Overexpression of DRG2 suppresses the growth of Jurkat T cells but does not induce apoptosis

DRG2 levels in prostate cancer cell lines predict response to PARP inhibitor during docetaxel treatment

PURPOSE

Developmentally regulated GTP-binding protein 2 (DRG2) regulates microtubule dynamics and G2/M arrest during docetaxel treatment. Poly ADP-ribose polymerase (PARP) acts as an important repair system for DNA damage caused by docetaxel treatment. This study investigated whether DRG2 expression affects response to PARP inhibitors (olaparib) using prostate cancer cell lines PC3, DU145, LNCaP-FGC, and LNCaP-LN3.

MATERIALS AND METHODS

The cell viability and DRG2 expression levels were assessed using colorimetric-based cell viability assay and western blot. Cells were transfected with DRG2 siRNA, and pcDNA6/V5-DRG2 was used to overexpress DRG2. Flow cytometry was applied for cell cycle assay and apoptosis analysis using the Annexing V cell death assay.

RESULTS

The expression of DRG2 was highest in LNCaP-LN3 and lowest in DU145 cells. Expressions of p53 in PC3, DU145, and the two LNCaP cell lines were null-type, high-expression, and medium-expression, respectively. In PC3 (DRG2 high, p53 null) cells, docetaxel increased G2/M arrest without apoptosis; however, subsequent treatment with olaparib promoted apoptosis. In DU145 and LNCaP-FGC (DRG2 low), docetaxel increased sub-G1 but not G2/M arrest and induced apoptosis, whereas olaparib had no additional effect. In LNCaP-LN3 (DRG2 high, p53 wild-type), docetaxel increased sub-G1 and G2/M arrest, furthermore olaparib enhanced cell death. Docetaxel and olaparib combination treatment had a slight effect on DRG2 knockdown PC3, but increased apoptosis in DRG2-overexpressed DU145 cells.

CONCLUSIONS

DRG2 and p53 expressions play an important role in prostate cancer cell lines treated with docetaxel, and DRG2 levels can predict the response to PARP inhibitors.

DRG2 Depletion Promotes Endothelial Cell Senescence and Vascular Endothelial Dysfunction

Endothelial cell senescence is involved in endothelial dysfunction and vascular diseases. However, the detailed mechanisms of endothelial senescence are not fully understood. Here, we demonstrated that deficiency of developmentally regulated GTP-binding protein 2 (DRG2) induces senescence and dysfunction of endothelial cells. DRG2 knockout (KO) mice displayed reduced cerebral blood flow in the brain and lung blood vessel density. We also determined, by Matrigel plug assay, aorta ring assay, and in vitro tubule formation of primary lung endothelial cells, that deficiency in DRG2 reduced the angiogenic capability of endothelial cells. Endothelial cells from DRG2 KO mice showed a senescence phenotype with decreased cell growth and enhanced levels of p21 and phosphorylated p53, γH2AX, senescence-associated β-galactosidase (SA-β-gal) activity, and senescence-associated secretory phenotype (SASP) cytokines. DRG2 deficiency in endothelial cells upregulated arginase 2 (Arg2) and generation of reactive oxygen species. Induction of SA-β-gal activity was prevented by the antioxidant N-acetyl cysteine in endothelial cells from DRG2 KO mice. In conclusion, our results suggest that DRG2 is a key regulator of endothelial senescence, and its downregulation is probably involved in vascular dysfunction and diseases.

