[Show abstract][Hide abstract] ABSTRACT: The pentadecapeptide gramicidin A, which is known to form highly conductive ion channels in a bilayer lipid membrane by assembling as transmembrane head-to-head dimers, can be modified by attaching a biotin group to its C-terminus through an aminocaproyl spacer. Such biotinylated gramicidin A analogues also form ion channels in a hydrophobic lipid bilayer, exposing the biotin group to the aqueous bathing solution. Interaction of the biotinylated gramicidin channels with (strept)avidin has previously been shown to result in the appearance of a long-lasting open state with a doubled transition amplitude in single-channel traces and a deceleration of the macroscopic current kinetics as studied by the sensitized photoinactivation method. Here this interaction was studied further by using streptavidin mutants with weakened biotin binding affinities. The Stv-F120 mutant, having a substantially reduced biotin binding affinity, exhibited an efficacy similar to that of natural streptavidin in inducing both double-conductance channel formation and deceleration of the photoinactivation kinetics of the biotinylated gramicidin having a long linker arm. The Stv-A23D27 mutant with a severely weakened biotin binding affinity was ineffective in eliciting the double-conductance channels, but decelerated noticeably the photoinactivation kinetics of the long linker biotinylated gramicidin. However, the marked difference in the effects of the mutant and natural streptavidins was smaller than expected on the basis of the substantially reduced biotin binding affinity of the Stv-A23D27 mutant. This may suggest direct interaction of this mutant streptavidin with a lipid membrane in the process of its binding to biotinylated gramicidin channels. The role of linker arm length in the interaction of biotinylated gramicidins with streptavidin was revealed in experiments with a short linker gramicidin. This gramicidin analogue appeared to be unable to form double-conductance channels, though several lines of evidence were indicative of its binding by streptavidin. The data obtained show the conditions under which the interaction of streptavidin with biotinylated gramicidin leads to the formation of the double-conductance tandem channels composed of two cross-linked transmembrane dimers.
[Show abstract][Hide abstract] ABSTRACT: A human plasma retinol-binding protein (RBP) mutant, named RBP-S, has been designed and produced in which the six native cysteine residues, involved in the formation of three disulfide bonds, have been replaced with serine. A hexa-histidine tag was also added to the C-terminus of RBP for ease of purification. The removal of the disulfide bonds led to a decrease in the affinity of RBP for all trans-retinol. Data indicates all-trans-retinol binds RBP and RBP-S with Kd = 4 x 10(-8) M and 1 x 10(-7) M, respectively, at approximately 20 degrees C. RBP-S has reduced stability as compared to natural RBP below pH 8.0 and at room temperature. Circular dichroism in the far-UV shows that there is a relaxation of the RBP structure upon the removal of its disulfide bonds. Circular dichroism in the near-UV shows that in the absence of the disulfide bonds, the optical activity of RBP is higher in the 310-330 nm than in the 280-290 nm range. This work suggests that the three native disulfide bonds aid in the folding of RBP but are not essential to produce a soluble, active protein.
Journal of Proteome Research 01/2003; 2(3):243-8. · 5.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The biotin-binding site of streptavidin was modified to alter its ligand-binding specificity. In natural streptavidin, the side chains of N23 and S27 make two of the three hydrogen bonds with the ureido oxygen of biotin. These two residues were mutated to severely weaken biotin binding while attempting to maintain the affinity for two biotin analogs, 2-iminobiotin and diaminobiotin. Redesigning of the biotin-binding site used the difference in local electrostatic charge distribution between biotin and these biotin analogs. Free energy calculations predicted that the introduction of a negative charge at the position of S27 plus the mutation N23A should disrupt two of the three hydrogen bonds between natural streptavidin and the ureido oxygen of biotin. In contrast, the imino hydrogen of 2-iminobiotin should form a hydrogen bond with the side chain of an acidic amino acid at position 27. This should reduce the biotin-binding affinity by approximately eight orders of magnitude, while leaving the affinities for these biotin analogs virtually unaffected. In good agreement with these predictions, a streptavidin mutant with the N23A and S27D substitutions binds 2-iminobiotin with an affinity (Ka) of 1 x 10(6) M-1, two orders of magnitude higher than that for biotin (1 x 10(4) M-1). In contrast, the binding affinity of this streptavidin mutant for diaminobiotin (2.7 x 10(4) M-1) was lower than predicted (2.9 x 10(5) M-1), suggesting the position of the diaminobiotin in the biotin-binding site was not accurately determined by modeling.
Proceedings of the National Academy of Sciences 12/1998; 95(23):13525-30. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Natural tetrameric streptavidin has two subunit interfaces; one is a strong interface between subunits in a tightly associated dimer, and the other is a weak interface between a pair of such dimers (dimer-dimer interface). To test whether strengthening the weak dimer-dimer interface could provide streptavidin with additional structural stability, covalent crosslinks were introduced between adjacent subunits through the dimer-dimer interface. Specific crosslinking sites were designed by site-directed mutations of His-127 residues that are in close proximity in natural streptavidin. The first and second streptavidin constructs have a disulfide bond and an irreversible covalent bond, respectively, between two Cys-127 residues across the dimer-dimer interface. The third variant is a hybrid tetramer consisting of two different streptavidin species, one having lysine and the other aspartic acid at position 127, which are covalently crosslinked. All streptavidin constructs with intersubunit crosslinks showed higher biotin-binding ability than natural core streptavidin after heat treatment. All of these crosslinked streptavidins retained bound biotin more stably than natural core streptavidin in guanidine hydrochloride at very acidic pH. These results suggest that the introduction of covalent bonds across the dimer-dimer interface enhances the overall stability of streptavidin.
[Show abstract][Hide abstract] ABSTRACT: A streptavidin mutant has been designed and produced that allows the specific, covalent immobilization of streptavidin on solid surfaces. This streptavidin mutant was constructed by fusing a six-residue sequence, containing a single cysteine, to the carboxyl terminus of streptavidin. Because this mutant has no other cysteine residues, the reactive sulfhydryl group of the cysteine residue serves as a unique immobilization site for conjugation using sulfhydryl chemistry. This streptavidin mutant was efficiently immobilized on maleimide-coated solid surfaces via its unique immobilization site. Characterization of the immobilized streptavidin mutant for the ability to bind to biotinylated macromolecules and the dissociation rates of bound biotin showed that the biotin-binding properties of this mutant were minimally affected by immobilization on solid surfaces. This streptavidin could be readily incorporated into a wide variety of solid-phase diagnostic tests and biomedical assays. This could enhance the performance of streptavidin-based solid-phase assay systems.
[Show abstract][Hide abstract] ABSTRACT: The present invention relates to streptavidin proteins and peptides having a altered physical properties such as an increased stability or increased or decreased affinity for binding biotin. The invention also relates to methods for the detection, identification, separation and isolation of targets using streptavidin proteins or peptides. Streptavidin with increased or reduced affinity allows for the use of the streptavidin-biotin coupling systems for detection and isolation systems wherein it is necessary to remove of one or the other of the binding partners. Such systems are useful for the purification of functional proteins and viable cells. The invention also relates to nucleic acids which encode these streptavidin proteins and peptides and to recombinant cells such as bacteria, yeast and mammalian cells which contain these nucleic acids.
[Show abstract][Hide abstract] ABSTRACT: Compounds and methods are described for producing streptavidin mutants with changed affinities. In particular, modifications to the sequence of the natural streptavidin gene is described to create amino acid substitutions resulting in greater affinity for biotin substitutes than for biotin.