Identification of a bipartite nuclear localization sequence necessary for nuclear import of 5-lipoxygenase.

5-Lipoxygenase catalyzes the synthesis of leukotrienes from arachidonic acid. This enzyme can reside either in the cytoplasm or the nucleus; its subcellular distribution is influenced by extracellular factors, and its nuclear import correlates with changes in leukotriene synthetic capacity. To identify sequences responsible for the nuclear import of 5-lipoxygenase, we transfected NIH 3T3 cells and RAW 264.7 macrophages with expression vectors encoding various 5-lipoxygenase constructs fused to green fluorescent protein. Overexpression of wild type 5-lipoxygenase with or without fusion to green fluorescent protein resulted in a predominantly intranuclear pattern of fluorescence, similar to the distribution of native 5-lipoxygenase in primary alveolar macrophages. Within the 5-lipoxygenase protein is a sequence (Arg(638)-Lys(655)) that closely resembles a bipartite nuclear localization signal. Studies using deletion mutants indicated that this region was necessary for nuclear import of 5-lipoxygenase. Analysis of mutants containing specific amino acid substitutions within this sequence confirmed that it was this sequence that was necessary for nuclear import of 5-lipoxygenase and that a specific arginine residue was critical for this function. As nuclear import of 5-lipoxygenase may regulate leukotriene production, natural or induced mutations in this bipartite nuclear localization sequence may also be important in affecting leukotriene synthesis.     

Identification of two novel nuclear import sequences on the 5-lipoxygenase protein

The nuclear import of 5-lipoxygenase modulates its capacity to produce leukotrienes from arachidonic acid. However, the molecular determinants of its nuclear import are unknown. Recently, we used structural and functional criteria to identify a novel import sequence at Arg(518) on human 5-lipoxygenase (Jones, S. M., Luo, M., Healy, A. M., Peters-Golden, M., and Brock, T. G. (2002) J. Biol. Chem. 277, 38550-38556). However, this analysis also indicated that other import sequences must exist. Here, we identify two additional sites, at Arg(112) and Lys(158), as nuclear import sequences. Both sites were found to be common to 5-lipoxygenases from different species but not found on other lipoxygenases. Both sites also appeared to be a part of structures that were predominantly random loops. Peptide sequences at these sites were sufficient to direct nuclear import of green fluorescent protein. Mutation of basic residues in these sites impaired nuclear import and combinations of mutations at different sites were additive in effect. Mutations in all three sites were required to disable nuclear accumulation of 5-lipoxygenase in all cells. Significantly, mutation in these sites did not inhibit catalytic function. Taken together, these results indicate that nuclear import of 5-lipoxygenase may reflect the combined functional effects of three discrete import sequences. Mutation of individual sites can, by itself, impair nuclear import, which in turn could impact arachidonic acid metabolism.     

Phosphorylation by protein kinase a inhibits nuclear import of 5-lipoxygenase

The enzyme 5-lipoxygenase initiates the synthesis of leukotrienes from arachidonic acid. Protein kinase A phosphorylates 5-lipoxygenase on Ser(523), and this reduces its activity. We report here that phosphorylation of Ser(523) also shifts the subcellular distribution of 5-lipoxygenase from the nucleus to the cytoplasm. Phosphorylation and redistribution of 5-lipoxygenase could be produced by overexpression of the protein kinase A catalytic subunit alpha, by pharmacological activators of protein kinase A, and by prostaglandin E(2). Mimicking phosphorylation by replacing Ser(523) with glutamic acid caused cytoplasmic localization; replacement of Ser(523) with alanine prevented phosphorylation and redistribution in response to protein kinase A activation. Because Ser(523) is positioned within the nuclear localization sequence-518 of 5-lipoxygenase, the ability of protein kinase A to phosphorylate and alter the localization of green fluorescent protein fused to the nuclear localization sequence-518 peptide was also tested. Site-directed replacement of Ser(523) with glutamic acid within the peptide impaired nuclear accumulation; overexpression of the protein kinase A catalytic subunit alpha and pharmacological activation of protein kinase caused phosphorylation of the fusion protein at Ser(523), and the phosphorylated protein was found chiefly in the cytoplasm. Taken together, these results indicate that phosphorylation of Ser(523) inhibits the nuclear import function of a nuclear localization sequence, resulting in the accumulation of 5-lipoxygenase enzyme in the cytoplasm. As cytoplasmic localization can be associated with reduced leukotriene synthetic capacity, phosphorylation of Ser(523) serves to inhibit leukotriene production by both impairing catalytic activity and by placing the enzyme in a site that is unfavorable for action.     