ZC3H15 Correlates with a Poor Prognosis and Tumor Progression in Melanoma

Zinc figure CCCH-type containing 15 (ZC3H15), also called developmentally regulated GTP-binding protein 1 (DRG1) family regulatory protein 1 (DFRP1), is a zinc finger containing protein. Despite playing a role in cellular signaling, it is found overexpressed in acute myeloid leukemia and also an independent prognostic marker in hepatocellular carcinoma patients. However, the biological effect of ZC3H15 in malignant melanoma (MM) remains unexplored. The expression of ZC3H15 in patients was analyzed using the R2: Genomics Analysis and Visualization Platform database. Immunohistochemical analysis, western blot, and qRT-PCR were used to detect ZC3H15 expression in melanoma tissues and cell lines. MTT, BrdU, flow cytometry assay, transwell, and western blot were performed to explore the proliferation, cell cycle, invasion, and migration of melanoma cells. We undertaken colony formation assay in vitro and tumor xenograft in vivo to detect the tumorigenicity of melanoma cells. In the present study, ZC3H15 was demonstrated highly expressed in melanoma tissues and cells. Elevated ZC3H15 impairs the survival of melanoma patients. Meanwhile, attenuation of ZC3H15 in melanoma cells inhibited cell proliferation and induced cycle arrest at G0/G1 phase. Consistently, the expression of cell cycle-related proteins cyclin dependent kinase 4 (CDK4), CDK6, and cyclin D1 (CCND1) was decreased while p21 was upregulated. Furthermore, we found the migration and invasion abilities were inhibited in ZC3H15-knockdown melanoma cells. In addition, downregulation of ZC3H15 resulted in inhibition of colony formation abilities in vitro and tumorigenesis in vivo. ZC3H15 promotes proliferation, migration/invasion, and tumorigenicity of melanoma cells. As a promising biomarker and therapeutic target in MM, ZC3H15 is worthy of further exploration.

Developmentally regulated GTPases: structure, function and roles in disease

GTPases are a large superfamily of evolutionarily conserved proteins involved in a variety of fundamental cellular processes. The developmentally regulated GTP-binding protein (DRG) subfamily of GTPases consists of two highly conserved paralogs, DRG1 and DRG2, both of which have been implicated in the regulation of cell proliferation, translation and microtubules. Furthermore, DRG1 and 2 proteins both have a conserved binding partner, DRG family regulatory protein 1 and 2 (DFRP1 and DFRP2), respectively, that prevents them from being degraded. Similar to DRGs, the DFRP proteins have also been studied in the context of cell growth control and translation. Despite these proteins having been implicated in several fundamental cellular processes they remain relatively poorly characterized, however. In this review, we provide an overview of the structural biology and biochemistry of DRG GTPases and discuss current understanding of DRGs and DFRPs in normal physiology, as well as their emerging roles in diseases such as cancer.

Developmentally regulated GTP-binding protein-2 regulates adipocyte differentiation

Developmentally regulated GTP-binding protein 2 (DRG2) participates in the regulation of proliferation and differentiation of multiple cells. However, whether DRG2 regulates adipocyte differentiation and related metabolic control remains elusive. This study revealed increases in body weight and adiposity in DRG2 transgenic (Tg) mice overexpressing DRG2. Consistent with these results, DRG2 Tg mice showed increased expression of genes involved in adipogenesis and lipid metabolism in the white adipose tissue. DRG2 was also identified to control adipogenesis by cooperating with peroxisome proliferator activated receptor-γ (PPAR-γ) in cultured adipocytes. Overall, the findings of the current study suggest that DRG2 plays an active role in regulating adipocyte differentiation, and thus participates in the development of obesity during exposure to a fat-rich diet.

DRG2 Accelerates Senescence via Negative Regulation of SIRT1 in Human Diploid Fibroblasts

