Multiple nuclear localization signals of the B-myb gene product

Overexpression of ScMYBAS1 alternative splicing transcripts differentially impacts biomass accumulation and drought tolerance in rice transgenic plants

Drought is the most significant environmental stress for agricultural production worldwide, and tremendous efforts have been made to improve crop yield under the increasing water scarcity. Transcription factors are major players in the regulation of water stress-related genes in plants. Recently, different MYB transcription factors were characterized for their involvement in drought response. A sugarcane R2R3-MYB gene (ScMYBAS1) and its four alternative forms of transcript (ScMYAS1-2, ScMYBAS1-3, ScMYBAS1-4 and ScMYBAS1-5) were identified in this study. The subcellular localization, in Nicotiniana benthamiana, of the TFs fused in frame with GFP revealed that ScMYBAS1-2-GFP and ScMYBAS1-3-GFP were observed in the nucleus. The overexpression of ScMYBAS1-2 and ScMYBAS1-3 spliced transcripts in rice promoted change in plant growth under both well-watered and drought conditions. The ScMYBAS1-2 and ScMYBAS1-3 transgenic lines revealed a higher relative water content (RWC) compared to the wild type before maximum stress under drought conditions. The ScMYBAS1-2 transgenic lines showed a reduction in biomass (total dry weight). Conversely, ScMYBAS1-3 showed an increased biomass (total dry weight) relative to the wild-type. The overexpression of ScMYBAS1-3 in rice transgenic lines showed involvement with drought tolerance and biomass and, for this reason, was considered a good target for plant transformation, particularly for use in developing genotypes with drought tolerance and biomass accumulation.

Iron-inducible nuclear translocation of a Myb3 transcription factor in the protozoan parasite Trichomonas vaginalis

In Trichomonas vaginalis, a novel nuclear localization signal spanning the folded R2R3 DNA-binding domain of a Myb2 protein was previously identified. To study whether a similar signal is used for nuclear translocation by other Myb proteins, nuclear translocation of Myb3 was examined in this report. When overexpressed, hemagglutinin-tagged Myb3 was localized to nuclei of transfected cells, with a cellular distribution similar to that of endogenous Myb3. Fusion to a bacterial tetracycline repressor, R2R3, of Myb3 that spans amino acids (aa) 48 to 156 was insufficient for nuclear translocation of the fusion protein, unless its C terminus was extended to aa 167. The conserved isoleucine in helix 2 of R2R3, which is important for Myb2's structural integrity in maintaining DNA-binding activity and nuclear translocation, was also vital for the former activity of Myb3, but less crucial for the latter. Sequential nuclear influx and efflux of Myb3, which require further extension of the nuclear localization signal to aa 180, were immediately induced after iron repletion. Sequence elements that regulate nuclear translocation with cytoplasmic retention, nuclear influx, and nuclear efflux were identified within the C-terminal tail. These results suggest that the R2R3 DNA-binding domain also serves as a common module for the nuclear translocation of both Myb2 and Myb3, but there are intrinsic differences between the two nuclear localization signals.

A highly organized structure mediating nuclear localization of a Myb2 transcription factor in the protozoan parasite Trichomonas vaginalis

Nuclear proteins usually contain specific peptide sequences, referred to as nuclear localization signals (NLSs), for nuclear import. These signals remain unexplored in the protozoan pathogen, Trichomonas vaginalis. The nuclear import of a Myb2 transcription factor was studied here using immunodetection of a hemagglutinin-tagged Myb2 overexpressed in the parasite. The tagged Myb2 was localized to the nucleus as punctate signals. With mutations of its polybasic sequences, 48KKQK51 and 61KR62, Myb2 was localized to the nucleus, but the signal was diffusive. When fused to a C-terminal non-nuclear protein, the Myb2 sequence spanning amino acid (aa) residues 48 to 143, which is embedded within the R2R3 DNA-binding domain (aa 40 to 156), was essential and sufficient for efficient nuclear import of a bacterial tetracycline repressor (TetR), and yet the transport efficiency was reduced with an additional fusion of a firefly luciferase to TetR, while classical NLSs from the simian virus 40 T-antigen had no function in this assay system. Myb2 nuclear import and DNA-binding activity were substantially perturbed with mutation of a conserved isoleucine (I74) in helix 2 to proline that altered secondary structure and ternary folding of the R2R3 domain. Disruption of DNA-binding activity alone by point mutation of a lysine residue, K51, preceding the structural domain had little effect on Myb2 nuclear localization, suggesting that nuclear translocation of Myb2, which requires an ordered structural domain, is independent of its DNA binding activity. These findings provide useful information for testing whether myriad Mybs in the parasite use a common module to regulate nuclear import.

