Stored messenger ribonucleoprotein particles in differentiated sclerotia of Physarum polycephalum

Type 2A phosphoprotein phosphatase is required for asexual development and pathogenesis of Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic, omnivorous plant pathogen with worldwide distribution. Sclerotia of S. sclerotiorum are pigmented, multihyphal structures that play a central role in the life and infection cycles of this pathogen. Plant infection depends on the formation of melanin-rich infection cushions, and secretion of hydrolytic enzymes and oxalic acid. Type 2A Ser/Thr phosphatases (PP2As) are involved in the regulation of a variety of cellular process. In the presence of cantharidin, a PP2A-specific inhibitor, hyphal elongation and sclerotia numbers were impaired whereas sclerotial size increased. We partially inactivated PP2A by antisense expression of the gene (pph1) encoding the PP2A catalytic subunit. When antisense expression was induced, almost complete cessation of fungal growth was observed, indicative of a crucial role for PP2A in fungal growth. RNAi-based gene silencing was employed to alter the expression of the 55-kDa R2 (B regulatory subunit). Isolates in which rgb1 RNA levels were decreased were slow growing, but viable. Melanin biosynthesis, infection-cushion production, and pathogenesis were significantly impaired in the rgb1 mutants, yet theses mutants were pathogenic on wounded leaves. Reduced ERK (extracellular signal-regulated kinases)-like mitogen-activated protein kinase (MAPK) function conferred a reduction in NADPH oxidase and PP2A activity levels, suggesting a functional link between MAPK, reactive oxygen species, and PP2A activity in S. sclerotiorum.

Characterization of a Trypanosoma cruzi poly(A)-binding protein and its genes

We have characterized the biochemical properties of a 66-kDa poly(A)-binding protein (PABP1) in the protozoan Trypanosoma cruzi and isolated two classes of cDNAs encoding the protein. In concordance, Southern blots showed the presence of 2 gene copies. The two cDNA classes differ in the length of adenosine-rich segments in the 5' untranslated region and in point changes scattered throughout the sequence, but their 1650-bp open reading frames encode identical proteins. A single mRNA of 5.5 kb was detected, indicating that the noncoding regions are unusually long. Both the mRNA and the protein are constitutively expressed in all stages of T. cruzi life cycle. The biochemical properties and sequence comparisons show that the T. cruzi PABP1 is similar to the PABP1 of other eukaryotic organisms. These results indicate that PABP1 has been conserved throughout eukaryotic evolution.

Identification and characterization of the poly(A)-binding proteins from the sea urchin: a quantitative analysis

Poly(A)-binding proteins (PABPs) are the best characterized messenger RNA-binding proteins of eucaryotic cells and have been identified in diverse organisms such as mammals and yeasts. The in vitro poly(A)-binding properties of these proteins have been studied intensively; however, little is known about their function in cells. In this report, we show that sea urchin eggs have two molecular weight forms of PABP (molecular weights of 66,000 and 80,000). Each of these has at least five posttranslationally modified forms. Both sea urchin PABPs are found in approximately 1:1 ratios in both cytoplasmic and nuclear fractions of embryonic cells. Quantification in eggs and embryos revealed that sea urchin PABPs are surprisingly abundant, composing about 0.6% of total cellular protein. This is 50 times more than required to bind all the poly(A) in the egg based on the binding stoichiometry of 1 PABP per 27 adenosine residues. We found that density gradient centrifugation strips PABP from poly(A) and therefore underestimates the amount of PABP complexed to poly(A)+ RNA in cell homogenates. However, large-pore gel filtration chromatography could be used to separate intact poly(A)-PABP complexes from free PABP. Using the gel filtration method, we found that the threefold increase in poly(A) content of the egg after fertilization is paralleled by an approximate fivefold increase in the amount of bound PABP. Furthermore, both translated and nontranslated poly(A)+ RNAs appear to be complexed to PABP. As expected from the observation that PABPs are so abundant, greater than 95% of the PABP of the cell is uncomplexed protein.