Accumulating evidence suggests that developmentally regulated GTP-binding protein 2 (DRG2), an evolutionarily conserved GTP-binding protein, plays an important role in regulating cell growth, inflammation, and mitochondria dynamics. However, the effect of DRG2 in aging remains unclear. In this study, we found that endogenous DRG2 protein expression is upregulated in oxidative stress-induced premature senescence models and tissues of aged mice. Ectopic expression of DRG2 significantly promoted senescence-associated β-galactosidase (SA-β-gal) activity and inhibited cell growth, concomitant with increase in levels of acetyl (ac)-p53 (Lys382), ac-nuclear factor-kB (NF-κB) p65 (Lys310), p21 Waf1/Cip1 , and p16 Ink4a and a decrease in cyclin D1. In this process, reactive oxygen species (ROS) and phosphorylation of H2A histone family member X (H2A.X), forming γ-H2A.X, were enhanced. Mechanistically, ectopic expression of DRG2 downregulated Sirtuin-1 (SIRT1), resulting in augmented acetylation of p53 and NF-κB p65. Additionally, DRG2 knockdown significantly abolished oxidative stress-induced premature senescence. Our results provide a possible molecular mechanism for investigation of cellular senescence and aging regulated by DRG2.

Developmentally regulated GTP-binding protein 2 levels in prostate cancer cell lines impact docetaxel-induced apoptosis

Purpose

This study aimed to confirm the association between developmentally regulated GTP-binding protein 2 (DRG2) expression and docetaxel-induced apoptosis and to determine whether prostate cancer responses to docetaxel treatment differ with DRG2 expression.

Materials and Methods

PC3, DU145, and LNCaP prostate cancer cell lines were used. The MTT assay was used to determine cell viability. Western blotting analysis was performed using anti-DRG2 antibodies. Cells were transfected with 50 nmol DRG2 siRNA using an siRNA transfection reagent for DRG2 knockdown. The cell cycle was analyzed by using flow cytometry, and apoptosis was detected by using the Annexin V cell death assay.

Results

DRG2 expression differed in each prostate cancer cell line. Docetaxel reduced DRG2 expression in a dose-dependent manner. Upon DRG2 knockdown in prostate cancer cells, an increase in the sub-G1 phase was observed without a change in the G1 or G2/M phases. When 4 nM docetaxel was administered to DRG2 knockdown prostate cancer cell lines, an increase in the sub-G1 phase was observed without increasing the G2/M phase, which was similar to that in DU145 cells before DRG2 knockdown. In PC3 and DU145 cell lines, DRG2 knockdown increased docetaxel-induced Annexin V (+) apoptosis by 8.7 and 2.7 times, respectively.

Conclusions

In prostate cancer cells, DRG2 regulates G2/M arrest after docetaxel treatment. In prostate cancer cells with DRG2 knockdown, apoptosis increases without G2/M arrest in response to docetaxel treatment. These results show that inhibition of DRG2 expression can be useful to enhance docetaxel-induced apoptosis despite low-dose administration in castration-resistant prostate cancer.

DRG2 supports the growth of primary tumors and metastases of melanoma by enhancing VEGF-A expression

Malignant metastatic melanoma (MM) is the most lethal of all skin cancers, but detailed mechanisms for regulation of melanoma metastasis are not fully understood. Here, we demonstrated that developmentally regulated GTP-binding protein 2 (DRG2) is required for the growth of primary tumors and for metastasis. DRG2 expression was significantly increased in MM compared with primary melanoma (PM) and dysplastic nevi. A correlation between DRG2 expression and poor disease-specific survival in melanoma patients was also identified. Furthermore, inhibition of DRG2 suppressed the binding of Hypoxia-inducible factor 1α to the VEGF-A promoter region, expression of vascular endothelial growth factor (VEGF)-A, and formation of endothelial cell tubes. In experimental mice, DRG2 depletion inhibited the growth of PM and lung metastases and increased survival. These results identify DRG2 as a critical regulator of VEGF-A expression and of growth of PMs and lung metastases.

