Intrinsically disordered protein

Proteins can exist in a trinity of structures: the ordered state, the molten globule, and the random coil. The five following examples suggest that native protein structure can correspond to any of the three states (not just the ordered state) and that protein function can arise from any of the three states and their transitions. (1) In a process that likely mimics infection, fd phage converts from the ordered into the disordered molten globular state. (2) Nucleosome hyperacetylation is crucial to DNA replication and transcription; this chemical modification greatly increases the net negative charge of the nucleosome core particle. We propose that the increased charge imbalance promotes its conversion to a much less rigid form. (3) Clusterin contains an ordered domain and also a native molten globular region. The molten globular domain likely functions as a proteinaceous detergent for cell remodeling and removal of apoptotic debris. (4) In a critical signaling event, a helix in calcineurin becomes bound and surrounded by calmodulin, thereby turning on calcineurin's serine/threonine phosphatase activity. Locating the calcineurin helix within a region of disorder is essential for enabling calmodulin to surround its target upon binding. (5) Calsequestrin regulates calcium levels in the sarcoplasmic reticulum by binding approximately 50 ions/molecule. Disordered polyanion tails at the carboxy terminus bind many of these calcium ions, perhaps without adopting a unique structure. In addition to these examples, we will discuss 16 more proteins with native disorder. These disordered regions include molecular recognition domains, protein folding inhibitors, flexible linkers, entropic springs, entropic clocks, and entropic bristles. Motivated by such examples of intrinsic disorder, we are studying the relationships between amino acid sequence and order/disorder, and from this information we are predicting intrinsic order/disorder from amino acid sequence. The sequence-structure relationships indicate that disorder is an encoded property, and the predictions strongly suggest that proteins in nature are much richer in intrinsic disorder than are those in the Protein Data Bank. Recent predictions on 29 genomes indicate that proteins from eucaryotes apparently have more intrinsic disorder than those from either bacteria or archaea, with typically > 30% of eucaryotic proteins having disordered regions of length > or = 50 consecutive residues.

Crystal structure of four-stranded Oxytricha telomeric DNA

The sequence d(GGGGTTTTGGGG) from the 3' overhang of the Oxytricha telomere has been crystallized and its three-dimensional structure solved to 2.5 A resolution. The oligonucleotide forms hairpins, two of which join to make a four-stranded helical structure with the loops containing four thymine residues at either end. The guanine residues are held together by cyclic hydrogen bonding and an ion is located in the centre. The four guanine residues in each segment have a glycosyl conformation that alternates between anti and syn. There are two four-stranded molecules in the asymmetric unit showing that the structure has some intrinsic flexibility.

Downregulation of Wnt2 and beta-catenin by siRNA suppresses malignant glioma cell growth

Increasing evidence suggests that aberrant activation of Wnt signaling is involved in tumor development and progression. Our earlier study on gene expression profile in human gliomas by microarray found that some members of Wnt family were overexpressed. To further investigate the involvement of Wnt signaling in gliomas, the expression of core components of Wnt signaling cascade in 45 astrocytic glioma specimens with different tumor grades was examined by reverse transcription-PCR and immunohistochemistry. Wnt2, Wnt5a, frizzled2 and beta-catenin were overexpressed in gliomas. Knockdown of Wnt2 and its key mediator beta-catenin in the canonical Wnt pathway by siRNA in human U251 glioma cells inhibited cell proliferation and invasive ability, and induced apoptotic cell death. Furthermore, treating the nude mice carrying established subcutaneous U251 gliomas with siRNA targeting Wnt2 and beta-catenin intratumorally also delayed the tumor growth. In both in vitro and in vivo studies, downregulation of Wnt2 and beta-catenin was associated with the decrease of PI3K/p-AKT expression, indicating the interplay between Wnt/beta-catenin and PI3K/AKT signaling cascades. In conclusion, the canonical Wnt pathway is of critical importance in the gliomagenesis and intervention of this pathway may provide a new therapeutic approach for malignant gliomas.