Bipartite Nuclear Localization Signals in the C Terminus of Human Topoisomerase IIα

DNA topoisomerase IIα is the intracellular target for several important chemotherapeutic agents, and drug-resistant human tumor cell lines have been described in which deletions in the C-proximal region of this enzyme are associated with its cytoplasmic localization. We have identified multiple potential bipartite nuclear localization signal (NLS) sequences in this region using a modified definition of the motif, and in the present study, we have expressed five of these as fusion proteins with β-galactosidase. Only one sequence (spanning amino acids 1454 to 1497) was sufficient to cause strong nuclear localization. Subsequent mutation analyses indicated that this NLS sequence was bipartite and that both domains contain more than two basic amino acids. Substitution of the lysine residue at position 1492 in the second basic domain with glutamine resulted in a fusion protein that localized inefficiently to the nucleus, indicating that all three basic residues in this domain are necessary. Our results confirm that a broader definition is required to detect all potential bipartite NLS motifs in a polypeptide sequence, although functional tests are still essential for identification of those sequences actually capable of directing nuclear localization.     

Multiple nuclear localization sequences allow modulation of 5-lipoxygenase nuclear import

The nuclear import of proteins typically requires the presence of a nuclear localization sequence (NLS). Some proteins have more than one NLS, but the significance of having multiple NLSs is unclear. The enzyme 5-lipoxygenase (5-LO) has three NLSs that, unlike the tight cluster of basic residues of the classical SV40 large T antigen NLS, contain dispersed basic residues. When attached to green fluorescent protein (GFP), individual 5-LO NLSs caused quantitatively and statistically less import than the SV40 NLS. Combined 5-LO NLSs produced nuclear import that was comparable to that of the SV40 NLS. As expected, GFP/NLS proteins displayed relatively uniform import in all cells. However, a fusion protein of GFP plus the 5-LO protein, modified to contain only one functional NLS, produced some cells with import and some cells without import. A GFP/5-LO fusion protein containing two functional NLSs produced four identifiable levels of nuclear import. Quantitative and visual analysis of a population of cells expressing the intact GFP/5-LO protein, with three intact NLSs, indicated five levels of nuclear import. This suggested that the subcellular distribution of 5-LO may vary widely in normal cells of the body. Consistent with this, immunohistochemical staining of lung sections found that individual macrophages, in situ, displayed cell-specific levels of import of 5-LO. Since nuclear accumulation is known to affect 5-LO activity, multiple NLSs may allow graded regulation of activity via controlled import. Multiple NLSs on other proteins may likewise allow fine control of protein action through modulation of the level of import.     

Co-localization of leukotriene a4 hydrolase with 5-lipoxygenase in nuclei of alveolar macrophages and rat basophilic leukemia cells but not neutrophils.

