No abstract

PRMT3 (protein arginine methyltransferase 3) is one of four type I arginine methyltransferases that catalyse the formation of asymmetric dimethylarginine. PRMT3 is unique in that its N-terminus harbours a C2H2 zinc-finger domain that is proposed to confer substrate specificity. In addition, PRMT3 is the only type I enzyme that is restricted to the cytoplasm. Known in vitro substrates for PRMT3 include GST-GAR (a glutathione S-transferase fusion protein containing the glycine- and arginine-rich N-terminal region of fibrillarin), Sam68 (Src-associated substrate during mitosis 68 kDa) and PABP-N1 [poly(A)-binding protein-N1; PABP2]. Here we report the identification of an in vivo substrate for mammalian PRMT3. We found that FLAG-tagged PRMT3 can 'pull down' a protein with a molecular mass of 30 kDa from HeLa cell extracts. MS identified this PRMT3-interacting protein as rpS2 (ribosomal protein S2). In vitro studies showed that the zinc-finger domain of PRMT3 is necessary and sufficient for binding to rpS2. In addition, rpS2 is methylated by PRMT3 in vitro and is also methylated in cell lines. Deletion analysis of the rpS2 amino acid sequence identified a N-terminal Arg-Gly repeat as the methylation site. Furthermore, both PRMT3 and rpS2 co-sediment with free ribosomal subunits. These studies implicate PRMT3 in ribosomal function and in the regulation of protein synthesis.

PRMT3 is a type I arginine methyltransferase that resides in the cytoplasm. A large proportion of this cystosolic PRMT3 is found associated with ribosomes. It is tethered to the ribosomes through its interaction with rpS2, which is also its substrate. Here we show that mouse embryos with a targeted disruption of PRMT3 are small in size but survive after birth and attain a normal size in adulthood, thus displaying Minute-like characteristics. The ribosome protein rpS2 is hypomethylated in the absence of PRMT3, demonstrating that it is a bona fide, in vivo PRMT3 substrate that cannot be modified by other PRMTs. Finally, the levels 40 S, 60 S, and 80 S monosomes and polyribosomes are unaffected by the loss of PRMT3, but there are additional as yet unidentified proteins that co-fractionate with ribosomes that are also dedicated PRMT3 substrates.Arginine methylation is a common posttranslational modification that occurs in both the nucleus and the cytoplasm (1). The methylation of arginine residues is catalyzed by at least two different classes of protein arginine methyltransferase (PRMT) 2 enzymes. The Type I enzymes catalyze the formation of asymmetric N G ,N G -dimethylarginine residues and the Type II enzyme catalyzes the formation of symmetric N G ,NЈ G -dimethylarginine residues. In mammals, there are five Type I enzymes (PRMT1,3,4, 6 and 8).Proteins that are substrates for the PRMTs usually contain glycine and arginine-rich patches, GAR motifs, that are the sites of methylation. The major pools of protein that are arginine methylated are the heterogeneous nuclear ribonucleoproteins, histones, and ribosomal proteins. Thirty years ago, HeLa cell ribosomal proteins were shown to be heavily lysine-and arginine-methylated (2), and further two-dimensional gel electrophoresis studies of purified ribosomes demonstrated that at least six prominent proteins are arginine methylated (3). PRMT3 is a ribosome-associated protein (4, 5) and may be responsible for much of the arginine methylation that occurs in this molecular machine.PRMT3 was identified as a PRMT1 binding protein in a yeast two-hybrid screen (6). However, gel filtration analysis of RAT1 cells demonstrated that PRMT3 occurs as a monomer and is not complexed in vivo with PRMT1 (6). PRMT3 also interacts with the tumor suppressor DAL-1 (7). This interaction inhibits PRMT3 enzymatic activity, both in in vitro reactions and in cell lines. At the organ level, PRMT3 is ubiquitously expressed (6), but in the brain it displays higher expression in neurons than in glial cells (8). PRMT3 is the only type 1 arginine methyltransferase that is localized exclusively to the cytoplasm (6, 9). Another unique property of PRMT3 is that it harbors a zinc finger domain at its N terminus. It has been proposed that this domain may play a role in the regulation of PRMT3 activity or in the recognition of PRMT3 substrates (6, 10). Deletion analysis studies demonstrated that PRMT3 lacking the zinc finger domain is still active in vitro, however the zinc finger minus enzyme loses its abil...