DRG2 knockdown induces Golgi fragmentation via GSK3β phosphorylation and microtubule stabilization

The perinuclear stacks of the Golgi apparatus maintained by dynamic microtubules are essential for cell migration. Activation of Akt (protein kinase B, PKB) negatively regulates glycogen synthase kinase 3β (GSK3β)-mediated tau phosphorylation, which enhances tau binding to microtubules and microtubule stability. In this study, experiments were performed on developmentally regulated GTP-binding protein 2 (DRG2)-stably knockdown HeLa cells to determine whether knockdown of DRG2 in HeLa cells treated with epidermal growth factor (EGF) affects microtubule dynamics, perinuclear Golgi stacking, and cell migration. Here, we show that DRG2 plays a key role in regulating microtubule stability, perinuclear Golgi stack formation, and cell migration. DRG2 knockdown prolonged the EGF receptor (EGFR) localization in endosome, enhanced Akt activity and inhibitory phosphorylation of GSK3β. Tau, a target of GSK3β, was hypo-phosphorylated in DRG2-knockdown cells and showed greater association with microtubules, resulting in microtubule stabilization. DRG2-knockdown cells showed defects in microtubule growth and microtubule organizing centers (MTOC), Golgi fragmentation, and loss of directional cell migration. These results reveal a previously unappreciated role for DRG2 in the regulation of perinuclear Golgi stacking and cell migration via its effects on GSK3β phosphorylation, and microtubule stability.

DRG2 Deficiency Causes Impaired Microtubule Dynamics in HeLa Cells

The developmentally regulated GTP binding protein 2 (DRG2) is involved in the control of cell growth and differentiation. Here, we demonstrate that DRG2 regulates microtubule dynamics in HeLa cells. Analysis of live imaging of the plus-ends of microtubules with EB1-EGFP showed that DRG2 deficiency (shDRG2) significantly reduced the growth rate of HeLa cells. Depletion of DRG2 increased 'slow and long-lived' subpopulations, but decreased 'fast and short-lived' subpopulations of microtubules. Microtubule polymerization inhibitor exhibited a reduced response in shDRG2 cells. Using immunoprecipitation, we show that DRG2 interacts with tau, which regulates microtubule polymerization. Collectively, these data demonstrate that DRG2 may aid in affecting microtubule dynamics in HeLa cells.

Elevated ZC3H15 increases HCC growth and predicts poor survival after surgical resection

Zinc finger CCCH-type containing 15 (ZC3H15), also known as DRG family regulatory protein 1 (DFRP1), is a highly conserved eukaryotic protein that associates with active translation machinery. The aim of our study was to explore the clinical relevance and intrinsic functions of ZC3H15 in hepatocellular carcinoma (HCC). We constructed a cohort with 261 tumor and matched normal tissues from HCC patients. ZC3H15 protein and mRNA levels were determined using immunohistochemistry, western blot analysis, and quantitative polymerase chain reaction. ZC3H15 was highly expressed in the majority of HCC cases, and high ZC3H15 levels were significantly associated with high serum a-fetoprotein (AFP) levels (>20 ng/mL) and vascular invasion. Kaplan-Meier and Cox regression data indicated that elevated ZC3H15 was an independent predictor for HCC-specific disease-free survival (hazards ratio [HR], 1.789; 95% confidence interval [95% CI], 1.298-2.466 [P=0.0004]) and overall survival (HR, 1.613; 95% CI, 1.120-2.322 [P=0.0101]). Interaction of ZC3H15 with TRAF2 increased activation of NFκB signaling. These results suggest ZC3H15 is an independent prognostic marker in HCC patients that is clinicopathologically associated with tumor invasion and serum AFP levels.

Weighted gene co-expression network analysis reveals potential genes involved in early metamorphosis process in sea cucumber Apostichopus japonicus

Sea cucumbers, one main class of Echinoderms, have a very fast and drastic metamorphosis process during their development. However, the molecular basis under this process remains largely unknown. Here we systematically examined the gene expression profiles of Japanese common sea cucumber (Apostichopus japonicus) for the first time by RNA sequencing across 16 developmental time points from fertilized egg to juvenile stage. Based on the weighted gene co-expression network analysis (WGCNA), we identified 21 modules. Among them, MEdarkmagenta was highly expressed and correlated with the early metamorphosis process from late auricularia to doliolaria larva. Furthermore, gene enrichment and differentially expressed gene analysis identified several genes in the module that may play key roles in the metamorphosis process. Our results not only provide a molecular basis for experimentally studying the development and morphological complexity of sea cucumber, but also lay a foundation for improving its emergence rate.

