Conformation of uteroglobin fragments

Coupe du Roi Bisection of Proteins. Spontaneous Tetramerization of Two Peptides That Span the Sequence of the Rabbit Uteroglobin Monomer

The study of dividing objects into isometric segments has yielded novel approaches to the synthesis of high-symmetry organic compounds. Reported herein is the first application of this concept to a protein, rabbit uteroglobin (UG). Bisection of UG into two identical homochiral segments led to the design of the heterodimeric 70mer peptide alpha(1,2)-S-S-alpha(3,4) that spans the sequence of the native UG monomer. The ability of this compound to form a globular 140mer tetramer consisting of two noncovalently bound heterodimers was assessed by ultracentrifugation at sedimentation equilibrium and by fluorescent spectroscopy. On the other hand, the monomeric peptides alpha(1,2)-SH and alpha(3,4)-SH were shown to selectively form the alpha(1,2)-S-S-alpha(3,4) heterodimer via spontaneous air oxidation in phosphate buffer at neutral pH.

The structural homology between uteroglobin and the pore-forming domain of colicin A suggests a possible mechanism of action for uteroglobin

Although the exact physiological function of uteroglobin is not known, it has been suggested that it may function by inhibiting phospholipase A2. We have found that the uteroglobin fold is embedded in that of the poreforming domain of colicin A. Colicin A is an antibiotic protein that kills sensitive Escherichia coli cells by forming a pore in their phospholipid membrane. The RMS deviation between the C alpha atoms after the structural alignment is 2.39 A for the 52 superimposed residues. In the alignment, uteroglobin helices 1, 2, 3, and 4 align with colicin A helices 6, 7, 3, and 4, respectively. The motif is strongly amphipathic in both proteins. On the basis of this common structural motif and of known experimental data on both proteins, we propose that UG binds to the membrane surface by lying on it monotopically. The phospholipase A2 inhibition would follow this initial binding step.

Conformation and molecular dynamics calculations on uteroglobin fragment 18-47

The conformational properties of fragment 18-47 of rabbit uteroglobin in aqueous solution containing SDS micelles were investigated by two-dimensional nmr spectroscopy and molecular dynamics calculations. The fragment comprises helices II and III and the beta-turn connecting the two helices. The nmr results and nmr-restrained molecular dynamics calculations showed that in the isolated fragment the elements of secondary structure present in the intact protein are preserved only in part. Specifically, a well-defined alpha-helix was found in the sequence 33-44, corresponding to helix III of uteroglobin, while the regions of helix II and beta-turn are characterized by high flexibility in the fragment.

Elucidating the folding problem of helical peptides using empirical parameters

Using an empirical analysis of experimental data we have estimated a set of energy contributions which accounts for the stability of isolated alpha-helices. With this database and an algorithm based on statistical mechanics, we describe the average helical behaviour in solution of 323 peptides and the helicity per residue of those peptides analyzed by nuclear magnetic resonance. Moreover the algorithm successfully detects the alpha-helical tendency, in solution, of a peptide corresponding to a beta-strand of ubiquitin.

Conformation and interactions of uteroglobin fragments 4-14 and 49-65 in aqueous solution containing surfactant micelles

The conformation of two fragments of rabbit uteroglobin is described. The peptides are PRFAHVIENLL and PQTTRENIMKLTEKIVK, corresponding to helices I and IV in the crystal structure. CD shows that both peptides interact with sodium dodecyl sulfate (SDS) micelles and change their conformation to an alpha-helix. The helical content estimated from the CD band at 222 nm is about 40% in each peptide. Surface tension measurements show that both peptides lower the critical micellar concentration (cmc) of SDS, with a more dramatic effect in the case of helix I. This peptide by itself acts as a surfactant, and is able to interact with SDS even below the observed cmc, forming beta aggregates. Proton magnetic resonance (1H-nmr) suggests that flexible helices are present. The longest helical stretches compatible with 1H-nmr data extend from Phe6 to Leu14 for helix I and from Arg53 to Ile63 for helix IV.