Multiple precursor proteins bind individual Tat receptor complexes and are collectively transported

The thylakoid t win a rginine protein t ranslocation (Tat) system is thought to have a multivalent receptor complex with each cpTatC‐Hcf106 pair constituting a signal peptide‐binding unit. Conceptual models suggest that translocation of individual precursor proteins occurs upon assembly of a Tha4 oligomer with a precursor‐occupied cpTatC‐Hcf106. However, results reported here reveal that multiple precursor proteins bound to a single receptor complex can be transported together. Precursor proteins that contain one or two cysteine residues readily formed intermolecular disulphide bonds upon binding to the receptor complex, resulting in dimeric and tetrameric precursor proteins. Three lines of evidence indicate that all members of precursor oligomers were specifically bound to a receptor unit. Blue native–polyacrylamide gel electrophoresis analysis showed that oligomers were present on individual receptor complexes rather than bridging two or more receptor complexes. Upon energizing the membrane, the dimeric and tetrameric precursors were transported across the membrane with efficiencies comparable with that of monomeric precursors. These results imply a novel aspect of Tat systems, whereby multiple precursor‐binding sites can act in concert to transport an interlinked oligo‐precursor protein.     

Glomerular basement membrane proteoglycans are derived from a large precursor

The basement membrane heparan sulfate proteoglycan produced by the Englebreth-Holm-Swarm (EHS) tumor and by glomeruli were compared by immunological methods. Antibodies to the EHS proteoglycan immunoprecipitated a single precursor protein (Mr = 400,000) from [35S]methionine-pulsed glomeruli, the same size produced by EHS cells. These antibodies detected both heparan sulfate proteoglycans and glycoproteins in extracts of unlabeled glomeruli and glomerular basement membrane. The proteoglycans contained core proteins of varying size (Mr = 150,000 to 400,000) with a Mr = 250,000 species being predominant. The glycoproteins are fragments of the core protein which lack heparan sulfate side chains. Antibodies to glomerular basement membrane proteoglycan immunoprecipitated the precursor protein (Mr = 400,000) synthesized by EHS cells and also reacted with most of the proteolytic fragments of the EHS proteoglycan. This antibody did not, however, react with the P44 fragment, a peptide situated at one end of the EHS proteoglycan core protein. These data suggest that the glomerular basement membrane proteoglycan is synthesized from a large precursor protein which undergoes specific proteolytic processing.     

Two subunits of the insect 26/29-kDa proteinase are probably derived from a common precursor protein

We previously identified the 26/29-kDa proteinase in the hemocytes of Sarcophaga peregrina (flesh fly) that appears to participate in elimination of foreign proteins in this insect [Eur. J. Biochem. 209, 939-944 (1992)]. Here, we report the cDNA cloning of this proteinase. The cDNA encodes a protein which includes both the 26- and 29-kDa subunit, strongly suggesting that the both subunits are derived from a single precursor protein. The 26- and 29-kDa subunit located at the amino-terminal and carboxyl-terminal of the precursor protein. The 29-kDa subunit itself appeared to be a proteinase, for this subunit had 52% sequence identity with Sarcophaga cathepsin L, while 26-kDa subunit had no significant similarity. We also showed that 26/29-kDa proteinase was insensitive to specific inhibitors of cathepsin L. These results indicate that this proteinase is a novel member of the papain family. We isolated similar cDNAs from Drosophila melanogaster and Periplaneta americana (cockroach), suggesting that this proteinase is conserved in a wide variety of insects and participates in their defense mechanisms.     

Flavonoids are differentially taken up and transported long distances in Arabidopsis

Flavonoids are synthesized in response to developmental and environmental signals and perform many functions in plants. Arabidopsis (Arabidopsis thaliana) roots grown in complete darkness do not accumulate flavonoids since the expression of genes encoding enzymes of flavonoid biosynthesis is light dependent. Yet, flavonoids accumulate in root tips of plants with light-grown shoots and light-shielded roots, consistent with shoot-to-root flavonoid movement. Using fluorescence microscopy, a selective flavonoid stain, and localized aglycone application to transparent testa mutants, we showed that flavonoids accumulated in tissues distal to the application site, indicating uptake and movement systems. This was confirmed by time-course fluorescence experiments and high-performance liquid chromatography. Flavonoid applications to root tips resulted in basipetal movement in epidermal layers, with subsequent fluorescence detected 1 cm from application sites after 1 h. Flavonoid application to midroot or cotyledons showed movement of flavonoids toward the root tip mainly in vascular tissue. Naringenin, dihydrokaempferol, and dihydroquercetin were taken up at the root tip, midroot, or cotyledons and traveled long distances via cell-to-cell movement to distal tissues, followed by conversion to quercetin and kaempferol. In contrast, kaempferol and quercetin were only taken up at the root tip. Using ATP-binding cassette (ABC) transporter and H(+)-ATPase inhibitors suggested that a multidrug resistance-associated protein ABCC transporter facilitated flavonoid movement away from the application site.     