The synthesis of leukotriene B(4) from arachidonic acid requires the sequential action of two enzymes: 5-lipoxygenase and leukotriene A(4) hydrolase. 5-Lipoxygenase is known to be present in the cytoplasm of some leukocytes and able to accumulate in the nucleoplasm of others. In this study, we asked if leukotriene A(4) hydrolase co-localizes with 5-lipoxygenase in different types of leukocytes. Examination of rat basophilic leukemia cells by both immunocytochemistry and immunofluorescence revealed that leukotriene A(4) hydrolase, like 5-lipoxygenase, was most abundant in the nucleus, with only minor occurrences in the cytoplasm. The finding of abundant leukotriene A(4) hydrolase in the soluble nuclear fraction was substantiated by two different cell fractionation techniques. Leukotriene A(4) hydrolase was also found to accumulate together with 5-lipoxygenase in the nucleus of alveolar macrophages. This result was obtained using both in situ and ex vivo techniques. In contrast to these results, peripheral blood neutrophils contained both leukotriene A(4) hydrolase and 5-lipoxygenase exclusively in the cytoplasm. After adherence of neutrophils, 5-lipoxygenase was rapidly imported into the nucleus, whereas leukotriene A(4) hydrolase remained cytosolic. Similarly, 5-lipoxygenase was localized in the nucleus of neutrophils recruited into inflamed appendix tissue, whereas leukotriene A(4) hydrolase remained cytosolic. These results demonstrate for the first time that leukotriene A(4) hydrolase can be accumulated in the nucleus, where it co-localizes with 5-lipoxygenase. As with 5-lipoxygenase, the subcellular distribution of leukotriene A(4) hydrolase is cell-specific and dynamic, but differences in the mechanisms regulating nuclear import must exist. The degree to which these two enzymes are co-localized may influence their metabolic coupling in the conversion of arachidonic acid to leukotriene B(4).     

Phosphorylation of serine 271 on 5-lipoxygenase and its role in nuclear export

The enzyme 5-lipoxygenase (5-LO) initiates the biosynthesis of leukotrienes, inflammatory mediators involved in immune diseases and defense. The subcellular localization of 5-LO is regulated, with nuclear import commonly leading to increased leukotriene production. We report here that 5-LO is constitutively phosphorylated on Ser-271 in transfected NIH 3T3 cells. This residue is nested in a classical nuclear export sequence, and phosphorylated Ser-271 5-LO was exclusively found in the nucleus by immunofluorescence and by fractionation techniques. Mutation of Ser-271 to Ala allowed nuclear export of 5-LO that was blocked by the specific nuclear export inhibitor leptomycin b, suggesting that phosphorylation of Ser-271 serves to interfere with exportin-1-mediated nuclear export. Consistent with previous reports that purified 5-LO can be phosphorylated on Ser-271 in vitro by MAPK-activated protein kinase 2, the nuclear export of 5-LO was increased by either treatment with the p38 inhibitor SB 203,580 or co-expression of a kinase-deficient p38 MAPK. Nuclear export of 5-LO can also be induced by KN-93, an inhibitor of Ca2+/calmodulin-dependent kinase II, and the effects of SB 203,580 plus KN-93 are additive. Finally, HeLa cells, which lack nuclear 5-LO, also lack constitutive phosphorylation of Ser-271. Taken together, these results indicate that the phosphorylation of Ser-271 serves to inhibit the nuclear export of 5-LO. This action works in concert with nuclear import, which is regulated by phosphorylation on Ser-523, to determine the subcellular distribution of 5-LO, which in turn regulates leukotriene biosynthesis.     

A cytotoxic ribonuclease variant with a discontinuous nuclear localization signal constituted by basic residues scattered over three areas of the molecule

Nuclear import of proteins is determined by specific signals that allow them to bind to receptors that mediate their energy-dependent transport through the nuclear pore. These signals are termed nuclear localization signals and do not constitute a specific consensus sequence. Among them, the most characterized correspond to monopartite and bipartite nuclear localization signals, which interact with the importin alpha/beta heterodimer. We previously described a cytotoxic variant of human pancreatic-ribonuclease that is actively transported into the nucleus. Here, we show that this protein interacts with importin alpha through different basic residues, including Lys1 and the arginine clusters 31-33 and 89-91. Although these residues are scattered along the sequence, they are close in the three-dimensional structure of the protein and their topological disposition strongly resembles that of a classical bipartite nuclear localization signal.     