Developmentally regulated GTP-binding protein 2 depletion leads to mitochondrial dysfunction through downregulation of dynamin-related protein 1

Mitochondrial dynamics, including constant fusion and fission, play critical roles in maintaining mitochondrial morphology and function. Here, we report that developmentally regulated GTP-binding protein 2 (DRG2) regulates mitochondrial morphology by modulating the expression of the mitochondrial fission gene dynamin-related protein 1 (Drp1). shRNA-mediated silencing of DRG2 induced mitochondrial swelling, whereas expression of an shRNA-resistant version of DRG2 decreased mitochondrial swelling in DRG2-depleted cells. Analysis of the expression levels of genes involved in mitochondrial fusion and fission revealed that DRG2 depletion significantly decreased the level of Drp1. Overexpression of Drp1 rescued the defect in mitochondrial morphology induced by DRG2 depletion. DRG2 depletion reduced the mitochondrial membrane potential, oxygen consumption rate (OCR), and amount of mitochondrial DNA (mtDNA), whereas it increased reactive oxygen species (ROS) production and apoptosis. Taken together, our data demonstrate that DRG2 acts as a regulator of mitochondrial fission by controlling the expression of Drp1.

DRG2 Regulates G2/M Progression via the Cyclin B1-Cdk1 Complex

Developmentally regulated GTP-binding protein 2 (DRG2) plays an important role in cell growth. Here we explored the linkage between DRG2 and G2/M phase checkpoint function in cell cycle progression. We observed that knockdown of DRG2 in HeLa cells affected growth in a wound-healing assay, and tumorigenicity in nude mice xenografts. Flow cytometry assays and [(3)H] incorporation assays indicated that G2/M phase arrest was responsible for the decreased proliferation of these cells. Knockdown of DRG2 elicited down-regulation of the major mitotic promoting factor, the cyclin B1/Cdk1 complex, but up-regulation of the cell cycle arresting proteins, Wee1, Myt1, and p21. These findings identify a novel role of DRG2 in G2/M progression.

Developmentally regulated GTP-binding protein 2 coordinates Rab5 activity and transferrin recycling

The small GTPase Rab5 regulates the early endocytic pathway of transferrin (Tfn), and Rab5 deactivation is required for Tfn recycling. Developmentally regulated GTP-binding protein 2 is required for interaction between Rab5 and RabGAP5 on endosomes and acts as a key regulator for Rab5 deactivation and Tfn recycling.

The small GTPase Rab5 regulates the early endocytic pathway of transferrin (Tfn), and Rab5 deactivation is required for Tfn recycling. Rab5 deactivation is achieved by RabGAP5, a GTPase-activating protein, on the endosomes. Here we report that recruitment of RabGAP5 is insufficient to deactivate Rab5 and that developmentally regulated GTP-binding protein 2 (DRG2) is required for Rab5 deactivation and Tfn recycling. DRG2 was associated with phosphatidylinositol 3-phosphate–containing endosomes. It colocalized and interacted with EEA1 and Rab5 on endosomes in a phosphatidylinositol 3-kinase–dependent manner. DRG2 depletion did not affect Tfn uptake and recruitment of RabGAP5 and Rac1 to Rab5 endosomes. However, it resulted in impairment of interaction between Rab5 and RabGAP5, Rab5 deactivation on endosomes, and Tfn recycling. Ectopic expression of shRNA-resistant DRG2 rescued Tfn recycling in DRG2-depleted cells. Our results demonstrate that DRG2 is an endosomal protein and a key regulator of Rab5 deactivation and Tfn recycling.

