Identification of single amino acid substitutions in the staphylococcal nuclease protein that enhance and diminish

Abstract

It has been inferred that residue changes that affect T cell recognition of synthetic peptides will have a similar effect in the intact protein. However, since small peptides do not require antigen processing It is possible that residue changes in synthetic peptides will not have an equivalent effect in the intact protein. Mutant proteins of staphylococcal nuclease (Nase) and 15mer synthetic peptides with corresponding substitutions were compared to determine if residue changes within an immunodominant epitope have an effect on the generation of naturally processed peptides. Five different substitutions in the synthetic peptide resulted In loss of reactivity of Individual Nase-specific clones. When the same single amino acid changes were made in the Intact protein, the naturally-processed peptides were also unable to stimulate the Nase-specific clones. However, two other substitutions in the synthetic peptide were stimulatory for a T cell clone even though the same changes in the intact protein were non-stimulatory. These results suggest that certain residue changes affect recognition of the naturally processed peptide but not the synthetic peptide with the same amino acid change. In addition, these results demonstrate that the effects of amino acid substitutions In synthetic peptides on T cell recognition may not always reflect the effects of these substitutions In the intact protein. Substitutions located outside Nase-specific T cell epitopes were also examined. Thirty different mutant proteins were all stimulatory. Moreover, a number of these mutants proteins were 50- to 100-fold more efficient in their stimulatory capacity than the native Nase protein. These results demonstrate that residue changes not directly involved in T cell epitope recognition can enhance the processing and/or presentation of T cell epitopes. These data support the concept that recognition of T cell epitopes may directly depend on the structure of the protein In which the recognition site is processed.