Identification and characterization of the poly(A)-binding proteins from the sea urchin: a quantitative analysis

Poly(A)-binding proteins (PABPs) are the best characterized messenger RNA-binding proteins of eucaryotic cells and have been identified in diverse organisms such as mammals and yeasts. The in vitro poly(A)-binding properties of these proteins have been studied intensively; however, little is known about their function in cells. In this report, we show that sea urchin eggs have two molecular weight forms of PABP (molecular weights of 66,000 and 80,000). Each of these has at least five posttranslationally modified forms. Both sea urchin PABPs are found in approximately 1:1 ratios in both cytoplasmic and nuclear fractions of embryonic cells. Quantification in eggs and embryos revealed that sea urchin PABPs are surprisingly abundant, composing about 0.6% of total cellular protein. This is 50 times more than required to bind all the poly(A) in the egg based on the binding stoichiometry of 1 PABP per 27 adenosine residues. We found that density gradient centrifugation strips PABP from poly(A) and therefore underestimates the amount of PABP complexed to poly(A)+ RNA in cell homogenates. However, large-pore gel filtration chromatography could be used to separate intact poly(A)-PABP complexes from free PABP. Using the gel filtration method, we found that the threefold increase in poly(A) content of the egg after fertilization is paralleled by an approximate fivefold increase in the amount of bound PABP. Furthermore, both translated and nontranslated poly(A)+ RNAs appear to be complexed to PABP. As expected from the observation that PABPs are so abundant, greater than 95% of the PABP of the cell is uncomplexed protein.

Tales of poly(A): a review

Until recently, evidence to support a translational role for the 3'-poly(A) tract of eukaryotic mRNAs has been mostly indirect, including: a correlation between the adenylation status of individual mRNAs and their translatability in vivo or in vitro, the demonstration that exogenously added poly(A) is a potent competitive inhibitor of the translation of poly(A)+mRNA, but not poly(A)-mRNAs in vitro, and a correlation between the abundance and stability of poly(A)-binding proteins (PABPs) and the rate of translational initiation in vivo. However, more recent studies demonstrate directly that poly(A)+mRNAs can initiate translation more efficiently than poly(A)-mRNAs, and indicate that this effect is: (i) targeted to the formation of 80S initiation complexes, and (ii) likely to be mediated by the cytoplasmic PABP. We suggest that the 3'-poly(A) tail should be considered a translational enhancer which may stimulate translational initiation in much the same way that transcriptional enhancers are thought to stimulate transcriptional initiation.

The function of proteins that interact with mRNA

Specific proteins are associated with mRNA in the cytoplasm of eukaryotic cells. The complement of associated proteins depends upon whether the mRNA is an integral component of the polysomal complex being translated, or, alternatively, whether it is part of the non-translated free mRNP fraction. By subjecting cells to ultraviolet irradiation in vivo to cross-link proteins to mRNA, mRNP proteins have been shown to be associated with specific regions of the mRNA molecule. Examination of mRNP complexes containing a unique mRNA has suggested that not all mRNA contain the same family of associated RNA binding proteins. The functions of mRNA associated proteins may include a role in providing stability for mRNA, and/or in modulating translation. With the recent demonstrations that both free and polysomal mRNPs are associated with the cytoskeletal framework, specific mRNP proteins may play a role in determining the subcellular localization of specific mRNPs.

Translational control and the cytoskeleton in Physarum polycephalum

Translationally active plasmodia of the syncytial slime mold Physarum polycephalum develop into translationally dormant sclerotia during starvation. Although functional mRNA and ribosomes exist in sclerotia, protein synthesis is suppressed at the level of initiation. To test the possibility that alterations in the cytoskeleton may limit protein synthesis, we have examined the distribution of polysomes and actin mRNA in the cytoskeletal (CSK) and soluble (SOL) fractions of Triton X-100-extracted plasmodia and sclerotia. Most of the polysomes and actin mRNA were located in the CSK of plasmodia, while most of the ribosomes and actin mRNA were located in the SOL of sclerotia. The results suggest that ribosomes and mRNA shift from the CSK to the SOL as protein synthesis is suppressed during starvation. Plasmodia and sclerotia can be induced to accumulate excess polysomes by treatment with low levels of the elongation inhibitor cycloheximide. Treatment of plasmodia with cycloheximide caused excess polysomes to accumulate in the SOL, suggesting that the CSK contains a limited capacity for binding translational components and that the association of polysomes with the cytoskeleton is not required for protein synthesis. Treatment of sclerotia with cycloheximide, however, caused polysomes and actin mRNA to accumulate in the CSK, suggesting that the sclerotial cytoskeleton, although depleted in ribosomes and mRNA, is capable of binding translational components. It is concluded that alterations in the sclerotial cytoskeleton are not involved in translational control.