Cell-associated episialin is a complex containing two proteins derived from a common precursor.

cDNA for the epithelial sialomucin episialin encodes a transmembrane molecule with a large extracellular domain, which mainly consists of repeats of 20 amino acids. Here we confirm the existence of a previously proposed proteolytic cleavage of episialin that occurs in the endoplasmic reticulum (Hilkens, J., and Buijs, F. (1988) J. Biol. Chem. 263, 4215-4222) and show that a similar cleavage takes place in in vitro translation systems. Using in vitro translation of truncated mRNAs, we map the cleavage site to a region located between 71 and 53 amino acids upstream of the transmembrane domain. Analysis of a mutant, in which this region has been deleted, indicates that the cleavage sites used in vitro and in vivo are identical or in close proximity. Both cleavage products remain associated although they are not linked through disulfide bonds. Therefore, the subunit derived from the N terminus, which represents the actual mucin-like domain, remains indirectly anchored to the cell membrane as a result of its interaction with the C-terminal subunit.     

Secreted hepatitis B surface antigen polypeptides are derived from a transmembrane precursor

Hepatitis B surface antigen (HBsAg), the major coat protein of hepatitis B virus, is also independently secreted from infected cells as a lipoprotein particle. Secretion proceeds without signal sequence removal or cleavage of other segments of the polypeptide. We have examined the synthesis and transport of HBsAg in cultured cells expressing the cloned surface antigen gene. Our results show that HBsAg is initially synthesized as a integral membrane protein. This transmembrane form is slowly converted to a secreted lipoprotein complex in the lumen of the endoplasmic reticulum via a series of definable intermediates, after which it is secreted from the cell. This unusual export process shares many features with the assembly and budding reactions of conventional enveloped animal viruses. However, it differs importantly in its absence of a requirement for the participation of nucleocapsid or other viral proteins.     

Different transforming growth factor-alpha species are derived from a glycosylated and palmitoylated transmembrane precursor

cDNA analysis has revealed that the 50 amino acid transforming growth factor-alpha (TGF-alpha) is derived from a 160 amino acid precursor. Antibodies to TGF-alpha and to a C-terminal portion of the precursor were used to study the biosynthesis and processing of the precursor. CHO cells transfected with a TGF-alpha expression vector secrete high levels of TGF-alpha; a mixture of species of about 18 kd is secreted in addition to the 50 amino acid form. These larger species are N-glycosylated and are derived from the same precursor as the smaller form. The C-terminal segment of the precursor remains anchored in the membrane and has covalently attached palmitate. The newly synthesized TGF-alpha precursor is thus a transmembrane protein that subsequently undergoes external proteolytic cleavages, releasing several TGF-alpha species.     

Ricin and Ricinus communis agglutinin subunits are all derived from a single-size polypeptide precursor

Antibodies have been raised in rabbits against the individually purified A and B subunits of the toxic castor bean lectin, ricin, and against the A' and B' subunits of Ricinus communis agglutinin type I. Each of the antisera recognised a single polypeptide species of Mr 60 500 when maturing castor bean endosperm mRNA was translated in vitro in a rabbit-reticulocyte-derived system. When dog pancreatic microsomal vesicles were included in the translational system, each subunit antiserum precipitated a group of 66 000-68 000-Mr core-glycosylated polypeptides which had been translocated into the lumen of the vesicles. The 60 500-Mr polypeptide appeared to be a common precursor to all four individual lectin subunits since (a) its glycosylated (66 000-68 000-Mr) forms were readily detected in the endoplasmic reticulum fraction isolated from maturing castor bean endosperm and (b) pulse-chase studies showed that the glycosylated precursors disappeared from the endoplasmic reticulum fraction with the concomittant appearance of authentic lectin subunits in a soluble protein fraction which included protein body matrix components. Antiserum prepared against whole R. communis agglutinin, type I, also precipitated the 65 000-Mr precursor in vitro and in vivo, but in addition precipitated a non-glycosylated 34 000-Mr polypeptide. This smaller protein is not a lectin subunit precursor, contradicting an earlier suggestion. It is most probably a precursor to the 2-S albumin storage proteins found in castor bean endosperm protein bodies.     

Two alpha-herpesvirus strains are transported differentially in the rodent visual system.

