Expression in mammalian cells, purification and characterization of recombinant human pancreatic ribonuclease

Consequences of the Endogenous N-Glycosylation of Human Ribonuclease 1

Ribonuclease 1 (RNase 1) is the most prevalent human homologue of the archetypal enzyme RNase A. RNase 1 contains sequons for N-linked glycosylation at Asn34, Asn76, and Asn88 and is N-glycosylated at all three sites in vivo. The effect of N-glycosylation on the structure and function of RNase 1 is unknown. By using an engineered strain of the yeast Pichia pastoris, we installed a heptasaccharide (Man₅GlcNAc₂) on the side chain of Asn34, Asn76, and Asn88 to produce the authentic triglycosylated form of human RNase 1. As a glutamine residue is not a substrate for cellular oligosaccharyltransferase, we used strategic asparagine-to-glutamine substitutions to produce the three diglycosylated and three monoglycosylated forms of RNase 1. We found that the N-glycosylation of RNase 1 at any position attenuates its catalytic activity but enhances both its thermostability and its resistance to proteolysis. N-Glycosylation at Asn34 generates the most active and stable glycoforms, in accord with its sequon being highly conserved among vertebrate species. These data provide new insight on the biological role of the N-glycosylation of a human secretory enzyme.

The solution structure and dynamics of human pancreatic ribonuclease determined by NMR spectroscopy provide insight into its remarkable biological activities and inhibition

Human pancreatic ribonuclease (RNase 1) is expressed in many tissues; has several important enzymatic and biological activities, including efficient cleavage of single-stranded RNA, double-stranded RNA and double-stranded RNA-DNA hybrids, digestion of dietary RNA, regulation of vascular homeostasis, inactivation of the HIV, activation of immature dendritic cells and induction of cytokine production; and furthermore shows potential as an anti-tumor agent. The solution structure and dynamics of uncomplexed, wild-type RNase 1 have been determined by NMR spectroscopy methods to better understand these activities. The family of 20 structures determined on the basis of 6115 unambiguous nuclear Overhauser enhancements is well resolved (pairwise backbone RMSD=1.07 A) and has the classic RNase A type of tertiary structure. Important structural differences compared with previously determined crystal structures of RNase 1 variants or inhibitor-bound complexes are observed in the conformation of loop regions and side chains implicated in the enzymatic as well as biological activities and binding to the cytoplasmic RNase inhibitor. Multiple side chain conformations observed for key surface residues are proposed to be crucial for membrane binding as well as translocation and efficient RNA hydrolysis. (15)N-(1)H relaxation measurements interpreted with the standard and our extended Lipari-Szabo formalism reveal rigid regions and identify more dynamic loop regions. Some of the most dynamic areas are key for binding to the cytoplasmic RNase inhibitor. This finding and the important differences observed between the structure in solution and that bound to the inhibitor are indications that RNase 1 to inhibitor binding can be better described by the "induced fit" model rather than the rigid "lock-into-key" mechanism. Translational diffusion measurements reveal that RNase 1 is predominantly dimeric above 1 mM concentration; the possible implications of this dimeric state for the remarkable biological properties of RNase 1 are discussed.

Intracellular route and mechanism of action of ERB-hRNase, a human anti-ErbB2 anticancer immunoagent

Human anti-ErbB2 immunoRNase with Erbicin fused to HP-RNase (ERB-hRNase) is a fully human immunoRNase made up of human pancreatic RNase fused to a human anti-ErbB2 scFv. It binds selectively with high affinity to ErbB2-positive cells, and specifically inhibits their proliferation, in vitro and in vivo. An investigation of its mechanism of action and its intracellular destination has revealed that ERB-hRNase depends on its RNase activity for cytotoxic action; it reaches the cytosol directly from the endosomal compartment; it is inhibited by the cytosolic RNase inhibitor (cRI), but the levels that ERB-hRNase reaches in the cytosol neutralize cRI, thus inducing cell death by apoptosis.

