MULTIPLE NATIVE-LIKE CONFORMATIONS TRAPPED VIA SELF-ASSOCIATION-INDUCED HYDROPHOBIC COLLAPSE OF THE 33-RESIDUE BETA-SHEET DOMAIN FROM PLATELET FACTOR-4

Native platelet factor 4 (PF4) (70 residues) has a hydrophobic three-s tranded anti-parallel beta-sheet domain on to which is folded an amphi pathic C-terminal alpha-helix and an aperiodic N-terminal domain. The 33-amino acid beta-sheet domain from PF4 (residues 23-55) has been syn thesized and studied by c.d. and n.m.r. At 10 DC and low concentration , peptide 23-55 appears to exist in aqueous solution in a random-coil distribution of highly flexible conformational states. Some preferred conformation, however, is observed, particularly within a relatively s table chain reversal from Leu-45 to Arg-49. As the peptide concentrati on and/or temperature is increased, a new conformational state(s) appe ars and intensifies as slowly exchanging (600 MHz H-1-n.m.r. chemical- shift time scale) random-coil resonances disappear. Hill plots of the concentration-dependence indicated mostly tetramer formation as found in native PF4. Although apparent resonance linewidths in aggregate sta te(s) are of the order of 100 Hz, sequence-specific assignments for mo st resonances could be made. N.m.r./nuclear Overhauser effect structur al analysis indicates the formation of multiple native-like anti-paral lel beta-sheet conformations, kinetically trapped via subunit-associat ion-induced hydrophobic collapse and stabilized by low-dielectric elec trostatic interactions among/between Gly-28 and Lys-50 in opposing sub units. Results are discussed in terms of protein folding.