Uptake and transneuronal passage of wild-type and attenuated strains of a swine alpha-herpesvirus (pseudorabies [PRV]) were examined in rat visual projections. Both strains of virus infected subpopulations of retinal ganglion cells and passed transneuronally to infect retino-recipient neurons in the forebrain. However, the location of infected forebrain neurons varied with the strain of virus. Intravitreal injection of wild-type virus produced two temporally separated waves of infection that eventually reached all known retino-recipient regions of the central neuraxis. By contrast, the attenuated strain of PRV selectively infected a functionally distinct subset of retinal ganglion cells with restricted central projections. The data indicate that projection-specific groups of ganglion cells are differentially susceptible to the two strains of virus and suggest that this sensitivity may be receptor mediated.     

Vertebrate and nematode genes coding for yolk proteins are derived from a common ancestor

One of the most obvious characteristics of the egg cells of oviparous animals is their large size resulting to a major extent from the deposition of nutritional reserves, mainly constituted of yolk proteins. In general, these are derived from a precursor called vitellogenin, which undergoes posttranslational modifications during secretion and during transport into and storage within the oocytes. Comparative analysis of the structural organization of the vitellogenin gene and of its product in different species shows that the vitellogenin gene is very ancient and that in vertebrates the gene may have more resemblance to the earliest gene than in invertebrates.     

Two African swine fever virus proteins derived from a common precursor exhibit different nucleocytoplasmic transport activities

African swine fever virus (ASFV), a large icosahedral deoxyvirus, is the causative agent of an economically relevant hemorrhagic disease that affects domestic pigs. The major purpose of the present study was to investigate the nuclear transport activities of the ASFV p37 and p14 proteins, which result from the proteolytic processing of a common precursor. Experiments were performed by using yeast-based nucleocytoplasmic transport assays and by analysis of the subcellular localization of different green fluorescent and Myc fusion proteins in mammalian cells. The results obtained both in yeast and mammalian cells clearly demonstrated that ASFV p14 protein is imported into the nucleus but not exported to the cytoplasm. The ability of p37 protein to be exported from the nucleus to the cytoplasm of both yeast and mammalian cells was also demonstrated, and the results clearly indicate that p37 nuclear export is dependent on the interaction of the protein with the CRM-1 receptor. In addition, p37 was shown to exhibit nuclear import activity in mammalian cells. The p37 protein nuclear import and export abilities described here constitute the first report of a nucleocytoplasmic shuttling protein encoded by the ASFV genome. Overall, the overlapping results obtained for green fluorescent protein fusions and Myc-tagged proteins undoubtedly demonstrate that ASFV p37 and p14 proteins exhibit nucleocytoplasmic transport activities. These findings are significant for understanding the role these proteins play in the replication cycle of ASFV.     

Multiple biological activities for two peptides derived from the nerve growth factor precursor

ProNGF can be cleaved proteolytically at dibasic residues and liberates two other peptides beside NGF, LIP1 a 29 amino acid (aa) peptide and LIP2 a 38 aa peptide. These peptides were found present in the rat intestine and shown to induce rapid phosphorylation of the Trk receptor in cell lines. The present study describes several novel biological properties for these peptides. They exert an anti-proliferative effect on the mitogenic activity of estrogen and IGF in MCF-7 cells. They protect against in vivo induction of excitotoxic lesions by the glutamatergic analogue ibotenate injected into the developing mouse brain and against in vitro NMDA-induced cell death in primary neuronal cultures. They bind to murine microglial cells and induce phosphorylation of Akt. These results suggest a role for LIP1 and LIP2 in cell survival.     

Mosquito vitellogenin subunits originate from a common precursor

Using a cell-free translation system, we demonstrated that the two subunits of mosquito vitellogenin (VG), 200 kDa and 65 kDa, originate from a common precursor. The precursor polypeptide of 220 kDa is a translation product specific to mRNA from vitellogenic mosquitoes. In immunoprecipitation analysis, the 220-kDa polypeptide was recognized by monoclonal antibodies directed either to the large or the small VG subunit. Peptide mapping showed homology between the 220-kDa polypeptide and both subunits, thus providing further proof that the 220-kDa product of translation is the precursor for both VG subunits. In the presence of microsomal membranes, the molecular size of the VG precursor increased to 235 kDa suggesting this as a first step in co-translational modifications of VG.     

The structurally similar neuropeptides adipokinetic hormone I and II are derived from similar, very small mRNAs

The grasshopper neuropeptides adipokinetic hormone (AKH) I and II were among the first of an extensive family of structurally similar arthropod hormones and neuroregulators to be isolated and sequenced. This paper reports the cloning of cDNAs derived from the unusually small mRNAs (550 bases) which code for the precursors of AKH I and II from Schistocerca nitans. Sequence analysis of the cDNAs indicates that AKH I and II are derived from small precursor proteins (63 and 61 amino acids) which are 55% identical in amino acid sequence. Each contains a 22-amino acid hydrophobic leader sequence followed by the AKH I or II sequence and an additional 28-amino acid carboxyl-terminal peptide of unknown function. Significant homology at the nucleic acid level (64% identity) is confined to the coding region of the mRNA sequences. Preliminary DNA blot analyses suggest that a single gene codes for each, and that the genes for AKH I and II may be linked. Genomic blots from various tissues fail to suggest that the high level of expression of AKH in the corpora cardiaca is due to tissue specific gene amplification.     