Identification and characterization of developmentally regulated mRNP proteins of Dictyostelium discoideum

The isolation of poly(A)+ polysomal and nonpolysomal RNPs by oligo(dT)-cellulose chromatography has led to the identification of more than 20 polypeptides that bind to the poly(A)+ mRNA in growing Dictyostelium cells. Most of these polypeptides were identified in experiments using short-wave UV light (254 nm) to crosslink specifically bound proteins to the RNA. Digestion of the RNPs with ribonucleases A and T1 prior to their application to oligo(dT)-cellulose permitted the isolation of the 3' poly(A)-protein complexes. In polysomal RNPs, two major polypeptides, with molecular weights of 31,000 (p31) and 31,500 (p31.5), are bound to poly(A). These proteins can also be purified from cytoplasmic extracts by affinity chromatography on poly(A)-Sepharose. Partial proteolytic digestion of p31 and p31.5 indicates that they are closely related. The UV-crosslinking experiments established that p31 and p31.5 bind to the non-poly(A) segments of mRNA as well. In nonpolysomal RNPs, p31 and a polypeptide with a molecular weight of 29,500 (p29.5) are the major species associated with poly(A). Partial proteolytic digestion of p29.5 indicates that it is closely related to p31 and p31.5. Only small amounts of p29.5 were observed in the polysomal poly(A)-protein complexes. Early in Dictyostelium development, when cellular translation activity is sharply reduced, most of the p29.5, p31 and p31.5 present is selectively degraded. These observations are consistent with a translational role for these proteins.

Role of mRNA 5'-terminal caps in translational dormancy of Physarum polycephalum

Translational regulation of protein synthesis accompanies sclerotization in Physarum polycephalum. Plasmodial and sclerotial poly(A)+ RNA were translated in a message-dependent wheat germ lysate in the presence of the cap analogue 7-methylguanosine-triphosphate to determine whether 5' structural alterations in mRNA accompany translational repression. The translation of plasmodial and sclerotial poly(A)+ RNA was reduced to identical levels suggesting that both RNA populations are capped. The 5'-termini of plasmodial and sclerotial poly(A)+ RNA were identified as m7G5'ppp5'Cm. Alterations in the 5'-cap of mRNA during sclerotization do not appear to be responsible for translational dormancy.

Poly(A)-containing messenger ribonucleoprotein complexes from sea urchin eggs and embryos: polypeptides associated with native and UV-crosslinked mRNPs

Fertilization of sea urchin eggs results in the rapid recruitment of stored messages into polyribosomes. Whether translational control in sea urchin eggs is mediated by macromolecules associated with the stored messages remains unknown, since preparations of messenger ribonucleoprotein complexes (mRNPs) were active in protein synthesis in a rabbit reticulocyte lysate. To facilitate the study of mRNPs, chromatography on oligo(dT)-cellulose was used to purify poly(A)-containing mRNPs from eggs and embryos of the sea urchin Strongylocentrotus purpuratus. Nonpolyribosomal mRNPs purified from eggs had a similar sedimentation in sucrose to unpurified mRNPs, a peak buoyant density in metrizamide of 1.22 g/cm3, and peak buoyant densities in Cs2SO4 in 1.42 g/cm3 after fixation with glutaraldehyde and 1.46 g/cm3 without fixation. Nonpolyribosomal mRNPs from eggs and zygotes contained 5-10 major proteins on sodium dodecylsulfate (SDS) polyacrylamide gels, and numerous minor bands. UV-irradiation of living eggs of the sea urchin Arbacia punctulata produced cross-linked mRNPs which contained a similar pattern of polypeptides to noncross-linked mRNPs. The polypeptides associated with embryonic polyribosomal mRNPs were also qualitatively similar to those present in nonpolyribosomal mRNPs, although stoichiometric differences may exist.