Bipartite nuclear localization signal of matrin 3 is essential for vertebrate cells

Matrin 3, a nuclear matrix protein has potential (1) to withhold promiscuously edited RNAs within the nucleus in cooperation with p54(nrb) and PSF, (2) to mediate NMDA-induced neuronal death, and (3) to modulate promoter activity of genes proximal to matrix/scaffold attachment region (MAR/SAR). We identified a bipartite nuclear localization signal (NLS) of chicken matrin 3 (cmatr3) at residues 583-602. By expressing green fluorescent protein (GFP) fused to the NLS mutant in chicken DT40 cells, we showed an essential role of the NLS for cell proliferation. Furthermore, we showed that both clusters of basic amino acids and a linker of the bipartite NLS were essential and sufficient for the nuclear import of GFP. Exogenous cmatr3 rescued the HeLa cells where human matrin 3 was suppressed by RNA interference, but cmatr3 containing deletions at either of the basic amino acid clusters or the linker could not.     

An importin alpha/beta-recognized bipartite nuclear localization signal mediates targeting of the human herpes simplex virus type 1 DNA polymerase catalytic subunit pUL30 to the nucleus

Although the 1235 amino acids human herpes simplex virus type 1 (HSV-1) DNA polymerase catalytic subunit, pUL30, is essential for HSV-1 replication in the nucleus of host cells, little information is available regarding its nuclear import mechanism. The present study addresses this issue directly, characterizing pUL30's nuclear import pathway for the first time using quantitative confocal laser scanning microscopy (CLSM) on living cells, and fluorescent binding assays. In addition to a previously described nuclear localization signal (NLS) located within the pUL30 binding site for the polymerase accessory protein (PAP) pUL42, that appears to be dispensable for nuclear targeting, pUL30 possesses three putative basic NLSs. Intriguingly, the core of pUL30-NLS2 (residues 1114-1120) is highly homologous to that of the recently described NLS, similarly located upstream of the PAP binding site, of the human cytomegalovirus (HCMV) DNA polymerase catalytic subunit, pUL54. Here we show for the first time that pUL30-NLS2 itself is only partially functional in terms of nuclear import due to residue P1118 present in position 3 of the NLS core. Intriguingly, pUL30-NLS2 together with pUL30-NLS3 (residues 1133-1136) represents a fully functional bipartite NLS (pUL30-NLSbip), required for nuclear targeting of pUL30, and able to confer nuclear localization on heterologous proteins by conferring high-affinity interaction with the importin (IMP) alpha/beta heterodimer. Since nuclear targeting of HSV-1 proteins forming the replication fork is crucial for viral replication, the pUL30-NLSbip emerges for the first time as a viable therapeutic target.     

Dissection of a nuclear localization signal

The regulated process of protein import into the nucleus of a eukaryotic cell is mediated by specific nuclear localization signals (NLSs) that are recognized by protein import receptors. This study seeks to decipher the energetic details of NLS recognition by the receptor importin alpha through quantitative analysis of variant NLSs. The relative importance of each residue in two monopartite NLS sequences was determined using an alanine scanning approach. These measurements yield an energetic definition of a monopartite NLS sequence where a required lysine residue is followed by two other basic residues in the sequence K(K/R)X(K/R). In addition, the energetic contributions of the second basic cluster in a bipartite NLS ( approximately 3 kcal/mol) as well as the energy of inhibition of the importin alpha importin beta-binding domain ( approximately 3 kcal/mol) were also measured. These data allow the generation of an energetic scale of nuclear localization sequences based on a peptide's affinity for the importin alpha-importin beta complex. On this scale, a functional NLS has a binding constant of approximately 10 nm, whereas a nonfunctional NLS has a 100-fold weaker affinity of 1 microm. Further correlation between the current in vitro data and in vivo function will provide the foundation for a comprehensive quantitative model of protein import.     

