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.     

CXXC domain of human DNMT1 is essential for enzymatic activity

DNA cytosine methylation is one of the major epigenetic gene silencing marks in the human genome facilitated by DNA methyltransferases. DNA cytosine-5 methyltransferase 1 (DNMT1) performs maintenance methylation in somatic cells. In cancer cells, DNMT1 is responsible for the aberrant hypermethylation of CpG islands and the silencing of tumor suppressor genes. Here we show that the catalytically active recombinant DNMT1, lacking 580 amino acids from the amino terminus, binds to unmethylated DNA with higher affinity than hemimethylated or methylated DNA. To further understand the binding domain of enzyme, we have used gel shift assay. We have demonstrated that the CXXC region (C is cysteine; X is any amino acid) of DNMT1 bound specifically to unmethylated CpG dinucleotides. Furthermore, mutation of the conserved cysteines abolished CXXC mediated DNA binding. In transfected COS-7 cells, CXXC deleted DNMT1 (DNMT1 (DeltaCXXC)) localized on replication foci. Both point mutant and DNMT1 (DeltaCXXC) enzyme displayed significant reduction in catalytic activity, confirming that this domain is crucial for enzymatic activity. A permanent cell line with DNMT1 (DeltaCXXC) displayed partial loss of genomic methylation on rDNA loci, despite the presence of endogenous wild-type enzyme. Thus, the CXXC domain encompassing the amino terminus region of DNMT1 cooperates with the catalytic domain for DNA methyltransferase activity.     

Glu300 of rat carboxypeptidase E is essential for enzymatic activity but not substrate binding or routing to the regulated secretory pathway

Several recently discovered members of the carboxypeptidase E (CPE) gene family lack critical active site residues that are conserved in other family members. For example, three CPE-like proteins contain a Tyr in place of Glu300 (equivalent to Glu270 of carboxypeptidase A and B). To investigate the importance of this position, Glu300 of rat CPE was converted into Gln, Lys, or Tyr, and the proteins expressed in Sf9 cells using the baculovirus system. All three mutants were secreted from the cells, but the media showed no enzyme activity above background levels. Wild-type CPE and the Gln300 point mutant bound to a p-aminobenzoyl-Arg-Sepharose affinity resin, and this binding was competed by an active site-directed inhibitor, guanidinoethylmercaptosuccinic acid. The affinity purified mutant CPE protein showed no detectable enzyme activity (<0.004% of wild-type CPE) toward dansyl-Phe-Ala-Arg. Expression of the Gln300 and Lys300 mutant CPE proteins in the NIT3 mouse pancreatic beta-cell line showed that these mutants are routed into secretory vesicles and secreted via the regulated pathway. Taken together, these results indicate that Glu300 of CPE is essential for enzyme activity, but not required for substrate binding or for routing into the regulated secretory pathway.     

(Arg)3 within the N-terminal domain of glucosidase I contains ER targeting information but is not required absolutely for ER localization

Glucosidase I is an endoplasmic reticulum (ER) type II membrane enzyme that cleaves the distal alpha1,2-glucose of the asparagine-linked GlcNAc2-Man9-Glc3 precursor. To identify sequence motifs responsible for ER localization, we prepared a protein chimera by transferring the cytosolic and transmembrane domain of glucosidase I to the luminal domain of Golgi-Man9-mannosidase. The GIM9 hybrid was overexpressed in COS 1 cells as an ER-resident protein that displayed alpha1,2-mannosidase activity, excluding the possibility that the glucosidase I-specific domains interfere with folding of the Man9-mannosidase catalytic domain. After substitution of the Args in position 7, 8, or 9 relative to the N-terminus by leucine, the GIM9 mutants were transported to the cell surface indicating that the (Arg)3 sequence functions as an ER-targeting motif. Cell surface expression was also observed after substitution of Arg-7 or Arg-8 but not Arg-9 in GIM9 by either lysine or histidine. Thus the side chain structure, including its positive charge, appears to be essential for signal function. Analysis of the N-linked glycans suggests that the (Arg)3 sequence mediates ER localization through Golgi-to-ER retrograde transport. Glucosidase I remained localized in the ER after truncation or mutation of the N-terminal (Arg)3 signal, in contrast to comparable GIM9 mutants. ER localization was also observed with an M9GI chimera consisting of the cytosolic and transmembrane domain of Man9-mannosidase and the glucosidase I catalytic domain. ER-specific targeting information must therefore be provided by sequence motifs contained within the glucosidase I luminal domain. This structural information appears to direct ER localization by retention rather than by retrieval, as concluded from N-linked Man9-GlcNAc2 being the major glycan released from the wild-type enzyme.     