Cytosolic RNase inhibitor only affects RNases with intrinsic cytotoxicity

Cytosolic RNase inhibitor binds to and neutralizes most members of the pancreatic type RNase superfamily. However, there are a few exceptions, e.g. amphibian onconase and bovine seminal RNase, and these are endowed with cytotoxic activity. Also, RNase variants created by mutagenesis to partially evade the RNase inhibitor acquire cytotoxic activity. These findings have led to the proposal that the cytosolic inhibitor acts as a sentry to protect mammalian cells from foreign RNases. We silenced the expression of the gene encoding the cytosolic inhibitor in HeLa cells and found that the cells become more sensitive to foreign cytotoxic RNases. However foreign, non-cytotoxic RNases remain non-cytotoxic. These results indicate that the cytosolic inhibitor neutralizes those foreign RNases that are intrinsically cytotoxic and have access to the cytosol. However, its normal physiological role may not be to guard against foreign RNases in general.

A Fully Human Antitumor ImmunoRNase Selective for ErbB-2-Positive Carcinomas

We report the preparation and characterization of a novel, fully human antitumor immunoRNase (IR). The IR, a human RNase and fusion protein made up of a human single chain variable fragment (scFv), is directed to the ErbB-2 receptor and overexpressed in many carcinomas. The anti-ErbB-2 IR, named hERB-hRNase, retains the enzymatic activity of the wild-type enzyme (human pancreatic RNase) and specifically binds to ErbB-2-positive cells with the high affinity (K(d) = 4.5 nm) of the parental scFv. hERB-hRNase behaves as an immunoprotoxin and on internalization by target cells becomes selectively cytotoxic in a dose-dependent manner at nanomolar concentrations. Administered in five doses of 1.5 mg/kg to mice bearing an ErbB-2-positive tumor, hERB-hRNase induced a dramatic reduction in tumor volume. hERB-hRNase is the first fully human antitumor IR produced thus far, with a high potential as a poorly immunogenic human drug devoid of nonspecific toxicity, directed against ErbB-2-positive malignancies.

Degradation of double-stranded RNA by human pancreatic ribonuclease: crucial role of noncatalytic basic amino acid residues

Under physiological salt conditions double-stranded (ds) RNA is resistant to the action of most mammalian extracellular ribonucleases (RNases). However, some pancreatic-type RNases are able to degrade dsRNA under conditions in which the activity of bovine RNase A, the prototype of the RNase superfamily, is essentially undetectable. Human pancreatic ribonuclease (HP-RNase) is the most powerful enzyme to degrade dsRNA within the tetrapod RNase superfamily, being 500-fold more active than the orthologous bovine enzyme on this substrate. HP-RNase has basic amino acids at positions where RNase A shows instead neutral residues. We found by modeling that some of these basic charges are located on the periphery of the substrate binding site. To verify the role of these residues in the cleavage of dsRNA, we prepared four variants of HP-RNase: R4A, G38D, K102A, and the triple mutant R4A/G38D/K102A. The overall structure and active site conformation of the variants were not significantly affected by the amino acid substitutions, as deduced from CD spectra and activity on single-stranded RNA substrates. The kinetic parameters of the mutants with double-helical poly(A).poly(U) as a substrate were determined, as well as their helix-destabilizing action on a synthetic DNA substrate. The results obtained indicate that the potent activity of HP-RNase on dsRNA is related to the presence of noncatalytic basic residues which cooperatively contribute to the binding and destabilization of the double-helical RNA molecule. These data and the wide distribution of the enzyme in different organs and body fluids suggest that HP-RNase has evolved to perform both digestive and nondigestive physiological functions.

J-modulated TROSY experiment extends the limits of homonuclear coupling measurements for larger proteins

This paper describes the use of a TROSY experimental scheme and its variant extended with a scaled J-modulation spin-echo sequence for accurate and sensitive measurement of homonuclear 3J(H(N)H(alpha)) coupling constants in larger proteins with uniform 15N labeling. Exclusive selection of the most slowly relaxing component of a 15N-1H multiplet by the TROSY approach leads to substantial improvement in resolution; this is a prerequisite for accurate measurement of couplings from the 1H multiplets directly along the 1H frequency dimension or from the J-scaled doublets along the 15N frequency dimension.