Identification and functional analysis of NOL7 nuclear and nucleolar localization signals

BACKGROUND

NOL7 is a candidate tumor suppressor that localizes to a chromosomal region 6p23. This locus is frequently lost in a number of malignancies, and consistent loss of NOL7 through loss of heterozygosity and decreased mRNA and protein expression has been observed in tumors and cell lines. Reintroduction of NOL7 into cells resulted in significant suppression of in vivo tumor growth and modulation of the angiogenic phenotype. Further, NOL7 was observed to localize to the nucleus and nucleolus of cells. However, the mechanisms regulating its subcellular localization have not been elucidated.

RESULTS

An in vitro import assay demonstrated that NOL7 requires cytosolic machinery for active nuclear transport. Using sequence homology and prediction algorithms, four putative nuclear localization signals (NLSs) were identified. NOL7 deletion constructs and cytoplasmic pyruvate kinase (PK) fusion proteins confirmed the functionality of three of these NLSs. Site-directed mutagenesis of PK fusions and full-length NOL7 defined the minimal functional regions within each NLS. Further characterization revealed that NLS2 and NLS3 were critical for both the rate and efficiency of nuclear targeting. In addition, four basic clusters within NLS2 and NLS3 were independently capable of nucleolar targeting. The nucleolar occupancy of NOL7 revealed a complex balance of rapid nucleoplasmic shuttling but low nucleolar mobility, suggesting NOL7 may play functional roles in both compartments. In support, targeting to the nucleolar compartment was dependent on the presence of RNA, as depletion of total RNA or rRNA resulted in a nucleoplasmic shift of NOL7.

CONCLUSIONS

These results identify the minimal sequences required for the active targeting of NOL7 to the nucleus and nucleolus. Further, this work characterizes the relative contribution of each sequence to NOL7 nuclear and nucleolar dynamics, the subnuclear constituents that participate in this targeting, and suggests a functional role for NOL7 in both compartments. Taken together, these results identify the requisite protein domains for NOL7 localization, the kinetics that drive this targeting, and suggest NOL7 may function in both the nucleus and nucleolus.

B-Myb-Dependent Regulation of c-Myc Expression by Cytosolic Phospholipase A2

Cytosolic phospholipase A(2) (cPLA(2)) cleaves membrane phospholipids to release arachidonic acid, initiating lipoxygenase and cyclooxygenase pathways. Mice lacking a gene for cPLA(2) suggested important roles of the protein in allergic responses, fertility, and neural cell death. Here we show that cPLA(2) negatively regulates c-Myc expression in a B-Myb-dependent manner. Overexpression of cPLA(2) protein but not a mutant cPLA(2) protein that lacks in vitro binding ability with B-Myb inhibits B-Myb-dependent c-myc gene expression. The inhibition was associated with physical interaction of B-Myb protein with cPLA(2) both in the cytoplasm and the nucleus. Binding site analysis demonstrated that both the N and C termini of cPLA(2) interact with B-Myb. Macrophage colony stimulating factor (MCSF) stimulated cPLA(2) redistribution into the nucleus and also association with B-Myb in human monocytes. Importantly, macrophages from mice with a disrupted cPLA(2) gene demonstrated significantly increased levels of c-Myc protein in the nucleus compared with cells from the wild-type mice, whereas B-Myb levels were similar in the cells from the cPLA(2)(+/+) and cPLA(2)(-/-) mice. Moreover, an introduction of cPLA(2) into cPLA(2)(-/-) mouse macrophages resulted in decreased c-Myc protein levels, and an inhibition of cPLA(2) expression by small interfering RNAs or antisense RNA increased the c-myc transcription in macrophage colony stimulating factor-activated human monocytes. These findings provide new insights into the function of cPLA(2) in B-Myb-dependent gene expression.