The enzymatic activity of CEM15/Apobec-3G is essential for the regulation of the infectivity of HIV-1 virion but not a sole determinant of its antiviral activity

Human immunodeficiency virus, type 1 (HIV-1) Vif protein plays an essential role in the regulation of the infectivity of HIV-1 virion. Vif functions to counteract an anti-HIV-1 cellular factor in non-permissive cells, CEM15/Apobec-3G, which shares a cytidine deaminase motif. CEM15/Apobec-3G deaminates dC to dU in the minus strand DNA of HIV-1, resulting in G to A hypermutation in the plus strand DNA. In this study, we have done the mutagenesis analysis on two cytidine deaminase motifs in CEM15/Apobec-3G and examined their antiviral functions as well as the DNA editing activity. Point mutations in the C-terminal active site such as E259Q and C291A almost completely abrogated the antiviral function, while those in the N-terminal active site such as E67Q and C100A retained this activity to a lesser extent as compared with that of the wild type. The DNA editing activities of E67Q and E259Q mutants were both retained but impaired to the same extent. This indicates that the enzymatic activity of this protein is essential but not a sole determinant of the antiviral activity. Furthermore, all the deletion mutants tested in this study lost the antiviral activity because of the loss of the activity for dimerization, suggesting that the entire protein structure is necessary for the antiviral function.     

The N-terminal domain of OmpATb is required for membrane translocation and pore-forming activity in mycobacteria

OmpATb is the prototype of a new family of porins in Mycobacterium tuberculosis and Mycobacterium bovis BCG. Although the pore-forming activity of this protein has been clearly established by using recombinant protein produced in Escherichia coli, characterization of the native porin has been hampered by the scarce amount of protein present in the M. tuberculosis detergent extracts. To this aim, we have developed a protocol to overproduce and obtain high yields of OmpATb in both Mycobacterium smegmatis and M. bovis BCG. The protein could be extracted and purified from the cell wall fraction and subsequently used for analysis of the pore-forming activity in multichannel and single-channel conductance experiments. Our results indicate that OmpATb produced in mycobacteria presents an average conductance value of 1,600+/-100 pS, slightly higher than that of OmpATb produced in E. coli, suggesting the occurrence of OmpATb in a highly ordered organization within the mycobacterial cell wall. In contrast to OmpATb, a truncated form lacking the first 72 amino acids (OmpATb73-326) was essentially found in the cytosol and was not active in planar lipid bilayers. This suggested that the N-terminal domain of OmpATb could participate in targeting of OmpATb to the cell wall. This was further confirmed by analyzing M. smegmatis clones expressing a chimeric protein consisting of a fusion between the N-terminal domain of OmpATb and the E. coli PhoA reporter. The present study shows for the first time that the N terminus of OmpATb is required for targeting the porin to the cell wall and also appears to be essential for its pore-forming activity.     

Human gastric lipase. The N-terminal tetrapeptide is essential for lipid binding and lipase activity

Human gastric lipase subjected to limited tryptic proteolysis lost its ability to hydrolyze emulsified long-chain triacylglycerol. Activity against a water-soluble substrate was however retained, indicating that proteolysis did not affect the active site. Sequence analysis revealed that trypsin specifically cleaved the linkage between lysine-4 and leucine-5. This cleavage rendered the enzyme unable to bind to emulsified triacylglycerol particles, e.g. human milk fat globules. We suggest that the N-terminal tetrapeptide, in particular lysine-4, is essential for the binding of human gastric lipase to lipid/water interfaces, and hence, for its physiological function.     