MicroRNA‑1915‑3p prevents the apoptosis of lung cancer cells by downregulating DRG2 and PBX2

Micro (mi)RNAs are short non‑coding RNA molecules, which post‑transcriptionally regulate gene expression and exert key roles in cell growth, differentiation and apoptosis. In the present study, the mechanism and the function of miR‑1915‑3p in the apoptotic regulation of lung cancer cell lines (NCI‑H441 and NCI‑H1650) were investigated. The expression analysis confirmed that the expression of miR‑1915‑3p was markedly decreased in the apoptotic cells. The overexpression of miR‑1915‑3p in the lung cancer cells prevented apoptosis induced by etoposide. Developmentally regulated GTP‑binding protein 2 (DRG2) and pre‑B cell leukemia homeobox 2 (PBX2) were identified as downstream targets of miR‑1915‑3p, which was shown to bind directly to the 3'‑untranslated region of DRG2 and PBX2, subsequently lowering their mRNA and protein expression levels. Co‑expression of miR‑1915‑3p and DRG2/PBX2 in the NCI‑H441 and NCI‑H1650 cells partly circumvented the effect of miR‑1915‑3p on apoptosis. The results in the present study revealed that miR‑1915‑3p functions as a silencer of apoptosis, which regulates lung cancer apoptosis via targeting DRG2/PBX2, and consequently this miRNA may be a putative therapeutic target in lung cancer.

Developmentally regulated GTP-binding protein 2 ameliorates EAE by suppressing the development of TH17 cells

Developmentally regulated GTP-binding protein 2 (DRG2) represents a novel subclass of GTP-binding proteins. We here report that transgenic overexpression of DRG2 in mice ameliorates experimental autoimmune encephalomyelitis (EAE), a murine model of multiple sclerosis (MS). The protective effect of DRG2 in EAE was mediated by the inhibition of the development of T(H)17 cells. DRG2 enhanced the activity of PPARγ, which led to an inhibition of the nuclear factor kappa B (NF-κB) activity and IL-6 production in antigen presenting cells and an inhibition of the development of T(H)17 cells. Our results demonstrate that DRG2 is an essential modulator of EAE.

Overexpression of developmentally regulated GTP-binding protein-2 increases bone loss

The developmentally regulated GTP-binding protein-2 (DRG2) is a novel subclass of GTP-binding proteins. Many functional characteristics of osteoclasts (OC) are associated with small GTPases. We hypothesized that DRG2 affects bone mass via modulating OC activity. Using DRG2 transgenic mice, we investigated the role of DRG2 in bone remodeling. DRG2 overexpression caused a decrease in bone mass and an increase in the number and activity of OC in vivo. DRG2 overexpression increased fusion, spreading, survival, and resorption activity of OC in vitro. Downregulation of DRG2 by siRNA decreased fusion, spreading, and survival of OC, supporting the observations found in DRG2 transgenic OC. Transgenic mature OCs were larger, with actin rings and higher ERK, Akt, Rac1 and Rho activities than wild-type OCs. Inhibition of these proteins abolished the effects of DRG2 on formation of large OCs with actin rings, implying that DRG2 affects cytoskeleton reorganization in a Rac1/Rho/ERK/Akt-dependent manner. In summary, DRG2 is associated with survival and cytoskeleton organization of OC under influence of macrophage colony-stimulating factor, and its overexpression leads to elevated bone resorptive activity of OC, resulting in bone loss.

Transcription factor Sp1 regulates basal transcription of the human DRG2 gene

Developmentally regulated GTP-binding protein 2 (DRG2) is an evolutionarily conserved GTP-binding protein. DRG2 mRNA expression has been confirmed in many animal and human tissues. DRG2 is thought to play an essential role in the control of cell growth and differentiation. However, transcriptional regulation of DRG2 is largely unknown. To investigate the mechanisms controlling DRG2 expression, we cloned 1509bp of the 5'-flanking sequence of this gene. Deletion analysis showed that the region between -113 and -70 is essential for the basal level expression of the DRG2 gene in K562 human erythroleukemic cells. Mutation of a putative stimulating protein 1 (Sp1) regulatory site located at position -108 resulted in a significant decline in DRG2 promoter activity. Electrophoretic mobility shift assay and chromatin immunoprecipitation analysis revealed that Sp1 binds to this site. Knockdown of Sp1 expression using siRNA inhibited the promoter activation as well as the endogenous DRG2 transcriptional level. Taken together, these results demonstrate that basal expression level of DRG2 is regulated by the Sp1 transcription factor.