Analysis of a developmentally regulated nuclear localization signal in Xenopus

The 289 residue nuclear oncoprotein encoded by the adenovirus 5 Ela gene contains two peptide sequences that behave as nuclear localization signals (NLS). One signal, located at the carboxy terminus, is like many other known NLSs in that it consists of a short stretch of basic residues (KRPRP) and is constitutively active in cells. The second signal resides within an internal 45 residue region of E1a that contains few basic residues or sequences that resemble other known NLSs. Moreover, this internal signal functions in injected Xenopus oocytes, but not in transfected Xenopus A6 cells, suggesting that it could be regulated developmentally (Slavicek et al. 1989. J. Virol. 63:4047). In this study, we show that the activity of this signal is sensitive to ATP depletion in vivo, efficiently directs the import of a 50 kD fusion protein and can compete with the E1a carboxy-terminal NLS for nuclear import. In addition, we have delineated the precise amino acid residues that comprise the second E1a NLS, and have assessed its utilization during Xenopus embryogenesis. Using amino acid deletion and substitution analyses, we show that the signal consists of the sequence FV(X)7-20MXSLXYM(X)4MF. By expressing in Xenopus embryos a truncated E1a protein that contains only the second NLS and by monitoring its cytoplasmic/nuclear distribution during development with indirect immunofluorescence, we find that the second NLS is utilized up to the early neurula stage. In addition, there appears to be a hierarchy among the embryonic germ layers as to when the second NLS becomes nonfunctional. For this reason, we refer to this NLS as the developmentally regulated nuclear localization signal (drNLS). The implications of these findings for early development are discussed.     

Dissection of the karyopherin alpha nuclear localization signal (NLS)-binding groove: functional requirements for NLS binding

Classical protein import, mediated by the binding of a classical nuclear localization signal (NLS) to the NLS receptor, karyopherin/importin alpha, is the most well studied nuclear transport process. Classical NLSs are either monopartite sequences that contain a single cluster of basic amino acids (Lys/Arg) or bipartite sequences that contain two clusters of basic residues separated by an unconserved linker region. We have created mutations in conserved residues in each of the three NLS-binding sites/regions in Saccharomyces cerevisiae karyopherin alpha (SRP1). For each mutant we have analyzed binding to both a monopartite and a bipartite NLS cargo in vitro. We have also expressed each karyopherin alpha mutant in vivo as the only cellular copy of the NLS receptor and examined the impact on cell growth and import of both monopartite and bipartite NLS-containing cargoes. Our results reveal the functional significance of specific residues within karyopherin alpha for NLS cargo binding. A karyopherin alpha variant with a mutation in the major NLS-binding site exhibits decreased binding to both monopartite and bipartite NLS cargoes, and this protein is not functional in vivo. However, we also find that a karyopherin alpha variant with a mutation in the minor NLS-binding site, which shows decreased binding only to bipartite NLS-containing cargoes, is also not functional in vivo. This suggests that the cell is dependent on the function of at least one bipartite NLS cargo that is imported into the nucleus by karyopherin alpha. Our experiments also reveal functional importance for the linker-binding region. This study provides insight into how changes in binding to cellular NLS sequences could impact cellular function. In addition, this work has led to the creation of conditional alleles of karyopherin alpha with well characterized defects in NLS binding that will be useful for identifying and characterizing novel NLS cargoes.     

Evaluation of the role of conserved His and Met residues among lipoxygenases by site-directed mutagenesis of recombinant human 5-lipoxygenase

