Binding of BiP to an assembly-defective protein in plant cells

The binding protein (BiP) has been implicated as a mediator of protein folding and assembly in the endoplasmic reticulum of mammalian cells and has often been found in stable association with structurally defective proteins. To acquire information on the activity of BiP in plant cells, we have expressed in tobacco protoplasts the wild type form and an assembly-defective form of bean phaseolin. Phaseolin (PHSL) is a soluble, trimeric, storage glycoprotein co-translationally inserted into the lumen of the endoplasmic reticulum and then transported along the secretory pathway to the protein storage vacuoles. We have previously shown that a PHSL mutant in which the last 59 amino acids have been deleted (Δ363PHSL) is unable to form trimers and is retained in a pre-Golgi compartment when synthesized in Xenopus oocytes. When transiently expressed in tobacco leaf protoplasts, wild-type PHSL is correctly glycosylated and assembles efficiently and rapidly into trimers. Δ363PHSL is also correctly glycosylated but does not trimerize. Tobacco BiP and Δ363PHSL are co-immunoselected using either anti-PHSL or anti-BiP antibodies. Under the same conditions, co-immunoselection of BiP with wild-type PHSL is not detectable. The BiP bound to Δ363PHSL can be released by treatment of the complex with ATP, indicating that the binding is related to the proposed function of BiP in protein folding and assembly in the endoplasmic reticulum. These data indicate that BiP stably binds structurally defective proteins in plant cells.     

Cytonuclear coevolution in a holoparasitic plant with highly disparate organellar genomes

Key Message

Contrasting substitution rates in the organellar genomes of Lophophytum agree with the DNA repair, replication, and recombination gene content. Plastid and nuclear genes whose products form multisubunit complexes co-evolve.

Abstract

The organellar genomes of the holoparasitic plant Lophophytum (Balanophoraceae) show disparate evolution. In the plastid, the genome has been severely reduced and presents a?>?85% AT content, while in the mitochondria most protein-coding genes have been replaced by homologs acquired by horizontal gene transfer (HGT) from their hosts (Fabaceae). Both genomes carry genes whose products form multisubunit complexes with those of nuclear genes, creating a possible hotspot of cytonuclear coevolution. In this study, we assessed the evolutionary rates of plastid, mitochondrial and nuclear genes, and their impact on cytonuclear evolution of genes involved in multisubunit complexes related to lipid biosynthesis and proteolysis in the plastid and those in charge of the oxidative phosphorylation in the mitochondria. Genes from the plastid and the mitochondria (both native and foreign) of Lophophytum showed extremely high and ordinary substitution rates, respectively. These results agree with the biased loss of plastid-targeted proteins involved in angiosperm organellar repair, replication, and recombination machinery. Consistent with the high rate of evolution of plastid genes, nuclear-encoded subunits of plastid complexes showed disproportionate increases in non-synonymous substitution rates, while those of the mitochondrial complexes did not show different rates than the control (i.e. non-organellar nuclear genes). Moreover, the increases in the nuclear-encoded subunits of plastid complexes were positively correlated with the level of physical interaction they possess with the plastid-encoded ones. Overall, these results suggest that a structurally-mediated compensatory factor may be driving plastid-nuclear coevolution in Lophophytum, and that mito-nuclear coevolution was not altered by HGT.

     

The Rate of Phaseolin Assembly Is Controlled by the Glucosylation State of Its N-Linked Oligosaccharide Chains

Many of the proteins that are translocated into the endoplasmic reticulum are glycosylated with the addition of a 14-saccharide core unit (Glc3Man9GlcNAc2) to specific asparagine residues of the nascent polypeptide. Glucose residues are then removed by endoplasmic reticulum-located glucosidases, with diglucosylated and monoglucosylated intermediates being formed. In this study, we used a cell-free system constituted of wheat germ extract and bean microsomes to examine the role of glucose trimming in the structural maturation of phaseolin, a trimeric glycoprotein that accumulates in the protein storage vacuoles of bean seeds. Removal of glucose residues from the N-linked chains of phaseolin was blocked by the glucosidase inhibitors castanospermine and N-methyldeoxynojirimycin. If glucose trimming was not allowed to occur, the assembly of phaseolin was accelerated. Conversely, polypeptides bearing partially trimmed glycans were unable to form trimers. The effect of castanospermine on the rate of assembly was much more pronounced for phaseolin polypeptides that have two glycans but was also evident when a single glycan chain was present, indicating that glycan clustering can modulate the effect of glucose trimming on the rate of trimer formation. Therefore, the position of glycan chains and their accessibility to the action of glucosidases can be fundamental elements in the control of the structural maturation of plant glycoproteins.     

Characterization and Genetic Study of the Ovine α S2 -Casein (CSN1S2) Allele B

An interesting and quite complex protein pattern has been described at ovine milk proteins but the genetic control of the variation observed was assessed only in few cases. The aim of this work was to characterize the ovine α _s2 -casein (CSN1S2) B variant, first observed in the Italian Gentile di Puglia, a fine-wooled ovine breed, and to investigate its occurrence in two further breeds, the Sarda and Camosciata, which are the most widespread dairy breeds in Italy. The B variant differs from the most common form A with two amino acid exchanges: Asp₇₅ → Tyr₇₅ and Ile₁₀₅ → Val₁₀₅. The first substitution, resulting in a loss of a negative charge, is responsible for the higher isoelectric point of the B protein variant, which allows its detection by isoelectric focusing electrophoresis (IEF). The occurrence of CSN1S2*B in Sarda and Comisana was demonstrated. Since the Asp₇₅ → Tyr₇₅ substitution modifies the protein electric charge, milk properties may result affected to some extent.     