The 5' ends of Hantaan virus (Bunyaviridae) RNAs suggest a prime-and-realign mechanism for the initiation of RNA synthesis

We examined the 5' ends of Hantaan virus (HTN) genomes and mRNAs to gain insight into the manner in which these chains were initiated. Like those of all members of the family Bunyaviridae described so far, the HTN mRNAs contained 5' terminal extensions that were heterogeneous in both length and sequence, presumably because HTN also "cap snatches" host mRNAs to initiate the viral mRNAs. Unexpectedly, however, almost all of the mRNAs contained a G residue at position -1, and a large fraction also lacked precisely one of the three UAG repeats at the termini. The genomes, on the other hand, commenced with a U residue at position +1, but only 5' monophosphates were found here, indicating that these chains may not have initiated with UTP at this position. Taken together, these unusual findings suggest a prime-and-realign mechanism of chain initiation in which mRNAs are initiated with a G-terminated host cell primer and genomes with GTP, not at the 3' end of the genome template but internally (opposite the template C at position +3), and after extension by one or a few nucleotides, the nascent chain realigns backwards by virtue of the terminal sequence repeats, before processive elongation takes place. For genome initiation, an endonuclease, perhaps that involved in cap snatching, is postulated to remove the 5' terminal extension of the genome, leaving the 5' pU at position +1.

Absence of the mid-sized neurofilament subunit decreases axonal calibers, levels of light neurofilament (NF-L), and neurofilament content

Neurofilaments (NFs) are prominent components of large myelinated axons and probably the most abundant of neuronal intermediate filament proteins. Here we show that mice with a null mutation in the mid-sized NF (NF-M) subunit have dramatically decreased levels of light NF (NF-L) and increased levels of heavy NF (NF-H). The calibers of both large and small diameter axons in the central and peripheral nervous systems are diminished. Axons of mutant animals contain fewer neurofilaments and increased numbers of microtubules. Yet the mice lack any overt behavioral phenotype or gross structural defects in the nervous system. These studies suggest that the NF-M subunit is a major regulator of the level of NF-L and that its presence is required to achieve maximal axonal diameter in all size classes of myelinated axons.

Crystal structure of human mitochondrial single-stranded DNA binding protein at 2.4 A resolution

We solved the crystal structure of the homotetrameric single-stranded DNA binding (SSB) protein from human mitochondria at a resolution of 2.4 A. The tetramer is formed by two dimers interacting head-to-head and shows D2 symmetry. Sequence-related tetrameric SSB proteins occur in prokaryotes and eukaryotic mitochondria; this is the first report of an atomic resolution structure of this type of protein. Using biochemical data and analysis of sequence homologies, we were able to correlate the functional properties with structure. We propose that ssDNA wraps around the tetrameric HsmtSSB protein through electropositive channels guided by flexible loops.

Glycosylation is necessary for the correct folding of human immunodeficiency virus gp120 in CD4 binding

Conflicting results have been reported regarding the role of carbohydrate on human immunodeficiency virus (HIV) envelope glycoprotein gp120 in CD4 receptor binding. Glycosylated, deglycosylated, and nonglycosylated forms of HIV type 1 (HIV-1) and HIV-2 gp120s were used to examine CD4 receptor-binding activity. Nonglycosylated forms of gp120 generated either by deletion of the signal sequence of HIV-1 gp120 or by synthesis in the presence of tunicamycin failed to bind to CD4. In contrast, highly mannosylated gp120 bound to soluble CD4 molecules well. Enzymatic removal of carbohydrate chains from glycosylated gp120 by endoglycosidase H or an endoglycosidase F/N glycanase mixture had no effect on the ability of gp120 to bind CD4. An experiment which measured the ability of gp120 to bind to CD4 as an assay of the proper conformation of gp120 showed that carbohydrate chains on gp120 are not required for the interaction between gp120 and CD4 but that N-linked glycosylation is essential for generation of the proper conformation of gp120 to provide a CD4-binding site.