The 5-, 12-, and 15-lipoxygenases contain a highly conserved sequence of the form His-(X)4-His-(X)4-His-(X)17-His-(X)8-His which represents a potential binding site for non heme iron to the protein. The importance of selected amino acids within this His cluster for the activity of human 5-lipoxygenase was investigated by site-directed mutagenesis using bacteria and insect cells expression systems. After single mutation of each of the 5 His residues at positions 363, 368, 373, 391, and 400 by Ser, Cys, or Lys, measurable levels of 5-lipoxygenase activity could be recovered in Escherichia coli only for the Ser363 and Cys363 mutants, with most amino acid substitutions causing a decrease in the levels of expression of the soluble protein. In contrast, 25-80% of soluble 5-lipoxygenase activity was recovered after the replacement of several of the hydrophobic amino acids in this region: Tyr384 by Ser or Phe; Phe394 by Trp and Val375 by Ala. Met436 could be replaced by Leu with little effect on 5-lipoxygenase activity or turnover inactivation half-time. High levels of mutant 5-lipoxygenases containing a Ser residue instead of His at each of the five positions were also expressed in Spodoptera frugiperda (Sf9) cells infected with recombinant baculovirus. The specific activity (58-75% of control) and the reaction time course of the Ser363, Ser391, and Ser400 mutants were comparable with that of native 5-lipoxygenase whereas inactive proteins were obtained for the Ser368 and Ser373 mutants. These results show that His368 and His373 residues are important for 5-lipoxygenase activity and that the other conserved His363, His391, His400, and Met436 residues are not crucial for the catalytic cycle or for the mechanism of self-inactivation of 5-lipoxygenase.     

The N-terminal "beta-barrel" domain of 5-lipoxygenase is essential for nuclear membrane translocation

5-Lipoxygenase is the key enzyme in the formation of leukotrienes, which are potent lipid mediators of asthma pathophysiology. This enzyme translocates to the nuclear envelope in a calcium-dependent manner for leukotriene biosynthesis. Eight green fluorescent protein (GFP)-lipoxygenase constructs, representing the major human and mouse enzymes within this family, were constructed and their cDNAs transfected into human embryonic kidney 293 cells. Of these eight lipoxygenases, only the 5-lipoxygenase was clearly nuclear localized and translocated to the nuclear envelope upon stimulation with the calcium ionophore. The N-terminal "beta -barrel" domain of 5-lipoxygenase, but not the catalytic domain, was necessary and sufficient for nuclear envelope translocation. The GFP-N-terminal 5-lipoxygenase domain translocated faster than GFP-5-lipoxygenase. beta-Barrel/catalytic domain chimeras with 12- and 15-lipoxygenase indicated that only the N-terminal domain of 5-lipoxygenase could carry out this translocation function. Mutations of iron atom binding ligands (His550 or deletion of C-terminal isoleucine) that disrupt nuclear localization do not alter translocation capacity indicating distinct determinants of nuclear localization and translocation. Moreover, data show that GFP-5-lipoxygenase beta-barrel containing constructs can translocate to the nuclear membrane whether cytoplasmic or nuclear localized. Thus, the predicted beta-barrel domain of 5-lipoxygenase may function like the C2 domain within protein kinase C and cytosolic phospholipase A(2) with unique determinants that direct its localization to the nuclear envelope.     

Structural basis for the specificity of bipartite nuclear localization sequence binding by importin-alpha

Importin-alpha is the nuclear import receptor that recognizes cargo proteins carrying conventional basic monopartite and bipartite nuclear localization sequences (NLSs) and facilitates their transport into the nucleus. Bipartite NLSs contain two clusters of basic residues, connected by linkers of variable lengths. To determine the structural basis of the recognition of diverse bipartite NLSs by mammalian importin-alpha, we co-crystallized a non-autoinhibited mouse receptor protein with peptides corresponding to the NLSs from human retinoblastoma protein and Xenopus laevis phosphoprotein N1N2, containing diverse sequences and lengths of the linker. We show that the basic clusters interact analogously in both NLSs, but the linker sequences adopt different conformations, whereas both make specific contacts with the receptor. The available data allow us to draw general conclusions about the specificity of NLS binding by importin-alpha and facilitate an improved definition of the consensus sequence of a conventional basic/bipartite NLS (KRX10-12KRRK) that can be used to identify novel nuclear proteins.     