The highly conserved eukaryotic DRG factors are required for efficient translation in a manner redundant with the putative RNA helicase Slh1

Eukaryotic and archaeal DRG factors are highly conserved proteins with characteristic GTPase motifs. This suggests their implication in a central biological process, which has so far escaped detection. We show here that the two Saccharomyces cerevisiae DRGs form distinct complexes, RBG1 and RBG2, and that the former co-fractionate with translating ribosomes. A genetic screen for triple synthetic interaction demonstrates that yeast DRGs have redundant function with Slh1, a putative RNA helicase also associating with translating ribosomes. Translation and cell growth are severely impaired in a triple mutant lacking both yeast DRGs and Slh1, but not in double mutants. This new genetic assay allowed us to characterize the roles of conserved motifs present in these proteins for efficient translation and/or association with ribosomes. Altogether, our results demonstrate for the first time a direct role of the highly conserved DRG factors in translation and indicate that this function is redundantly shared by three factors. Furthermore, our data suggest that important cellular processes are highly buffered against external perturbation and, consequently, that redundantly acting factors may escape detection in current high-throughput binary genetic interaction screens.

Structure of the human protein kinase MPSK1 reveals an atypical activation loop architecture

The activation segment of protein kinases is structurally highly conserved and central to regulation of kinase activation. Here we report an atypical activation segment architecture in human MPSK1 comprising a β sheet and a large α-helical insertion. Sequence comparisons suggested that similar activation segments exist in all members of the MPSK1 family and in MAST kinases. The consequence of this nonclassical activation segment on substrate recognition was studied using peptide library screens that revealed a preferred substrate sequence of X-X-P/V/I-ϕ-H/Y-T^∗-N/G-X-X-X (ϕ is an aliphatic residue). In addition, we identified the GTPase DRG1 as an MPSK1 interaction partner and specific substrate. The interaction domain in DRG1 was mapped to the N terminus, leading to recruitment and phosphorylation at Thr100 within the GTPase domain. The presented data reveal an atypical kinase structural motif and suggest a role of MPSK1 regulating DRG1, a GTPase involved in regulation of cellular growth.

Identification of DRG family regulatory proteins (DFRPs): specific regulation of DRG1 and DRG2

DRG1 and DRG2 comprise a highly conserved subfamily of GTP-binding proteins and are thought to act as critical regulators of cell growth. Their abnormal expressions may trigger cell transformation or cell cycle arrest. Our aim is to clarify their physiological functions and regulatory mechanisms. Here we report identification of novel proteins, DRG family regulatory protein (DFRP) 1 and DFRP2, which regulate expression of DRG proteins through specific binding. In transient transfection experiments, DFRP1 specifically binds DRG1, and DFRP2 preferentially binds DRG2. DFRPs provide stability to the target DRG proteins through physical association, possibly by blocking the poly-ubiquitination that would precede proteolysis of DRG proteins. DFRPs are highly conserved in eucaryotes, and the expression patterns of dfrp1 and drg1 transcripts in Xenopus embryos and tissues are similar, indicating that these genes work cooperatively in various types of eukaryotic cells. Immunofluorescence experiments have revealed that the interaction between DRG1 and DFRP1 may occur in the cytoplasm. We generated dfrp1- knockout cells and found that endogenous expression of DRG1 is regulated by DFRP1, confirming that DFRP1 is a specific up-regulator of DRG1 in vivo. On the basis of these results, we propose that DRG1 and DRG2 are regulated differently despite their structural similarities.