Requirement of heavy neurofilament subunit in the development of axons with large calibers

Neurofilaments (NFs) are prominent components of large myelinated axons. Previous studies have suggested that NF number as well as the phosphorylation state of the COOH-terminal tail of the heavy neurofilament (NF-H) subunit are major determinants of axonal caliber. We created NF-H knockout mice to assess the contribution of NF-H to the development of axon size as well as its effect on the amounts of low and mid-sized NF subunits (NF-L and NF-M respectively). Surprisingly, we found that NF-L levels were reduced only slightly whereas NF-M and tubulin proteins were unchanged in NF-H-null mice. However, the calibers of both large and small diameter myelinated axons were diminished in NF-H-null mice despite the fact that these mice showed only a slight decrease in NF density and that filaments in the mutant were most frequently spaced at the same interfilament distance found in control. Significantly, large diameter axons failed to develop in both the central and peripheral nervous systems. These results demonstrate directly that unlike losing the NF-L or NF-M subunits, loss of NF-H has only a slight effect on NF number in axons. Yet NF-H plays a major role in the development of large diameter axons.

Lack of sequence homology among RNAs of avian leukosis-sarcoma viruses, reticuloendotheliosis viruses, and chicken endogenous RNA-directed DNA polymerase activity

The relatedness of the RNAs of the three avian systems, including six avian leukosis-sarcoma viruses, four reticuloendotheliosis viruses, and the microsome fraction of normal uninfected chicken embryo cells, containing RNA and a DNA polymerase have been studied by nucleic acid hybridization. All six avian leukosis-sarcoma viruses have closely related nucleotide sequences; and all four reticuloendotheliosis viruses have closely related nucleotide sequences. But, almost no similarities were detected between the RNAs of avian leukosis-sarcoma viruses and reticuloendotheliosis viruses. The RNA template of the endogenous RNA-directed DNA polymerase activity of normal uninfected chicken cells had no detectable relationship to RNAs of avian leukosis-sarcoma and reticuloendotheliosis viruses.

Conformational changes induced in the endoplasmic reticulum luminal domain of calnexin by Mg-ATP and Ca2+

The type I membrane protein calnexin functions as a molecular chaperone for secretory glycoproteins in the endoplasmic reticulum with ATP and Ca2+ as two of the cofactors involved in substrate binding. Protease protection experiments with intact canine rough microsomes showed that amino acid residues 1-462 of calnexin are located within the lumen of the endoplasmic reticulum. Expression using the baculovirus Sf9 insect cell system of a recombinant truncated calnexin corresponding to residues 1-462 (calnexin delta TMC) revealed an association in vivo with a coexpressed secretory glycoprotein substrate, human immunodeficiency virus type I gp120. For the in vitro characterization of calnexin delta TMC, we purified this secreted form to homogeneity from the medium of Sf9 cells. We demonstrate that the properties of the purified calnexin delta TMC correspond to those of full-length calnexin in canine microsomes with at least one intramolecular disulfide bond and binding to 45Ca2+. Calnexin delta TMC underwent a marked and reversible conformational change following Ca2+ binding as measured by its resistance to proteinase K digestion of a 60-kDa fragment and also by the change from an oligomeric form of calnexin delta TMC to a monomeric form. We also found that calnexin bound Mg-ATP leading to a conformational change from a monomeric to an oligomeric form that coincided as with markedly increased proteinase sensitivity. Our results identify the luminal domain of calnexin as responsible for binding substrates, Ca2+, and Mg-ATP. Because Ca2+ and ATP are required in vivo for the maintenance of calnexin-substrate interactions, conformational changes in the luminal domain of calnexin induced by Ca2+ and Mg-ATP are relevant to the in vivo function of calnexin as a molecular chaperone.

The influence of a polymorphism at position -857 of the tumour necrosis factor gene on clinical response to etanercept therapy in rheumatoid arthritis

OBJECTIVES

We aimed to test whether polymorphisms in the etanercept target genes TNFA and LTA are associated with clinical responses to etanercept therapy in RA patients.

METHODS

Clinical responses of 70 patients treated with etanercept were determined according to the ACR criteria. We genotyped 13 single-nucleotide polymorphisms (SNPs) within TNFA and LTA and tested whether they influenced the responses to 12 weeks of etanercept therapy. Univariate and multivariate analyses were performed to compare allele, genotype and haplotype distributions between responders and non-responders.

