Barnase and barstar are trivial names of the extracellular RNase and its intracellular inhibitor produced by Bacillus amyloliquefaciens. Inhibition involves the formation of a very tight one-to-one complex of the two proteins. With the crystallographic solution of the structure of the barnasebarstar complex and the development of methods for measuring the free energy of binding, the pair can be used to study protein-protein recognition in detail. In this report, we describe the isolation of suppressor mutations in barstar that compensate for the loss in interaction energy caused by a mutation in barnase. Our suppressor search is based on in vivo selection for barstar variants that are able to protect host cells against the RNase activity of those barnase mutants not properly inhibited by wild-type barstar. This approach utilizes a plasmid system in which barnase expression is tightly controlled to keep the mutant barnase gene silent. When expression of barnase is turned on, failure to form a complex between the mutant barnase and barstar has a lethal effect on host cells unless overcome by substitution of the wild-type barstar by a functional suppressor derivative. A set of barstar suppressors has been identified for barnase mutants with substitutions in two amino acid positions (residues 102 and 59), which are critically involved in both RNase activity and barstar binding. The mutations selected as suppressors could not have been predicted on the basis of the known protein structures. The single barstar mutation with the highest information content for inhibition of barnase (H102K) has the substitution Y30W. The reduction in binding caused by the R59E mutation in barnase can be partly reversed by changing Glu-76 of barstar, which forms a salt bridge with the Arg-59 in the wild-type complex, to arginine, thus completing an interchange of the two charges.Much of biological existence is determined by unique interactions at the molecular level. A vast array of molecular recognitions are specific protein-protein interactions between enzymes and their substrates, enzymes and their inhibitors, the assembly of identical or different subunits into larger structures, etc. What is the nature of the interactions involved in protein-protein recognition? Which forces stabilize protein complexes? How plastic are the interfaces? One approach to this problem is to determine which changes in one partner will compensate for weakened binding caused by mutation in the other.We have developed a system to do just this, using in vivo selection to find such complementing mutations. The gene for barnase, the extracellular RNase of Bacillus amyloliquefaciens, is lethal when expressed in Escherichia coli without concurrent expression of its intracellular polypeptide inhibitor barstar. The genes for both proteins have been cloned in E. coli plasmids and both can be produced in quantity as plasmidThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertis...
The adenosine diphosphate (ADP)-ribosyltransferase, Vip2 (vegetative insecticidal protein), from Bacillus cereus in combination with another protein from the same organism, Vip1, has insecticidal activity against western corn rootworm larvae. The Vip2 protein exerts its intracellular poisoning effect by modifying actin and preventing actin polymerization. Due to the nature of this toxin, expression of Vip2 in planta is lethal. In this work, we attempted to build an enzyme precursor (proenzyme, zymogen) that would silently reside in one biological system (e.g. plants or yeast) and be activated in the other (insect larvae). Our approach involved engineering a random propeptide library at the C-terminal end of Vip2 and selecting for malfunctional enzyme variants in yeast. A selected proenzyme (proVip2) possesses reduced enzymatic activity as compared with the wild-type Vip2 protein, but remains a potent toxin toward rootworm larvae. In addition, upon analysis of the digestive fate of the engineered enzyme precursor in rootworm larvae, we demonstrated that 'zymogenized' Vip2 can be proteolytically activated by rootworm digestive enzyme machinery. This report represents an example of applying a protein engineering strategy for the creation of a plant-tolerated, zymogen-like form of an otherwise toxic protein. This approach may outline a novel path to address challenges associated with utilizing toxic proteins in certain biotechnological applications.
We report the design of a new tightly controlled barnase system which allows the existence of the barnase gene in host cells without a signal sequence. When expression of barnase is turned on by gene inversion in vivo, the lethal effect of barnase (or its mutants) is not compromised either by coexpression of its polypeptide inhibitor (barstar), or by extracellular secretion. This serves as a rapid, sensitive in vivo test for the detection of any very low residual activity of the barnase mutants. Active-site mutants His102Lys, Glu73Asp and Arg87Lys, and a mutant which greatly reduces the stability and yield of protein, Arg83Lys, produce enough activity to be detectable by this test. In contrast, when expressed on a secretion vector, these mutants do not yield detectable activity in a solution assay. Truly inactive mutants, such as those of His102 to Gly, Ala or Leu, were completely harmless when expressed in this system.
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