CONSTRAINING BETA-TURN STRUCTURE IN MODEL IMMUNOGENS

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Globular protein molecules are largely composed of beta-sheet and alpha- helical secondary structures separated by sharp changes in chain trajectory at four residue beta-turns or larger loops on the surface of the molecule. These betaturn and loop structures can often be recognized in protein sequences, aiding in secondary structure prediction and in the identification of potential linear peptide epitopes for antibody production. While antipeptide antibodies which recognize a full protein chain have been prepared in a number of cases, the resulting monoclonal antibodies often have a low affinity for the folded protein from which the synthetic peptide immunogen sequence was derived. A protocol was proposed to use a second protein (staphylococcal nuclease) as a "host" to constrain the "guest" peptide immunogen into a native structure by incorporating that sequence into a hybrid protein at an appropriate site. Work will continue on two hybrid protein systems to develop this method and to understand its success or failure in structural and physical terms. Results during the last three years indicate that there is a strong relationship between the sequence and type of a beta-turn which can be dominant over globular protein context effects. Investigations will continue to define the relationship between amino acid sequence and beta-turn type. These experiments should provide an improvement in our ability to predict betaturn sites in globular proteins and should define protein engineering design principles for the hybrid proteins above and other new protein molecules in general. NMR and x-ray crystallography experiments will continue in an effort to define the physical basis by which a cis peptide bond is favored in a type VI beta-turn of staphylococcal nuclease. This system provides an opportunity to observe the influence of amino acid sequence on the equilibrium between two beta-turn types on the surface of a globular protein. A genetic and crystallographic analysis will continue to define sequences consistent with a type I' beta-turn in nuclease, and the physical and structural basis for these sequences preferences.