Multiple calmodulin mRNA species are derived from two distinct genes.

We have observed three calmodulin mRNA species in rat tissues. In order to know from how many expressed genes they are derived, we have investigated the genomic organization of calmodulin genes in the rat genome. From a rat brain cDNA library, we obtained two kinds of cDNAs (pRCM1 and pRCM3) encoding authentic calmodulin. DNA sequence analysis of these cDNA clones revealed substitutions of nucleotides at 73 positions of 450 nucleotides in the coding region, although the amino acid sequences of these calmodulins are exactly the same. DNA sequences in the 5' and 3' noncoding regions are quite different between these two cDNAs. From these results, we conclude that they are derived from two distinct bona fide calmodulin genes, CaMI (pRCM1) and CaMII (pRCM3). Total genomic Southern hybridization suggested four distinct calmodulin-related genes in the rat genome. By cloning and sequencing the calmodulin-related genes from rat genomic libraries, we demonstrated that the other two genes are processed pseudogenes generated from the CaMI (lambda SC9) and CaMII (lambda SC8) genes, respectively, through an mRNA-mediated process of insertions. Northern blotting showed that the CaMI gene is transcribed in liver, muscle, and brain in similar amounts, whereas the CaMII gene is transcribed mainly in brain. S1 nuclease mapping indicated that the CaMI gene produced two mRNA species (1.7 and 4 kilobases), whereas the CaMII gene expressed a single mRNA species (1.4 kilobases).     

Drosophila fat body protein P6 and alcohol dehydrogenase are derived from a common ancestral protein

Summary Drosophila melanogaster alcohol dehydrogenase is an example of convergent evolution: it is not related to the ADHs of other organisms, but to short-chain dehydrogenases, which until now have been found only in bacteria and in mammalian steroid hormone metabolism. We present evidence that theDrosophila ADH is phylogenetically more closely related to P6, another highly expressed protein from the fat body ofDrosophila, than it is to the short-chain dehydrogenases. The polypeptide sequence of P6 was inferred from DNA sequence analysis. Both ADH and P6 polypeptides have retained a high structural similarity with respect to the Chou-Fasman prediction of secondary structure and hydropathy. P6 is also homologous to the 25-kd protein from the fat body ofSarcophaga peregrina, whose sequence we have reexamined. The evolution of the P6-ADH family of proteins is characterized by a dramatic increase in the methionine content of P6. Methionine accounts for 20% of P6 amino acids. This is in contrast with the absence of this amino acid in mature ADH. There is evidence that P6 and the 25-kd protein have undergone a parallel and independent enrichment in methionine. When corrected for this, the rate of amino acid replacement shows that the P6-25-kd lineage diverged from insect ADH shortly before the divergence of the ADH gene (Adh) from its 3-duplication (Adhdup).     

Multiple catalase genes are differentially regulated in Aspergillus nidulans

Detoxification of hydrogen peroxide is a fundamental aspect of the cellular antioxidant responses in which catalases play a major role. Two differentially regulated catalase genes, catA and catB, have been studied in Aspergillus nidulans. Here we have characterized a third catalase gene, designated catC, which predicts a 475-amino-acid polypeptide containing a peroxisome-targeting signal. With a molecular mass of 54 kDa, CatC shows high similarity to other small-subunit monofunctional catalases and is most closely related to catalases from other fungi, Archaea, and animals. In contrast, the CatA (approximately 84 kDa) and CatB (approximately 79 kDa) enzymes belong to a family of large-subunit catalases, constituting a unique fungal and bacterial group. The catC gene displayed a relatively constant pattern of expression, not being induced by oxidative or other types of stress. Targeted disruption of catC eliminated a constitutive catalase activity not detected previously in zymogram gels. However, a catalase activity detected in catA catB mutant strains during late stationary phase was still present in catC and catABC null mutants, thus demonstrating the presence of a fourth catalase, here named catalase D (CatD). Neither catC nor catABC triple mutants showed any developmental defect, and both mutants grew as well as wild-type strains in H(2)O(2)-generating substrates, such as fatty acids, and/or purines as the sole carbon and nitrogen sources, respectively. CatD activity was induced during late stationary phase by glucose starvation, high temperature, and, to a lesser extent, H(2)O(2) treatment. The existence of at least four differentially regulated catalases indicates a large and regulated capability for H(2)O(2) detoxification in filamentous fungi.