Background & Aims Prograstrin induces proliferation in colon crypts by activating p65NF-κ B and β-catenin. We investigated whether Annexin A2 (AnxA2), a progastrin receptor, activates NF-κB and β-catenin in vivo. Method ANXA2-null (ANXA2− /−) and wild-type (ANXA2+/+) mice were studied, along with clones of progastrin-responsive HEK-293 cells that stably expressed full-length progastrin (HEK-mGAS) or an empty-vector (HEK-C). Small interfering RNA was used to downregulate AnxA2, p65NF-κB, and β-catenin in cells. Results Proliferation and activation of p65 and β-catenin increased significantly in HEK-mGAS, compared with HEK-C clones. HEK-mGAS cells had a 2–4-fold increase in relative levels of c-Myc, COX-2, CyclinD1, DCAMKL+1, and CD44, compared with HEK-C clones. Down-regulation of AnxA2 in HEK-mGAS clones reduced activation of NF-κB and β-catenin, as well as levels of DCAMKL+1. Surprisingly, downregulation of β-catenin had no effect on activation of p65NF-κB, whereas down-regulation of p65 significantly reduced activation of β-catenin in HEK-mGAS clones. Loss of either p65 or β-catenin significantly reduced proliferation of HEK-mGAS clones, indicating that both factors are required for the proliferative effects of progastrin. Lengths of colon crypts and levels of p65, β-catenin, DCAMKL+1, and CD44 were significantly higher in ANXA2+/+ mice compared to corresponding values measured in either ANXA2− /− mice injected with progastrin or ANXA2+/+ and ANXA2− /− mice injected with saline. Conclusions AnxA2 expression is required for the biological effects of progastrin in vivo and in vitro, and mediates the stimulatory effect of progastrin on p65NF-κ, β-catenin, and the putative stem-cell markers DCAMKL+1 and CD44. AnxA2 might therefore mediate the hyperproliferative and co-carcinogenic effects of progastrin.

We recently reported that overexpression of progastrin in embryonic epithelial cells (HEKmGAS-cells) increased proliferation of the cells, compared to that of control HEKC-cells. Here we report the novel finding that tumorigenic and metastatic potential of HEKmGAS cells is also increased significantly, compared to that of HEKC cells. Cell-surface associated annexinA2 (CS-ANXA2) binds progastrin and is over-expressed on cancer-cells, allowing us to successfully use fluorescently-labeled progastrin-peptide for enumerating metastatic lesions of transformed/cancer cells in vivo. Next, we examined the hypothesis that increased tumorigenic/metastatic potential of isogenic HEKmGAS vs HEKC cells maybe due to transformed-phenotype of stem-cells. FACSorting/FACScanning of cells demonstrated significant increases in percent DCLK1/Lgr5 positive stem-cells, co-expressing CD44/CS-ANXA2, in HEKmGAS vs HEKC-cells. Distinct differences were noted in morphology of HEKC vs HEKmGAS spheroidal growths on non-adherent cultures (selective for stem cells). HEKC-spheroids were rounded with distinct perimeters (basement membranes?), while HEKmGAS-spheroids were amorphous, with no perimeters. Relative levels of DCLK1/Lgr5/CD44 and AnnexinA2/β-catenin/pNFκBp65/metalloproteinases were significantly increased in HEKmGAS vs HEKC-cells, growing either as mono-layer cultures, 3D-spheroids (in vitro), or xenografts (in vivo). Interestingly, HEKC-cells enriched for CS-ANXA2, developed amorphous spheroids, while down-regulation of ANXA2 in HEKmGAS-clones, resulted in loss of matrixmetalloproteinases and re-formation of rounded spheroids, suggesting high levels of CS-ANXA2/matrixmetalloproteinases may impact spheroid morphology. Down-regulation of DCLK1 significantly attenuated activation of β-catenin, with loss of proliferation of HEKmGAS and HEKC-cells, suggesting DCLK1 is required for maintaining proliferation of cells. Conclusions Our results suggest the novel possibility that transformed stem-cells, unlike non-transformed stem-cells, co-express stem-cell-markers DCLK1 and CD44 with CS-ANXA2.