstract
Abstract Abstract 3403 RUNX1 is a transcription factor that is required for definitive hematopoietic development, and helps regulate long term hematopoietic stem cell self-renewal, platelet production, and lymphocyte development during adult hematopoiesis. RUNX1 is known to be modified via phosphorylation, acetylation, ubiquitination and methylation, for example on R208 and R210 by PRMT1, which activates its activating function. We continue to investigate how the methylation of RUNX1 by other protein arginine methyl transferases (PRMTs) regulates its function. Loop 9 of the DNA binding domain (the Runt domain) of RUNX1 contains an SGRGK sequence that is also present on the tails of histone H2A and H4. The histone tails of H4 and H2A can be methylated by a purified PRMT5 complex in vitro. An enzymatically active in vitro PRMT5 complex capable of methylating histones and SM proteins requires two subunits: both PRMT5 and MEP50, a WD 40 repeat domain protein. Nevertheless, this purified PRMT5/MEP50 complex cannot methylate the DNA binding domain of the RUNX1 protein in vitro. We show that RUNX1 also can be symmetrically methylated at R142 within the SGRGK motif in vitro by a nuclear PRMT5/MEP50 complex which also contains COPR5. We show after RUNX1 is methylated on R142 within the nucleus of HEL cells, RUNX1 is exported to the cytoplasm in a CRM1 dependent manner, as the export of methylated RUNX1 is blocked by lemptomycin B. CRM1 interacts with PRMT5, supporting that PRMT5 mediated arginine methylation tags protein for nuclear export. Therefore, PRMT5 not only involves in epigenetic regulation by methylation of histones but also it can directly controls the level of transcription factor proteins within the nucleus. Polycytocemia Vera patients who express the Jak2V617F mutation have low PRMT5 activity due to JAK2V617F mediated PRMT5 phosphorylation (Liu et al 2011). How Jak2 signaling affects RUNX1 methylation and RUNX1 localization within the nucleus is still under investigation. By controlling the amount of RUNX1 available within the cell nucleus, PRMT5 may regulate lineage differentiation potential and growth potential of hematopoietic stem and progenitor cells. The nuclear localization of RUNX1 can be changed through post translational modification such as arginine methylation in addition to point mutations and translocations involving RUNX1 found patients with leukemia and pre-leukemic diseases. Disclosures: No relevant conflicts of interest to declare.