Identification of a specific tyrosine residue in Bryodin 1 distinct from the active site but required for full catalytic and cytotoxic activity

Abstract

Bryodin 1 (BDl) is a type I ribosome-inactivating protein (RIP) with low inherent animal toxicity. It has been cloned recently and the recombinant protein (rBD1) has been produced and crystallized. To gain insight into the relationship of rBDl structure and function, we investigated the role of sequences in a region (residues 128-156) that exhibits homology with membrane interactive sequences and is not part of the enzymatically defined active site. Progressive deletions representing a-helical turns within these residues were generated; mutant rBDl proteins were expressed in Escherichia coli and demonstrated increasing losses of enzymatic activity. Point mutations were also generated within this region to replace Y140, Y141, and Y142 with either alanine or lysine. Mutants at position 140 or 142 retained full enzymatic activity, whereas A141 and K141 mutants were > 19-fold less potent. In cytotoxicity assays, the rBDl point mutants at Y141 were >80-fold less potent than either rBDl or mutants at residues 140 or 142. However, when introduced into the anti-CD40 single-chain immunotoxin rBDIG28-5 sFv, the A140 and A141 point mutations led to decreased cytotoxicity toward CD40 positive cell lines. These data indicate that Y141 plays an important role in the enzymatic activity of BDI and that Y140, although not essential for catalytic activity, is required for full BDI function. Because residues 140 and 141 are distinct from residues implicit in the active site, they may be involved in ribosomal and/or membrane interactions or in intracellular trafficking of the toxin and immunotoxin.Keywords: bryodin; immunotoxin; membrane interaction; mutagenesis; ribosome-inactivating protein Bryodin-1 (BDI), a type 1 RIP isolated from Bryonia dioica (Stirpe et al., 1986), may have a therapeutic advantage over other RIPs due to its lower toxicity in vivo (Siegall et al., 1994). Type 1 RIPs have a common N-glycosidase activity, a basic isoelectric point, and molecular mass of 26-30 kDa (Barbieri et al., 1993). Type I RIPs from different plants show high sequence and structural homology with each other as well as with the A chain of the type 2 RIPs ricin and abrin. Alignment of sequences from BDl and other RIPs indicates five regions of highest homology, several of which are clustered around the active site cleft (Shaw et al., 1992). The role of sequences in other regions has not yet been elucidated.In order to better understand the function of BDl sequences in one of the undescribed regions that was also interesting as a potential mediator of membrane interactions, a mutational analysis was performed within BDI residues 128-156. This region is on the solvent surface of BDl according to the X-ray crystal structure (H. Klei & H. Einspahr, pers. comm.) and is not thought to be part of the active site of RIPs (Katzin et al., 1991 arated by a turn containing three consecutive tyrosines. One, two, or all three of these tyrosines are conserved among many RIPs.To investigate the contribution of this domain to rBDl activity, three pr...

Invariant Ser211 is involved in the catalysis of PD‐L4, type I RIP from Phytolacca dioica leaves

“…The mechanism of action of the enzyme is fully elucidated and residues involved in the catalytic reaction have been described 50, 51. Other highly conserved residues in close proximity to the active site cleft have already been reported, as has also the structural implication of noncatalytic cleft amino acids in RIPs catalysis and inhibition of protein synthesis 52–55…”

Ribosome-Inactivating Proteins of Bougainvillea glabra Uncovered Polymorphism and Active Site Divergence

“…BI6 was found to be missing the active residues of tryptophan (i.e., Q8’s Trp198), while the other isoforms retained the five highly conserved active amino acid residues of the RIP family in Bougainvillea . The common characteristic of these auxiliary sites is the ability to stabilize the active site catalysis of adenine and provide good conditions for reaction with substrates by interacting with the phosphate groups of the RNA backbone [ 39 , 40 , 41 , 42 ]. After the protonation of adenine, the conserved active amino acid Tyr70 side chain was rotated, and adenine was embedded in the active region, facilitating the next adenosine removal reaction.…”

Reliable sequence determination of ribosome- inactivating proteins by combining electrospray mass spectrometry and Edman degradation

“…Secondary structure prediction studies, performed on the saporin‐S9 sequence (data not reported), have shown that some of the amino acid substitutions, especially H248, may affect the secondary structure. Although the influence of single‐site mutation on the conversion of a β‐strand to an α‐helix has been observed in the crystal structure of the 98 amino acid Escherichia coli protein Fis,28 further studies are needed on saporin‐S9 to confirm the structural changes and correlate them with the differences in biological activities. In this regard, the possibility offered by the MS approach in the rapid acquisition of sequence data by homology comparison with known proteins allows easy gathering of the primary structure for medium‐ and high‐mass proteins.…”