B-Myb repressor function is regulated by cyclin A phosphorylation and sequences within the C-terminal domain

B-Myb is a widely expressed member of the myb oncogene family that has been shown to act as either an activator or repressor of gene transcription in a cell-type-specific fashion. For example, in aortic smooth muscle cells B-Myb represses transcription of the alpha2(V) collagen gene. Recently, phosphorylation of B-Myb by cyclin A was shown to enhance greatly its ability to transactivate. Here, we have tested the effects of cyclin A on the ability of B-Myb to repress. We report that coexpression of cyclin A abolished repression of the alpha2(V) collagen promoter, whereas a dominant-negative cyclin-dependent kinase 2 (cdk2) enhanced repression by ectopic and endogenous B-Myb protein. Mutation of 10 of 22 putative cyclin A sites, which greatly reduces the effects of cyclin A on transactivation by B-Myb, had no effect on the ability of cyclin A to alleviate B-Myb-mediated repression of alpha2(V) collagen promoter activity. Furthermore, the stability of the mutant B-Myb protein was largely unaffected by cyclin A, although ectopic expression of cyclin A enhanced the rate of decay of wild-type B-Myb protein. Thus, the mechanisms of repression and activation appear distinct, for example, mediated by different critical phosphorylation sites or protein-protein interactions. B-Myb mutants with either deletion of aa 374-581 (B-Myb-Mut3) or C-terminal truncation beyond aa 491 (B-Myb-491) positively regulated alpha2(V) collagen promoter activity, and were not affected by cyclin A. Thus, our findings indicate that the ability of B-Myb to function as a repressor of matrix promoter activity is abolished by cyclin A, and maps the sites mediating negative regulation by B-Myb to the region between aa 491 and 582.

Effects of B-Myb on gene transcription: phosphorylation-dependent activity ans acetylation by p300

The transcription factor B-Myb is a cell-cycle regulated phosphoprotein involved in cell cycle progression through the transcriptional regulation of many genes. In this study, we show that the promoter of the fibroblast growth factor-4 (FGF-4) gene is strongly activated by B-Myb in HeLa cells and it can serve as a novel diagnostic tool for assessing B-Myb activity. Specifically, B-Myb deletion mutants were examined and domains of B-Myb required for activation of the FGF-4 promoter were identified. Using phosphorylation-deficient mutant forms of B-Myb, we also show that phosphorylation is essential for B-Myb activity. Moreover, a mutant form of B-Myb, which lacks all identified phosphorylation sites and which has little activity, can function as a dominant-negative and suppress wild-type B-Myb activity. Acetylation is another post-translational modification known to affect the activity of other Myb family members. We show that B-Myb is acetylated by the co-activator p300. We also show that the bromo and histone acetyltransferase domains of p300 are sufficient to interact with and acetylate B-Myb. These data indicate that phosphorylation of B-Myb is an essential modification for activity and that acetylation of B-Myb may play a role in B-Myb activity.

Novel anther-specific myb genes from tobacco as putative regulators of phenylalanine ammonia-lyase expression

Two cDNA clones (NtmybAS1 and NtmybAS2) encoding MYB-related proteins with strong sequence similarity to petunia (Petunia hybrida) PhMYB3 were isolated from a tobacco (Nicotiana tabacum cv Samsun) pollen cDNA library. Northern blot and in situ hybridization revealed that NtmybAS transcripts are specifically expressed in both sporophytic and gametophytic tissues of the anther including tapetum, stomium, vascular tissue, and developing pollen. Random binding site selection assays revealed that NtMYBAS1 bound to DNA sequences closely resembling consensus MYB binding sites MBSI and MBSIIG, with a higher affinity for MBSI. Transient expression analyses of the N-terminal MYB domain demonstrated the presence of functional nuclear localization signals, and full-length NtMYBAS1 was able to activate two different phenylalanine ammonia-lyase promoters (PALA and gPAL1) in tobacco leaf protoplasts. Similar analysis of truncated NtmybAS1 cDNAs identified an essential, C-terminal trans-activation domain. Further in situ hybridization analyses demonstrated strict co-expression of NtmybAS and gPAL1 in the tapetum and stomium. Despite abundant expression of NtmybAS transcripts in mature pollen, gPAL1 transcripts were not detectable in pollen. Our data demonstrate that NtMYBAS1 is a functional anther-specific transcription factor, which is likely to be a positive regulator of gPAL1 expression and phenylpropanoid synthesis in sporophytic, but not in gametophytic, tissues of the anther.