In vitro evolution of a dimeric variant of human pancreatic ribonuclease

Site-directed mutagenesis of human pancreatic RNase (HP-RNase) was used as a model system for investigating the genetic events underlying the evolutionary origins of protein oligomers. HP-RNase is a monomeric enzyme with no natural tendency to oligomerize (K(d) for any dimers in solution of >280 mM). Nevertheless, deletion of five amino acid residues in the loop linking the N-terminal helix of HP-RNase to the rest of the protein was found to drive polypeptide chains to fold into dimers. These dimers could not be dissociated by heating at 70 degrees C, and small amounts of monomer were detected only in highly diluted samples. Measurement of dimer and monomer concentrations under equilibrium conditions yielded a K(d) of 1.5 microM. This implies that the deletion increases the protein propensity to dimerize at least 5.2 orders of magnitude. Moreover, the HP-RNase dimers were found to be over 4.6 orders of magnitude more stable than the dimers of bovine pancreatic RNase A obtained by lyophilization from acetic acid (K(d) > 73 mM). Cross-linking experiments with divinyl sulfone indicated that the HP-RNase dimers are stabilized by the exchange between subunits of their N-terminal helices. This generates composite active sites, i.e., each contributed by two subunit chains, that retain full enzymatic activity. Overall, these results show that a deletion of few residues in a key region of a monomeric protein can be the primary event irreversibly leading to oligomerization of the protein through the swap of a secondary structure element between protomers.

Production of engineered human pancreatic ribonucleases, solving expression and purification problems, and enhancing thermostability

Human pancreatic ribonuclease, the homolog of bovine pancreatic ribonuclease, has a significant therapeutic potential. Its study has been hindered by the difficulty of obtaining the enzyme in a pure and homogeneous form, either from human source or using heterologous expression. Engineering of different variants of human pancreatic ribonuclease has allowed us to study and overcome some problems encountered during its heterologous production in an Escherichia coli system and its purification from inclusion bodies. The 5'-end region of the mRNA that encodes the enzyme is critical for obtaining high expression levels. The results also suggest the importance of the proline 50 residue in the recovery yields of human pancreatic ribonuclease. All the variants produced are pure and homogeneous. Their homogeneity has been demonstrated by cation-exchange and reversed-phase chromatography and by mass spectrometry analysis. Moreover, enhancement of human pancreatic ribonuclease thermal stability is observed when residues R4, K6, Q9, D16, and S17 are changed to the corresponding residues of bovine seminal ribonuclease.

A dimeric mutant of human pancreatic ribonuclease with selective cytotoxicity toward malignant cells

Monomeric human pancreatic RNase, devoid of any biological activity other than its RNA degrading ability, was engineered into a dimeric protein with a cytotoxic action on mouse and human tumor cells, but lacking any appreciable toxicity on mouse and human normal cells. This dimeric variant of human pancreas RNase selectively sensitizes to apoptotic death cells derived from a human thyroid tumor. Because of its selectivity for tumor cells, and because of its human origin, this protein represents a potentially very attractive, novel tool for anticancer therapy.

Human pancreatic ribonuclease--deletion of the carboxyl-terminal EDST extension enhances ribonuclease activity and thermostability

Mammalian ribonucleases constitute one of the fastest evolving protein families in nature. The addition of a four-residue carboxyl-terminal tail: Glu-Asp-Ser-Thr (EDST) in human pancreatic ribonuclease (HPR) in comparison with bovine pancreatic RNase (RNase A) could have adaptive significance in humans. We have cloned and expressed human pancreatic ribonuclease in Escherichia coli to probe the influence of the four-residue extension and neighboring C-terminal residues on the biochemical properties of the enzyme. Removal of the C-terminal extension from HPR yielded an enzyme, HPR-(1-124)-peptide, with enhanced ability to cleave poly(C). HPR-(1-124)-peptide also exhibited a steep increase in thermal stability mimicking that known for RNase A. Wild-type HPR had significantly low thermal stability compared to RNase A. The study identifies the C-terminal boundary in the human pancreatic ribonuclease required for efficient catalysis.

Nucleotide sequence encoding human pancreatic ribonuclease

A cDNA coding for human pancreatic ribonuclease was isolated from a pancreas cDNA library and sequenced. This cDNA (1620 bp) includes an entire open reading frame encoding mature protein (128 aa) following a signal peptide (28 aa) as well as 5'- and 3'-untranslated regions.