Catalysis of peptide dissociation from class II MHC-peptide complexes

Certain peptides such as dynorphin A [dynA-(1-13)] enhance the release of a ntigenic peptides bound to class II MHC molecules at neutral pH. This enhan ced release has been termed push off. Previous work has shown that the anti genic pigeon cytochrome c peptide PCC-(89-104) has at least two conformatio nal isomers when bound to the class II MHC protein I-E-k. We have according ly studied the push off of PCC-(89-101) from the complex PCC-(89-104)/I-E-k to see whether these isomeric conformations are distinguished by the push- off effect. A comparison of the association and dissociation kinetics of PC C-(89-104)/I-E-k in the presence of dynA-(1-13) shows that dynA-(1-13) does not simply replace PCC-(89-104) but rather acts catalytically. The major p roduct is peptide-free I-E-k, which is receptive to further peptide binding . Evidence is presented that a two peptide-one MHC complex is formed in sol ution. This ternary complex represents the first step of the mechanism of p ush off. F-19 NMR data are presented that indicate that dynA-(1-13) interac ts specifically with only one of the two isomeric complexes of PCC-(89-101) and I-E-k. A push-off mechanism is proposed in which dynA-(1-13) binds out side the peptide binding groove. In a second step, the dissociation of one of the two isomers is specifically enhanced, Thus the push-off effect may b e useful for identifying conformational isomers and for separating them.