The zinc ion in the HNH motif of the endonuclease domain of colicin E7 is not required for DNA binding but is essential for DNA hydrolysis

The HNH motif was originally identified in the subfamily of HNH homing endonucleases, which initiate the process of the insertion of mobile genetic elements into specific sites. Several bacteria toxins, including colicin E7 (ColE7), also contain the 30 amino acid HNH motif in their nuclease domains. In this work, we found that the nuclease domain of ColE7 (nuclease-ColE7) purified from Escherichia coli contains a one-to-one stoichiometry of zinc ion and that this zinc-containing enzyme hydrolyzes DNA without externally added divalent metal ions. The apo-enzyme, in which the indigenous zinc ion was removed from nuclease-ColE7, had no DNase activity. Several divalent metal ions, including Ni²⁺, Mg²⁺, Co²⁺, Mn²⁺, Ca²⁺, Sr²⁺, Cu²⁺ and Zn²⁺, re-activated the DNase activity of the apo-enzyme to various degrees, however higher concentrations of zinc ion inhibited this DNase activity. Two charged residues located at positions close to the zinc-binding site were mutated to alanine. The single-site mutants, R538A and E542A, showed reduced DNase activity, whereas the double-point mutant, R538A + E542A, had no observable DNase activity. A gel retardation assay further demonstrated that the nuclease-ColE7 hydrolyzed DNA in the presence of zinc ions, but only bound to DNA in the absence of zinc ions. These results demonstrate that the zinc ion in the HNH motif of nuclease-ColE7 is not required for DNA binding, but is essential for DNA hydrolysis, suggesting that the zinc ion not only stabilizes the folding of the enzyme, but is also likely to be involved in DNA hydrolysis.     

The C-terminal flexible domain of the heme chaperone CcmE is important but not essential for its function

CcmE is a heme chaperone active in the cytochrome c maturation pathway of Escherichia coli. It first binds heme covalently to strictly conserved histidine H130 and subsequently delivers it to apo-cytochrome c. The recently solved structure of soluble CcmE revealed a compact core consisting of a beta-barrel and a flexible C-terminal domain with a short alpha-helical turn. In order to elucidate the function of this poorly conserved domain, CcmE was truncated stepwise from the C terminus. Removal of all 29 amino acids up to crucial histidine 130 did not abolish heme binding completely. For detectable transfer of heme to type c cytochromes, only one additional residue, D131, was required, and for efficient cytochrome c maturation, the seven-residue sequence (131)DENYTPP(137) was required. When soluble forms of CcmE were expressed in the periplasm, the C-terminal domain had to be slightly longer to allow detection of holo-CcmE. Soluble full-length CcmE had low activity in cytochrome c maturation, indicating the importance of the N-terminal membrane anchor for the in vivo function of CcmE.     

3-Phosphoglycerate dehydrogenase from Corynebacterium glutamicum: the C-terminal domain is not essential for activity but is required for inhibition by L-serine

The serA gene of Corynebacterium glutamicum coding for 3-phosphoglycerate dehydrogenase (PGDH) was isolated and functionally characterized. It encodes a polypeptide of 530 aminoacyl residues (aa), which is substantially longer than the corresponding Escherichia coli polypeptide of 410 aa. The difference is largely due to an additional stretch of aa in the carboxy- (C)-terminal part of the polypeptide. Overexpression of serA in C. glutamicum results in a 16-fold increase in specific PGDH activity to 2.1 U/mg protein, with activity being inhibited by high concentrations of L-serine. A set of muteins that were progressively truncated at the C-terminal end was constructed. When overexpressed, mutein SerADelta197 showed a specific PGDH dehydrogenase activity of 1.3 U/mg protein, with the activity no longer being sensitive to L-serine. Gel filtration experiments showed that wild type PGDH is a homotetramer, whereas mutein SerADelta197 constitutes a dimer. Thus, the specific regulatory features of C. glutamicum PGDH are due to the C-terminal part of the polypeptide, which can be deleted with almost no effect on the catalytic activity of the enzyme.     

The N-terminal cytokine binding domain of LIFR is required for CNTF binding and signaling

Ciliary neurotrophic factor (CNTF) forms a functional receptor complex containing the CNTF receptor, gp130, and the leukemia inhibitory factor receptor (LIFR). However, the nature and stoichiometry of the receptor-mediated interactions in this complex have not yet been fully resolved. We show here that signaling by CNTF, but not by LIF or oncostatin M (OSM), was abolished in cells overexpressing a LIFR mutant with the N-terminal cytokine binding domain deleted. Our results illustrate molecular differences between the CNTF active receptor complex and those of LIF and OSM and provide further support for the hexameric model of the CNTF receptor complex.     

Aminolevulinate synthase: lysine 313 is not essential for binding the pyridoxal phosphate cofactor but is essential for catalysis.

