Purification and characterization of a non-S-RNase and S-RNases from styles of Japanese pear (Pyrus pyrifolia)

Apple S-RNase triggers inhibition of tRNA aminoacylation by interacting with a soluble inorganic pyrophosphatase in growing self-pollen tubes in vitro

Apple exhibits S-RNase-based self-incompatibility (SI), in which S-RNase plays a central role in rejecting self-pollen. It has been proposed that the arrest of pollen growth in SI of Solanaceae plants is a consequence of the degradation of pollen rRNA by S-RNase; however, the underlying mechanism in Rosaceae is still unclear. Here, we used S₂ -RNase as a bait to screen an apple pollen cDNA library and characterized an apple soluble inorganic pyrophosphatase (MdPPa) that physically interacted with S-RNases. When treated with self S-RNases, apple pollen tubes showed a marked growth inhibition, as well as a decrease in endogenous soluble pyrophosphatase activity and elevated levels of inorganic pyrophosphate (PPi). In addition, S-RNase was found to bind to two variable regions of MdPPa, resulting in a noncompetitive inhibition of its activity. Silencing of MdPPa expression led to a reduction in pollen tube growth. Interestingly, tRNA aminoacylation was inhibited in self S-RNase-treated or MdPPa-silenced pollen tubes, resulting in the accumulation of uncharged tRNA. Furthermore, we provide evidence showing that this disturbance of tRNA aminoacylation is independent of RNase activity. We propose an alternative mechanism differing from RNA degradation to explain the cytotoxicity of the S-RNase apple SI process.

Molecular and genetic characterization of a self-compatible apple cultivar, 'CAU-1'

In this study, we characterized a naturally occurring self-compatible apple cultivar, 'CAU-1' (S₁S₉), and studied the underlying mechanism that causes its compatibility. Analyses of both fruit set rate and seed number after self-pollination or cross-pollination with 'Fuji' (S₁S₉), and of pollen tube growth, demonstrated that 'CAU-1' is self-compatible. Genetic analysis by S-RNase PCR-typing of selfed progeny of 'CAU-1' revealed the presence of all progeny classes (S₁S₁, S₁S₉, and S₉S₉). Moreover, no evidence of S-allele duplication was found. These findings support the hypothesis that loss of function of an S-locus unlinked pollen-part mutation (PPM) expressed in pollen, rather than a natural mutation in the pollen-S gene (S₁- and S₉- haplotype), leads to SI breakdown in 'CAU-1'. In addition, there were no significant differences in pollen morphology or fertility between 'Fuji' and 'CAU-1'. However, we found that the effect of S₁- and S₉-RNase on the SI behavior of pollen could not be addressed better in 'CAU-1' than in 'Fuji'. Furthermore, we found that a pollen-expressed hexose transporter, MdHT1, interacted with S-RNases and showed significantly less expression in 'CAU-1' than in 'Fuji' pollen tubes. These findings support the hypothesis that MdHT1 may participate in S-RNase internalization during the SI process, and decrease of MdHT1 expression in 'CAU-1' hindered the release of self S-RNase into the cytoplasm of pollen tubes, thereby protecting pollen from the cytotoxicity of S-RNase, finally probably resulting in self-compatibility. Together, these findings indicate that S-locus external factors are required for gametophytic SI in the Rosaceae subtribe Pyrinae.

Identification and characterization of a novel chitinase with antifungal activity from 'Baozhu' pear (Pyrus ussuriensis Maxim.)

A novel chitinase from the 'Baozhu' pear was found, purified, and characterized in this report. This chitinase was a monomer with a molecular mass of 28.9 kDa. Results of the internal peptide sequence analyses classify this chitinase as a class III chitinase. In the enzymatic hydrolytic assay, this chitinase could hydrolyze chitin derivatives into di-N-acetylchitobiose (GlcNAc2) as a major product in the initial phase, as well as hydrolyze GlcNAc2 into N-acetylglucosamine (GlcNAc), which represents both chitobiosidase and β-N-acetylglucosaminase activity. Biological analyses showed that this chitinase exhibits strong antifungal activity toward agricultural pathogenic fungi. In total, chitinase from 'Baozhu' pear is a novel bifunctional chitinase that could be a potential fungicide in the biological control of plant diseases.

Apple MdABCF assists in the transportation of S-RNase into pollen tubes

Self-incompatibility (SI) is a reproductive isolation mechanism in flowering plants. Plants in the Solanaceae, Rosaceae and Plantaginaceae belong to the gametophytic self-incompatibility type. S-RNase, which is encoded by a female-specific gene located at the S locus, degrades RNA in the pollen tube and causes SI. Recent studies have provided evidence that S-RNase is transported non-selectively into the pollen tube, but have not specified how this transportation is accomplished. We show here that the apple (Malus domestica) MdABCF protein, which belongs to group F of the ABC transporter family, assists in transportation of S-RNase into the pollen tube. MdABCF is located in the pollen tube membrane and interacts with S-RNase. S-RNase was unable to enter the pollen tube when MdABCF was silenced by antisense oligonucleotide transfection. Our results show that MdABCF assists in transportation of either self or non-self S-RNase into the pollen tube. Moreover, MdABCF coordinates with the cytoskeleton to transport S-RNase. Blockage of S-RNase transport disrupts self-incompatibility in this system.