Subcellular distribution of ADAR1 isoforms is synergistically determined by three nuclear discrimination signals and a regulatory motif

ADAR1 is an RNA-specific adenosine deaminase that edits RNA sequences. We have demonstrated previously that different ADAR1 isoforms are induced during acute inflammation. Here we show that the mouse ADAR1 isoforms are differentially localized in cellular compartments and that their localization is controlled by several independent signals. Nuclear import of the full-length ADAR1 is predominantly regulated by a nuclear localization signal at the C terminus (NLS-c), which consists of a bipartite basic amino acid motif plus the last 39 residues of ADAR1. Deletion of the NLS-c causes the truncated ADAR1 protein to be retained in the cytoplasm. The addition of this sequence to pyruvate kinase causes the cytoplasmic protein to be localized within the nucleus. The localization of nuclear ADAR1 is determined by a dynamic balance between the nucleolar binding activity of the nucleolar localization signal (NoLS) in the middle of the protein and the exporting activity of the nuclear exporter signal (NES) near the N terminus. The NoLS consists of a typical monopartite cluster of basic residues followed by the third double-stranded RNA-binding domain. These signals act independently; however, NES function can be completely silenced by the NLS-c when a regulatory motif within the catalytic domain and the NoLS are deleted. Thus, the intracellular distribution of the various ADAR1 isoforms is determined by NLS-c, NES, NoLS, and a regulatory motif.     

Alterations in leukotriene synthase activity of the human 5-lipoxygenase by site-directed mutagenesis affecting its positional specificity

The positional specificity of arachidonic acid oxygenation is currently the decisive parameter for classification of lipoxygenases. Although the mechanistic basis of lipoxygenase specificity is not completely understood, sequence determinants for the positional specificity have been identified for various isoenzymes. In this study we altered the positional specificity of the human 5-lipoxygenase by multiple site-directed mutagenesis and assayed the leukotriene A(4) synthase activity of the mutant enzyme species with (5S,6E,8Z,11Z,14Z)-5-hydroperoxy-6,8,11,14-eicos atetraenoic acid (5S-HpETE) as substrate. The wild-type 5-lipoxygenase converts 5S-HpETE almost exclusively to leukotriene A(4) as indicated by the dominant formation of leukotriene A(4) hydrolysis products. Since leukotriene synthesis involves a hydrogen abstraction from C(10), it was anticipated that the 15-lipoxygenating quadruple mutant F359W + A424I + N425M + A603I might not exhibit a major leukotriene A(4) synthase activity. Surprisingly, we found that this quadruple mutant exhibited a similar leukotriene synthase activity as the wild-type enzyme in addition to its double oxygenation activity. The leukotriene synthase activity of the 8-lipoxygenating double mutant F359W + A424I was almost twice as high, and similar amounts of leukotriene A(4) hydrolysis products and double oxygenation derivatives were detected with this enzyme species. These data indicate that site-directed mutagenesis of the human 5-lipoxygenase that leads to alterations in the positional specificity favoring arachidonic acid 15-lipoxygenation does not suppress the leukotriene synthase activity of the enzyme. The residual 8-lipoxygease activity of the mutant enzyme and its augmented rate of 5-HpETE conversion may be discussed as major reasons for this unexpected result.     

A bipartite nuclear localization signal is required for p53 nuclear import regulated by a carboxyl-terminal domain.

Abnormal p53 cellular localization has been considered to be one of the mechanisms that could inactivate p53 function. To understand the regulation of p53 cellular trafficking, we have previously identified two p53 domains involved in its localization. A basic domain, Lys(305)-Arg(306), is required for p53 nuclear import, and a carboxyl-terminal domain, namely the cytoplasmic sequestration domain (CSD) from residues 326-355, could block the nuclear import of Lys(305) or Arg(306) mutated p53. To characterize further the function of these two domains, we demonstrate in this report that the previously described major nuclear localization signal works together with Lys(305)-Arg(306) to form a bipartite and functional nuclear localization sequence (NLS) for p53 nuclear import. The CSD could block the binding of p53 to the NLS receptor, importin alpha, and reduce the efficiency of p53 nuclear import in MCF-7, H1299, and Saos-2 cells. The blocking effect of the CSD is not due to the enhancement of nuclear export or oligomerization of the p53. These results indicate that the CSD can regulate p53 nuclear import by controlling access of the NLS to importin alpha binding.