RESULTS

Association of the -857C/T SNP at the TNFA promoter was marginally significant when patients were divided into responders and non-responders according to improvement criteria ACR20 or ACR70. When ACR70 responders (the best responders) were compared with ACR20 non-responders (the worst responders), however, the association was prominent [odds ratio (OR) = 12, 95% confidence interval (CI) = 1.4-105, P = 0.0077, P(corrected) = 0.054], as the frequency of the T allele was 5% in the ACR20 non-responders but 39% in the ACR70 responders. Moreover, the ratio of ACR70 responder number to ACR20 non-responder number among T-allele carriers was >10-fold higher than in the C-allele homozygotes (OR = 12, 95% CI = 1.2-120, P = 0.033).

CONCLUSIONS

RA patients with the T allele of TNFA -857C/T SNP respond better to etanercept therapy than homozygotes for the C allele, indicating that, when the results have been confirmed, this SNP could become a useful genetic marker for predicting responses.

Early postnatal development of corpus callosum and corticospinal white matter assessed with quantitative tractography

BACKGROUND AND PURPOSE

The early postnatal period is perhaps the most dynamic phase of white matter development. We hypothesized that the early postnatal development of the corpus callosum and corticospinal tracts could be studied in unsedated healthy neonates by using novel approaches to diffusion tensor imaging (DTI) and quantitative tractography.

MATERIALS AND METHODS

Isotropic 2 x 2 x 2 mm(3) DTI and structural images were acquired from 47 healthy neonates. DTI and structural images were coregistered and fractional anisotropy (FA), mean diffusivity (MD), and normalized T1-weighted (T1W) and T2-weighted (T2W) signal intensities were determined in central midline and peripheral cortical regions of the white matter tracts of the genu and splenium of the corpus callosum and the central midbrain and peripheral cortical regions of the corticospinal tracts by using quantitative tractography.

RESULTS

We observed that central regions exhibited lower MD, higher FA values, higher T1W intensity, and lower T2W intensity than peripheral cortical regions. As expected, MD decreased, FA increased, and T2W signal intensity decreased with increasing age in the genu and corticospinal tract, whereas there was no significant change in T1W signal intensity. The central midline region of the splenium fiber tract has a unique pattern, with no change in MD, FA, or T2W signal intensity with age, suggesting different growth trajectory compared with the other tracts. FA seems to be more dependent on tract organization, whereas MD seems to be more sensitive to myelination.

CONCLUSIONS

Our novel approach may detect small regional differences and age-related changes in the corpus callosum and corticospinal white matter tracts in unsedated healthy neonates and may be used for future studies of pediatric brain disorders that affect developing white matter.

Proteins of vesicular stomatitis virus. I. Polyacrylamide gel analysis of viral antigens

Infection of L cells with vesicular stomatitis virus results in the release, into the cell-free fluid, of four antigenic components separable by rate zonal centrifugation on sucrose gradients. The largest antigens are the infectious (B) particle and a shorter noninfectious, autointerfering (T) particle. The two small antigens are characterized by sedimentation coefficients of approximately 20S and 6S. Treatment of purified B or T particles with sodium deoxycholate results in the release from the particle of a nucleoprotein core which can be purified on sucrose gradient and which has a sedimentation coefficient characteristic of the virus from which it arose. Utilizing purified antigens labeled with (14)C-amino acids during growth, we examined the protein constituents of each antigen by acrylamide-gel electrophoresis. The proteins of B and T particles are identical, each containing one minor (virus protein 1) and three major (virus proteins 2, 3, and 4) proteins, numbered in order of increasing mobility. Virus protein 3 originates from the nucleoprotein core, whereas proteins 2 and 4 come from the coat. The origin of virus protein 1 is not known. The 20S antigen contains a single protein equivalent to virus protein 3, whereas the 6S antigen shows a single protein which is similar to, but probably distinct from, virus protein 2.

