Metal-induced changes in the fluorescence properties of tyrosine and tryptophan site-specific mutants of oncomodulin

Hydrophobic ligand binding properties of the human lipocalin apolipoprotein M

Apolipoprotein M (apoM) is a plasma protein associated mainly with HDL. ApoM is suggested to be important for the formation of prebeta-HDL, but its mechanism of action is unknown. Homology modeling has suggested apoM to be a lipocalin. Lipocalins share a structurally conserved beta-barrel, which in many lipocalins bind hydrophobic ligands. The aim of this study was to test the ability of apoM to bind different hydrophobic substances. ApoM was produced both in Escherichia coli and in HEK 293 cells. Characterization of both variants with electrophoretic and immunological methods suggested apoM from E. coli to be correctly folded. Intrinsic tryptophan fluorescence of both apoM variants revealed that retinol, all-trans-retinoic acid, and 9-cis-retinoic acid bound (dissociation constant = 2-3 microM), whereas other tested substances (e.g., cholesterol, vitamin K, and arachidonic acid) did not. The intrinsic fluorescence of two apoM mutants carrying single tryptophans was quenched by retinol and retinoic acid to the same extent as wild-type apoM, indicating that the environment of both tryptophans was affected by the binding. In conclusion, the binding of retinol and retinoic acid supports the hypothesis that apoM is a lipocalin. The physiological relevance of this binding has yet to be elucidated.

Effects of metal binding affinity on the chemical and thermal stability of site-directed mutants of rat oncomodulin

Tryptophan fluorescence was used to study the stability and unfolding behavior of several single tryptophan mutants of the metal-binding protein rat oncomodulin (OM); F102W, Y57W, Y65W and the engineered protein CDOM33 which had the 12 residues of the CD loop replaced with a more potent metal binding site. Both the thermal and the chemical stability were improved upon binding of metal ions with the order apo < Ca2+ < Tb3+. During thermal denaturation, the transition midpoints (T(un)) of Y65W was the lowest, followed by Y57W and F102W. The placement of the Trp residue in the F-helix in F102W made the protein slightly more thermostable, although the fluorescence response was readily affected by chemical denaturants, which acted through the disruption of hydrogen bonds at the C-terminal end of the F-helix. Under both thermal and chemical denaturation, the engineered protein showed the highest stability. This indicated that increasing the number of metal ligating oxygens in the binding site, either by using a metal ion with a higher coordinate number (i.e., Tb3+) which binds more carboxylate ligands, or by providing more ligating groups, as in the CDOM33 replacement, produces notable improvements in protein stability.

Structural features of phi29 single-stranded DNA-binding protein. I. Environment of tyrosines in terms of complex formation with DNA

The single-stranded DNA-binding protein (SSB) of Bacillus subtilis phage phi29 is absolutely required for viral DNA replication in vivo. About approximately 95% of the intrinsic tyrosine fluorescence of phi29 SSB is quenched upon binding to ssDNA, making tyrosine residues strong candidates to be directly involved in complex formation with ssDNA. Thus, we have studied the spectroscopic properties of the phi29 SSB tyrosines (Tyr-50, Tyr-57, and Tyr-76) using steady-state and time-resolved fluorescence measurements. phi29 SSB tyrosines do not seem to be highly restricted by strong interactions with neighbor residues, as suggested by (i) the high value of the average quantum yield of the phi29 SSB fluorescence emission (phiF = 0.067 +/- 0.010), (ii) the fast motions of the tyrosine side chains (phi(short) = 0.14 +/- 0.06 ns), and (iii) the lack of tyrosinate emission at neutral pH. Stern-Volmer analysis of the quenching by acrylamide and I- indicates that phi29 SSB tyrosines are surrounded by a negatively charged environment and located in a relatively exposed protein domain, accessible to the solvent and, likely, to ssDNA. Changes in the intrinsic fluorescence upon ssDNA binding allowed us to determine that temperature has an opposite effect on the thermodynamic parameters K (intrinsic binding constant) and omega (cooperativity) defining phi29 SSB-poly(dT) interaction, the effective DNA binding constant, K(eff) = K omega, being largely independent of temperature. Altogether, the fluorescent properties of phi29 SSB tyrosines are consistent with a direct participation in complex formation with ssDNA.

A protease assay using time-resolved lanthanide luminescence from an engineered calcium binding protein substrate

OBJECTIVES

The objective of this work was to demonstrate the utility of luminescence from lanthanides bound to a mutant of the Ca2+ binding protein, oncomodulin, to monitor protease activity.

DESIGN AND METHODS

A mutant of oncomodulin with a cysteine residue at position 57 located in the CD binding loop was conjugated to a salicylic acid group. The luminescence of Tb3+ resulting from electronic energy transfer from the salicylic acid group was monitored using time resolved lanthanide luminescence in the presence of proteolytic enzymes.

RESULTS

Low detection limits for subtilisin (150 pg), chymotrypsin (2.5 ng), cathepsin B (3.5 ng), and HIV-1 protease (25 ng) were found.

CONCLUSION

The simplicity of the assay coupled with its high level of sensitivity make it useful for the detection of protease at very low concentrations.

Human alpha and beta parvalbumins. Structure and tissue-specific expression

alpha and beta parvalbumins are Ca(2+)-binding proteins of the EF-hand type. We determined the protein sequence of human brain alpha parvalbumin by mass spectrometry and cloned human beta parvalbumin (or oncomodulin) from genomic DNA and preterm placental cDNA. beta parvalbumin differs in 54 positions from alpha parvalbumin and lacks the C-terminal amino acid 109. From MS analyses of alpha and beta parvalbumins we conclude that parvalbumins generally lack posttranslational modifications. alpha and beta parvalbumins were differently expressed in human tissues when analyzed by immunoblotting and polymerase-chain-reaction techniques. Whereas alpha parvalbumin was found in a number of adult human tissues, beta parvalbumin was restricted to preterm placenta. The pattern of alpha parvalbumin expression also differs in man compared to other vertebrates. For example, in rat, alpha parvalbumin was found in extrafusal and intrafusal skeletal-muscle fibres whereas, in man, alpha parvalbumin was restricted to the muscle spindles. Different functions for alpha and beta parvalbumins are discussed.

A new intrinsic fluorescent probe for proteins. Biosynthetic incorporation of 5-hydroxytryptophan into oncomodulin

The tryptophan analog, 5-hydroxytryptophan (5HW), has a significant absorbance between 310-320 nm, which allows it to act as an exclusive fluorescence probe in protein mixtures containing a large number of tryptophan residues. Here for the first time a method is reported for the biosynthetic incorporation of 5HW into an expressed protein, the Y57W mutant of the Ca2+ binding protein, oncomodulin. Fluorescence anisotropy and time-resolved fluorescence decay measurements of the interaction between anti-oncomodulin antibodies and the 5HW-incorporated oncomodulin conveniently provide evidence of complex formation and epitope identification that could not be obtained with the natural amino acid. This report demonstrates the significant potential for the use of 5HW as an intrinsic probe in the study of structure and dynamics of protein-protein interactions.