THE PROCESSING ROUTES DETERMINED BY NEGATIVELY CHARGED RESIDUES IN DR1-RESTRICTED T-CELL DETERMINANTS

The presentation pathways followed by DR1-restricted determinants from the fusion protein of measles virus were studied, By assessing the ca pacity of various APC preparations to stimulate fusion protein-specifi c B cells, ii was shown that the determinant contained within the fusi on protein sequence 254-268 (F254) relies on newly synthesized DR1 pro tein for its presentation, By contrast, the determinant contained with in the fusion protein sequence 314-328 (F314) is captured by DR1 prote in recycled from the plasma membrane, In vitro binding analyses showed that the F254 and F314 peptides optimally bind to DR1 at pH 4 and pH 5, respectively, In addition, it was found that binding of the F254 pe ptide to DR1 is much poorer at pH 7 than at pH 4, while binding of the F314 peptide was decreased only moderately at pH 7 as compared with p H 5, Substitution of the glutamic acid 261 for an alamine in the F254 peptide resulted in an analogue with an improved capacity of binding t o DR1 at neutral pH. By contrast, replacement of the valine 319 by a g lutamic acid in the F314 peptide generated al analogue with a decrease d binding capacity at pH 7, These findings indicated that determinants that do mot bear acidic residues are captured efficiently by DR1 mole cules over a broader range of pH than determinants containing acidic r esidues, Binding analyses between DR1 and Four additional peptides fur ther supported this conclusion, Altogether, these results suggested th at acidic residues, by tuning the optimal pH for the assembly of pepti de-DR1 complexes, determine the processing pathway followed by the det erminants.