T cell-specific FADD-deficient mice: FADD is required for early T cell development

FADD/Mort1, initially identified as a Fas-associated death-domain containing protein, functions as an adapter molecule in apoptosis initiated by Fas, tumor necrosis factor receptor-I, DR3, and TRAIL-receptors. However, FADD likely participates in additional signaling cascades. FADD-null mutations in mice are embryonic-lethal, and analysis of FADD(-)/- T cells from RAG-1(-)/- reconstituted chimeras has suggested a role for FADD in proliferation of mature T cells. Here, we report the generation of T cell-specific FADD-deficient mice via a conditional genomic rescue approach. We find that FADD-deficiency leads to inhibition of T cell development at the CD4(-)CD8(-) stage and a reduction in the number of mature T cells. The FADD mutation does not affect apoptosis or the proximal signaling events of the pre-T cell receptor; introduction of a T cell receptor transgene fails to rescue the mutant phenotype. These data suggest that FADD, through either a death-domain containing receptor or a novel receptor-independent mechanism, is required for the proliferative phase of early T cell development.

Evidence for non-V3-specific neutralizing antibodies that interfere with gp120/CD4 binding in human immunodeficiency virus 1-infected humans

Total anti-gp120 antibodies (total anti-gp120 Abs) were purified from a pool of four human immunodeficiency virus-positive (HIV+) sera by affinity chromatography on a gp120SF2-Sepharose column and exhibited both type- and group-specific neutralizing activities. To dissect the epitope specificity of the group-specific neutralizing antibodies, CD4 attachment site-specific antibodies (CD4-site Abs) were isolated from total anti-gp120 Abs by using a CD4-blocked gp120SF2-Sepharose column. The CD4-site Abs exhibited group-specific neutralizing activities. Another approach to dissecting type- and group-specific neutralizing activities of total anti-gp120 Abs was to separate the third variable region (V3)-specific antibodies (V3-region Abs) from non-V3-region-specific antibodies (non-V3 Abs). The results indicated that V3-region Abs exhibited type-specific neutralizing activities, whereas non-V3 Abs exhibited group-specific neutralizing activities. By comparing the neutralizing activities of V3-region Abs to those of non-V3 Abs, we concluded that V3-region Abs are more effective than non-V3 Abs in neutralizing a specific HIV isolate. Collectively, this study indicates that group-specific neutralizing anti-gp120 antibodies are specific for the CD4 attachment site.

Oligonucleotide fingerprints of RNA species obtained from rhabdoviruses belonging to the vesicular stomatitis virus subgroup

The relationships among the genomes of various rhabdoviruses belonging to the vesicular stomatitis virus subgroup were analyzed by an oligonucleotide fingerprinting technique. Of 10 vesicular stomatitis viruses, Indiana serotype (VSV Indiana), obtained from various sources, either no, few, or many differences were observed in the oligonucleotide fingerprints of the 42S RNA species extracted from standard B virions. Analyses of the oligonucleotides obtained from RNA extracted from three separate preparations of VSV Indiana defective T particles showed that their RNAs contain fewer oligonucleotides than the corresponding B particle RNA species. The fingerprints of RNA obtained from five VSV New Jersey serotype viruses were easily distinguished from those of the VSV Indiana isolates. Three of the VSV New Jersey RNA fingerprints were similar to each other but quite different from those of the other two viruses. The RNA fingerprints of two Chandipura virus isolates (one obtained from India and one from Nigeria) were also unique, whereas the fingerprint of Cocal virus RNA was unlike that of the serologically related VSV Indiana.

Proposal for genetic characterisation of wild-type mumps strains: Preliminary standardisation of the nomenclature

Though mumps virus (MuV) is a monotypic virus, genetic variation between strains has been described. Viruses have been placed into genotypes designated A-L based on the nucleotide sequence of the small hydrophobic (SH) gene, which is the most variable gene in the mumps genome. Molecular characterisation of MuV is an important component of mumps surveillance because it can help identify the transmission pathways of the virus as well as distinguish between wild-type and vaccine strains. Here, we propose a standardized nomenclature and an analysis protocol for the genetic characterisation of mumps strains to facilitate expansion of molecular epidemiological studies. In addition to assigning standard reference strains for the recognized genotypes of MuV, a convention is proposed for naming for strains and criteria to designate a new genotype.