The microtubule cytoskeleton and pollen tube Golgi vesicle system are required for in vitro S-RNase internalization and gametic self-incompatibility in apple

S-RNase is the female determinant of gametophytic self-incompatibility in apple and is usually considered to be the reason for rejection of pollen. In this study, we investigated the role of microtubules (MTs) in internalization of S-RNases by pollen tubes cultured in vitro. The results showed that S-RNase was imported into the pollen tube where it inhibits pollen tube growth, and that S-RNase is co-localized with the Golgi vesicle during the internalization process. Moreover, MT depolymerization is observed following accumulation of S-RNases in the pollen cytosol. On the other hand, S-RNase was prevented from entering the pollen tube when the pollen was treated with the actin filament (AF) inhibitor latrunculin A (LatA), the MT inhibitor oryzalin, or the MT stabilizer taxol at subtoxic concentrations. These hindered the construction of the MT, with pollen tubes capable of growth under these conditions. Pollen tubes showed improved growth in self-pollinated styles that were pre-treated with taxol. This suggests that cytoskeleton antagonists can prevent S-RNase-mediated inhibition of pollen tubes in vivo by blocking S-RNase internalization. These results suggest that an intact and dynamic cytoskeleton is required for the in vitro internalization of S-RNase, as shown by the effects of various cytoskeleton inhibitors. S-RNase internalization takes place via a membrane/cytoskeleton-based Golgi vesicle system, which can also affect self-incompatibility in apple.

A novel ribonuclease with HIV-1 reverse transcriptase inhibitory activity from the edible mushroom Hygrophorus russula

A 28-kDa ribonuclease, with an optimum pH of 4.0 and an optimum temperature at 58 °C, was isolated from fruiting bodies of the edible mushroom Hygrophorus russula. It was purified by ion exchange chromatography on carboxymethyl-cellulose, dithyaminoethyl-cellulose, quaternary amine-sepharose and sulphopropyl-sepharose, followed by fast protein liquid chromatography gel filtration on Superdex 75. The N-terminal amino acid sequence was ASAGG which showed homology to those of other fungal RNases to some degree. It exerted the highest RNase activity on poly C and poly U. The Michaelis constant (K(m)) value of the RNase on yeast tRNA was 3.6 μM, and the maximal velocity (V(max)) was 0.6 μM. The RNase activity was suppressed by some ions including Fe(2+) and Zn(2+) ions. The RNase inhibited the activity of HIV-1 reverse transcriptase with an IC(50) of 4.64 μM.

Ribonucleases of different origins with a wide spectrum of medicinal applications

Ribonucleases (RNases) are a type of nucleases that catalyze the degradation of RNA into smaller components. They exist in a wide range of life forms from prokaryotes to eukaryotes. RNase-controlled RNA degradation is a determining factor in the control of gene expression, maturation and turnover, which are further associated with the progression of cancers and infectious diseases. Over the years, RNases purified from multiple origins have drawn increasing attention from medical scientists due to their remarkable antitumor properties. In this review, we present a brief summary of the representative RNases of fungal, bacterial, plant, and animal origins and outline their potential medicinal value in the treatment of tumor and AIDS. Among them, the most clinically promising RNases are mushroom RNases, Binase and Barnase from bacteria, ginseng RNases, and Onconase from frog (Rana pipiens). Fast developing protein engineering of RNases, which display more potent cytotoxic activity on and greater selectivity for malignant cells, has also aroused the interest of researchers. The multiple anti-cancer mechanisms of RNases are also included. To sum up, these inspiring studies unveil a new perspective for RNases as potential therapeutic agents.

Petunia nectar proteins have ribonuclease activity

Plants requiring an insect pollinator often produce nectar as a reward for the pollinator's visitations. This rich secretion needs mechanisms to inhibit microbial growth. In Nicotiana spp. nectar, anti-microbial activity is due to the production of hydrogen peroxide. In a close relative, Petunia hybrida, limited production of hydrogen peroxide was found; yet petunia nectar still has anti-bacterial properties, suggesting that a different mechanism may exist for this inhibition. The nectar proteins of petunia plants were compared with those of ornamental tobacco and significant differences were found in protein profiles and function between these two closely related species. Among those proteins, RNase activities unique to petunia nectar were identified. The genes corresponding to four RNase T2 proteins from Petunia hybrida that show unique expression patterns in different plant tissues were cloned. Two of these enzymes, RNase Phy3 and RNase Phy4 are unique among the T2 family and contain characteristics similar to both S- and S-like RNases. Analysis of amino acid patterns suggest that these proteins are an intermediate between S- and S-like RNases, and support the hypothesis that S-RNases evolved from defence RNases expressed in floral parts. This is the first report of RNase activities in nectar.