Regulation of DNA binding activity and nuclear transport of B-Myb in Xenopus oocytes

DNA binding activity and nuclear transport of B-Myb in Xenopus oocytes are negatively regulated. Two distinct sequence elements in the C-terminal portion of the protein are responsible for these different inhibitory activities. A C-terminal Xenopus B-Myb protein fragment inhibits the DNA binding activity of the N-terminal repeats in trans, indicating that intramolecular folding may result in masking of the DNA binding function. Xenopus B-Myb contains two separate nuclear localization signals (NLSs), which, in Xenopus oocytes, function only outside the context of the full-length protein. Fusion of an additional NLS to the full-length protein overcomes the inhibition of nuclear import, suggesting that masking of the NLS function rather than cytoplasmic anchoring is responsible for the negative regulation of Xenopus B-Myb nuclear transfer. During Xenopus embryogenesis, when inhibition of nuclear import is relieved, Xenopus B-myb is preferentially expressed in the developing nervous system and neural crest cells. Within the developing neural tube, Xenopus B-myb gene transcription occurs preferentially in proliferating, non-differentiated cells.

Nuclear import and export of influenza virus nucleoprotein

Influenza virus nucleoprotein (NP) shuttles between the nucleus and the cytoplasm. A nuclear localization signal (NLS) has been identified in NP at amino acids 327 to 345 (J. Davey et al., Cell 40:667-675, 1985). However, some NP mutants that lack this region still localize to the nucleus, suggesting an additional NLS in NP. We therefore investigated the nucleocytoplasmic transport of NP from influenza virus A/WSN/33 (H1N1). NP deletion constructs lacking the 38 N-terminal amino acids, as well as those lacking the 38 N-terminal amino acids and the previously identified NLS, localized to both the cytoplasm and the nucleus. Nuclear localization of a protein containing amino acids 1 to 38 of NP fused to LacZ proved that these 38 amino acids function as an NLS. Within this region, we identified two basic amino acids, Lys7 and Arg8, that are crucial for NP nuclear import. After being imported into the nucleus, the wild-type NP and the NP-LacZ fusion construct containing amino acids 1 to 38 of NP were both transported back to the cytoplasm, where they accumulated. These data indicate that NP has intrinsic structural features that allow nuclear import, nuclear export, and cytoplasmic accumulation in the absence of any other viral proteins. Further, the information required for nuclear import and export is located in the 38 N-terminal amino acids of NP, although other NP nuclear export signals may exist. Treatment of cells with a protein kinase C inhibitor increased the amounts of nuclear NP, whereas treatment of cells with a phosphorylation stimulator increased the amounts of cytoplasmic NP. These findings suggest a role of phosphorylation in nucleocytoplasmic transport of NP.

The regulation of protein transport to the nucleus by phosphorylation

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The mechanism of nuclear transport of natural or artificial transport substrates in digitonin-permeabilized cells

Characterization of nuclear protein transport in digitonin-permeabilized cells revealed that the number of the nuclear localization signal sequences (NLS) within the transport substrate basically influences the mechanism of the transport reaction. Phycoerythrine-NLS transport substrate carrying a maximum of 4–5 conjugated NLSs/subunit, or Bsp methyltransferase-NLS fusion protein were efficiently transported into the nuclei of digitonin-permeabilized cultured cells without any exogenously added cytosolic protein. All the characteristic properties of in vivo nuclear transport are faithfully reproduced with these transport substrates: (i) the transport requires a functional NLS in the transported protein, a transport-incompetent mutant NLS being ineffective; (ii) the transport is energy dependent; (iii) the wild type nuclear localization peptide efficiently competes for transport, while the transport-incompetent mutant peptide does not; and (iv) wheat germ agglutinin inhibits this transport reaction. Nuclear transport observed with these substrates was not due to any damage of the nuclear membrane or inefficient extraction of the cytosolic proteins during the permeabilization of the cells. The nuclear transport was proportional to the number of conjugated NLSs. Nuclear transport of phycoerythrine carrying 7–8 conjugated NLSs/subunit required the addition of exogenous cytosolic proteins. This transport also fulfilled all the characteristic properties of an authentic nuclear transport. Nuclear transport with different combinations of transport substrates further supported the assumption that distinct transport mechanisms operate for different substrates. From a mixture of PE-NLS7-8 and Bsp methyltransferase-NLS, the highly conjugated substrate was completely retained in the cytoplasm in the absence of exogenous cytosol, while Bsp methyltransferase-NLS was efficiently transported. Exogenous cytosol promoted the nuclear transport of the highly conjugated substrate.