Inhibition of self-binding antibodies (autobodies) by a VH-derived peptide

The self-binding properties of a dominant idiotypic antibody (T15) and a minor idiotypic antibody (M603), both specific for phosphorylcholine, were examined as models of self-binding antibodies (autobodies). Observed differences in the self-binding affinity of T15 and M603 relate to variable sequence differences in their respective heavy and light chains. A molecular recognition theory based on the translation of coding and noncoding DNA strands was used to identify complementary amino acid sequences responsible for self-binding. The second hypervariable region of the heavy chain domain, extending into the third framework region, was predicted as the primary self-binding locus. Among peptides synthesized with different variable heavy and light chain regions, a 24-residue peptide spanning the second hypervariable and third framework regions of the heavy chain of T15 was nearly as effective as phosphorycholine in inhibiting the self-binding complexes.

Reticuloendotheliosis virus nucleic acid sequences in cellular DNA

Reticuloendotheliosis virus 60S RNA labeled with (125)I, or reticuloendotheliosis virus complementary DNA labeled with (3)H, were hybridized to DNAs from infected chicken and pheasant cells. Most of the sequences of the viral RNA were found in the infected cell DNAs. The reticuloendotheliosis viruses, therefore, replicate through a DNA intermediate. The same labeled nucleic acids were hybridized to DNA of uninfected chicken, pheasant, quail, turkey, and duck. About 10% of the sequences of reticuloendotheliosis virus RNA were present in the DNA of uninfected chicken, pheasant, quail, and turkey. None were detected in DNA of duck. The specificity of the hybridization was shown by competition between unlabeled and (125)I-labeled viral RNAs and by determination of melting temperatures. In contrast, (125)I-labeled RNA of Rous-associated virus-O, an avian leukosis-sarcoma virus, hybridized 55% to DNA of uninfected chicken, 20% to DNA of uninfected pheasant, 15% to DNA of uninfected quail, 10% to DNA of uninfected turkey, and less than 1% to DNA of uninfected duck.

Control of expression, glycosylation, and secretion of HIV-1 gp120 by homologous and heterologous signal sequences

The HIV-1 gp120 signal sequence of 30 amino acids is longer than most glycoprotein signal sequences and contains an average of 5 positively charged amino acids. The HIV-1 gp120 gene with its natural signal sequence expressed in any prokaryotic or eukaryotic expression systems showed extremely low levels of synthesis and secretion. However, deletion of the HIV-1 gp120 signal sequence results in production of large quantities of a nonglycosylated form of gp120 in Spodoptera frugiperda cells. Substitution of the gp120 natural signal sequences with the signal sequences from honeybee mellitin or murine interleukin 3 promotes a high level of expression of a glycosylated form of gp120 and efficient secretion. These heterologous signal sequences contain one (mellitin) or no (IL-3) positively charged amino acids and led us to investigate the role of the positively charged amino acids in the signal sequence of HIV-1 gp120. Four charge-altered forms of the gp120 signal sequence of HIV-1 were constructed by site-directed mutagenesis in which the positively charged amino acids were sequentially substituted with neutral amino acids. The results of these experiments showed that the expression and secretion of gp120 was progressively increased by eliminating the positively charged amino acids in a stepwise fashion. However, the substitution of all positively charged amino acids resulted in the accumulation of nonglycosylated gp120 within the cells with decreased amounts of the glycosylated form of gp120. These results demonstrate that the positively charged amino acids in the signal sequence of HIV-1 gp120 are key factors in determining its poor expression and secretion. Analyses of intracellular transport and folding of gp120 further indicate that the presence of a highly charged, uncleaved signal sequence is an important factor limiting transport of gp120 from the rough ER to the Golgi apparatus.