5-Aminolevulinate synthase is the first enzyme of the heme biosynthetic pathway in animals and some bacteria. Lysine-313 of the mouse erythroid aminolevulinate synthase was recently identified to be linked covalently to the pyridoxal 5'-phosphate cofactor (Ferreira GC, Neame PJ, Dailey HA, 1993, Protein Sci 2:1959-1965). Here we report on the effect of replacement of aminolevulinate synthase lysine-313 by alanine, histidine, and glycine, using site-directed mutagenesis. Mutant enzymes were purified to homogeneity, and the purification yields were similar to those of the wild-type enzyme. Although their absorption spectra indicate that the mutant enzymes bind pyridoxal 5'-phosphate, they bind noncovalently. However, addition of glycine to the mutant enzymes led to the formation of external aldimines. The formation of an external aldimine between the pyridoxal 5'-phosphate cofactor and the glycine substrate is the first step in the mechanism of the aminolevulinate synthase-catalyzed reaction. In contrast, lysine-313 is an essential catalytic residue, because the K313-directed mutant enzymes have no measurable activity. In summary, site-directed mutagenesis of the aminolevulinate synthase active-site lysine-313, to alanine (K313A), histidine (K313H), or glycine (K313G) yields enzymes that bind the pyridoxal 5'-phosphate cofactor and the glycine substrate to produce external aldimines, but which are inactive. This suggests that lysine-313 has a functional role in catalysis.     

The inhibitor of apoptosis protein-binding domain of Smac is not essential for its proapoptotic activity

Smac/DIABLO, a recently identified inhibitor of apoptosis protein (IAP)-binding protein, is released from the mitochondria during apoptosis and reportedly potentiates apoptosis by relieving the inhibition of IAPs on caspases. We now describe the molecular characterization of Smac beta, an alternatively spliced form of Smac, which lacks the mitochondrial-targeting sequence found in Smac and has a cortical distribution in both human embryonic kidney 293 and breast epithelial tumor MCF-7 cells. Smac beta, which binds IAPs in vitro, does not bind IAPs in intact cells due to cellular processing and removal of its NH(2)-terminal IAP-binding domain. Despite its inability to interact with IAPs in cells, processed Smac beta is proapoptotic, as demonstrated by its ability to potentiate apoptosis induced by both death receptor and chemical stimuli. Furthermore, expression of a NH(2)-terminally truncated Smac mutant (Delta75), which lacks the entire IAP-interacting domain, potentiates apoptosis to the same extent as Smac and Smac beta. Our data support the hypothesis that the main proapoptotic function of Smac and Smac beta is due to a mechanism other than IAP binding.     

The rubella virus RNA binding activity of human calreticulin is localized to the N-terminal domain.

The rubella virus RNA 3' cis-acting element, which is essential for viral negative-strand RNA synthesis, is specifically bound by autophosphorylated calreticulin. Autophosphorylation in recombinant human calreticulin occurs on serine and threonine residues. The RNA-binding and autophosphorylation activities were localized to the N-terminal 180 amino acids. Furthermore, N-terminal deletions revealed that the RNA-binding activity of calreticulin is abrogated upon deletion of the first 10 residues, whereas the autophosphorylation activity resides between amino acids 60 and 180. These results indicate that both the rubella virus RNA-binding and autophosphorylation activities of calreticulin are present in the N-terminal domain.     

The cytoplasmic domain of the Plasmodium falciparum ligand EBA-175 is essential for invasion but not protein trafficking

The invasion of host cells by the malaria parasite Plasmodium falciparum requires specific protein-protein interactions between parasite and host receptors and an intracellular translocation machinery to power the process. The transmembrane erythrocyte binding protein-175 (EBA-175) and thrombospondin-related anonymous protein (TRAP) play central roles in this process. EBA-175 binds to glycophorin A on human erythrocytes during the invasion process, linking the parasite to the surface of the host cell. In this report, we show that the cytoplasmic domain of EBA-175 encodes crucial information for its role in merozoite invasion, and that trafficking of this protein is independent of this domain. Further, we show that the cytoplasmic domain of TRAP, a protein that is not expressed in merozoites but is essential for invasion of liver cells by the sporozoite stage, can substitute for the cytoplasmic domain of EBA-175. These results show that the parasite uses the same components of its cellular machinery for invasion regardless of the host cell type and invasive form.