The N-terminal ricin propeptide influences the fate of ricin A-chain in tobacco protoplasts

The plant toxin ricin is synthesized in castor bean seeds as an endoplasmic reticulum (ER)-targeted precursor. Removal of the signal peptide generates proricin in which the mature A- and B-chains are joined by an intervening propeptide and a 9-residue propeptide persists at the N terminus. The two propeptides are ultimately removed in protein storage vacuoles, where ricin accumulates. Here we have demonstrated that the N-terminal propeptide of proricin acts as a nonspecific spacer to ensure efficient ER import and glycosylation. Indeed, when absent from the N terminus of ricin A-chain, the non-imported material remained tethered to the cytosolic face of the ER membrane, presumably by the signal peptide. This species appeared toxic to ribosomes. The propeptide does not, however, influence catalytic activity per se or the vacuolar targeting of proricin or the rate of retrotranslocation/degradation of A-chain in the cytosol. The likely implications of these findings to the survival of the toxin-producing tissue are discussed.     

Trimer formation determines the rate of influenza virus haemagglutinin transport in the early stages of secretion in Xenopus oocytes

We have previously shown that influenza haemagglutinin (HA) acquires Endo H resistance en route to the cell surface after microinjection of its mRNA into Xenopus oocytes (Ceriotti, A. and A. Colman. 1989. J. Cell Biol. 109:1439-1444.) In this paper we use the injection of varying amounts of mRNA (0.05-5 ng/oocyte) to effect a 30-fold change in HA protein synthesis within the oocyte. Using the Endo H assay as an indicator of protein movement from the ER to the medial Golgi we find that this movement is reduced, sometimes dramatically, when intracellular HA levels fall. This reduction in movement is closely correlated with a decreased rate of trimer formation as assessed both by trypsin resistance and sedimentation analysis, leading us to conclude that trimer formation is not only, as has been shown before essential for ER-Golgi complex movement, but is the major rate limiting step in this movement. Interestingly at least 50% of unassembled HA monomers that accumulate after low HA synthesis can be rescued into trimers over 24 h later, after a second injection of concentrated HA mRNA. In contrast when we repeated this experiment with another membrane protein, the human low density lipoprotein, or with murine secretory immunoglobulin we found that the rate of movement was insensitive to the protein concentration. This latter result seemed surprising since earlier work had shown that unassembled IgG heavy chains (like monomeric HA) remain in the oocyte ER; however in these present experiments we have been unable to detect any unassembled heavy chains even at the lowest expression levels, indicating that tetramerization of Ig is much faster than trimerization of HA.     

Accumulation of a sulphur-rich seed albumin from sunflower in the leaves of transgenic subterranean clover (Trifolium subterraneum L.)

A gene encoding a sulphur-rich, sunflower seed albumin (23% cysteine plus methionine) was modified to contain the promoter for the 35S RNA of cauliflower mosaic virus, in order to obtain leaf expression in transgenic plants. In addition, a sequence encoding an endoplasmic reticulum-retention signal was added to the 3 end of the coding region so as to stabilize the protein by diverting it away from the vacuole. The modified gene was introduced into subterranean clover (T. subterraneum L.) and its expression was detected by northern and western blots and by immunogold localization. The albumin was accumulated in the lumen of the endoplasmic reticulum, and, among six independent, transformed lines, it accumulated in the leaves of T₀ transgenic plants at varying levels up to 0.3% of the total extractable protein. The level of accumulation of the sunflower albumin increased with increasing leaf age, and in the older leaves of the most highly expressing plants of the T₁ generation it reached 1.3% of total extractable protein. Expression of the SSA gene was stable in the first and second generation progeny. These results indicate that there is potential for significantly improving the nutritional value of subterranean clover for ruminant animals such as sheep by expressing genes that code for sulphur-rich, rumen-stable proteins in leaves.     

Redox regulation of glutenin subunit assembly in the plant endoplasmic reticulum

The glutenin fraction of wheat storage proteins consists of large polymers in which high‐ and low‐molecular‐weight subunits are connected by inter‐chain disulfide bonds. We found that assembly of a low‐molecular‐weight glutenin subunit in the endoplasmic reticulum is a rapid process that leads to accumulation of various oligomeric forms, and that this assembly is sensitive to perturbation of the cellular redox environment. In endoplasmic reticulum‐derived microsomes, low‐molecular‐weight glutenin subunits are subjected to hyper‐polymerization, indicating that cytosolic factors play an important role in limiting polymer size. Addition of physiological concentrations of reduced glutathione is sufficient to maintain the original polymerization pattern of the glutenin subunits upon cytosol dilution. Furthermore, we show that a low‐molecular‐weight glutenin subunit can be glutathionylated in endoplasmic reticulum‐derived microsomes, and that it can be directly reduced by glutathione in vitro. These results indicate that glutenin polymerization is sensitive to changes in the redox state of the cell, and suggest that the presence of a reducing cytosolic environment plays an important role in regulating disulfide bond formation in the endoplasmic reticulum of plant cells.     

Ricin B chain targeted to the endoplasmic reticulum of tobacco protoplasts is degraded by a CDC48- and vacuole-independent mechanism

The B chain of ricin was expressed and delivered to the endoplasmic reticulum of tobacco protoplasts where it disappeared with time in a manner consistent with degradation. This turnover did not occur in the vacuoles or upon secretion. Indeed, several lines of evidence indicate that, in contrast to the turnover of endoplasmic reticulum-targeted ricin A chain in the cytosol, the bulk of expressed ricin B chain was degraded in the secretory pathway.