Modulation of the redox potential of the [Fe(SCys)(4)] site in rubredoxin by the orientation of a peptide dipole

Rubredoxins (Rds) may be separated into two classes based upon the correlation of their reduction potentials with the identity of residue 44; those with Ala44 have reduction potentials that are approximately 50 mV higher than those with Val44. The smaller side chain volume occupied by Ala44 relative to that occupied by Val44 has been proposed to explain the increase in the reduction potential, based upon changes in the Gly43-Ala44 peptide bond orientation and the distance to the [Fe(SCys)(4)] center in the Pyrococcus furiosus (Pf) Rd crystal structure compared to those of Gly43-Val44 in the Clostridium pasteurianum (Cp) Rd crystal structure. As an experimental test of this hypothesis, single-site Val44 <--> Ala44 exchange mutants, [V44A]Cp and [A44V]Pf Rds, have been cloned and expressed. Reduction potentials of these residue 44 variants and pertinent features of the X-ray crystal structure of [V44A]Cp Rd are reported. Relative to those of wild-type Cp and Pf Rds, the V44A mutation in Cp Rd results in an 86 mV increase in midpoint reduction potential and the [A44V] mutation in Pf Rd results in a 95 mV decrease in midpoint reduction potential, respectively. In the crystal structure of [V44A]Cp Rd, the peptide bond between residues 43 and 44 is approximately 0.3 A closer to the Fe center and the hydrogen bond distance between the residue 44 peptide nitrogen and the Cys42 gamma-sulfur decreases by 0.32 A compared to the analogous distances in the wild-type Cp Rd crystal structure. The results described herein support the prediction that the identity of residue 44 alone determines whether a Rd reduction potential of about -50 or 0 mV is observed.

The number of identical kringle IV repeats in apolipoprotein(a) affects its processing and secretion by HepG2 cells

A variable number of 5.6-kilobase kringle IV repeats in the human apolipoprotein(a) (apo(a)) gene results in a size polymorphism of the protein and correlates inversely with the plasma levels of the atherogenic lipoprotein(a) (Lp(a)). In order to analyze whether this association reflects a direct effect of kringle IV repeat number on Lp(a) plasma concentration, we have studied the expression of recombinant apo(a) (r-apo(a)) isoforms in the human hepatocarcinoma cell line HepG2. Following transient transfection of apo(a) cDNA expression plasmids that differed only in the number of kringle IV repeats, we observed a gradual decrease of Lp(a) in the medium of the cells with an increasing number of kringle IV repeats, mimicking the relationship present in humans in vivo. The analysis of apo(a) protein in the lysate and in the medium of cells that were transfected with a plasmid encoding an apo(a) isoform with 22 kringles revealed a predominant intracellular precursor with little secretion of the mature apo(a) protein. In contrast, transfection of a plasmid encoding an isoform with 11 kringles led to effective secretion of the mature peptide into the medium, indicating differential processing rates of apo(a) isoforms in the secretory path way. The intracellular accumulation of an apo(a) precursor in the endoplasmic reticulum was demonstrated by cell fractionation and [35S]Met metabolic labeling/temperature block experiments using HepG2 cells stably transfected with recombinant apo(a). The direct and causal effect of kringle IV repeat number on the expression of recombinant apo(a) in HepG2 cells, and presumably liver cells, provides a novel mechanism for the genetic regulation of the concentration of a protein.

FADD-deficient T cells exhibit a disaccord in regulation of the cell cycle machinery

FADD is an adapter protein that was originally isolated as a transducer of apoptotic signals for death domain-containing receptors. However, FADD-deficient mice are embryonic lethal and FADD(-/-) T cells developed from FADD(-/-) embryonic stem cells in the RAG-1(-/-) hosts lack the full potential to proliferate when stimulated through their T-cell receptor complex, suggesting that FADD protein might play a dualistic role in mediating not only cell death signaling but other non-apoptotic cellular pathways as well. Here we show that a substantial number of freshly isolated FADD(-/-) peripheral T cells are cycling but are defective in their co-stimulatory response when stimulated. Analysis of several cell cycle proteins shows normal down-regulation of p27 inhibitor but increased levels of p21, decreased levels of cyclin D2, and constitutive activation of several cyclin-dependent kinases in activated T cells. These data suggest that FADD is involved in the regulation of cell cycle machinery in T lymphocytes.