Biochemistry and Molecular Pharmacology
First Thesis Advisor
Robert E. Humphreys
Peptide Hydrolases, Antigen-Presenting Cells, Pharmacology
The overall goal of my thesis research was to understand better the mechanisms that control antigen processing and presentation by class II MHC molecules. Towards this goal I investigated ways in which the physical structure and post-translational modifications of the class II MHC alpha and beta chains and associated molecules might serve to regulate antigen processing and presentation. Specifically, I investigated (1) a hypothesis that Ii might aid binding of foreign antigenic peptides to the class II MHC foreign antigen binding site (desetope), and the application of this hypothesis to the prediction of class II-presented peptides; (2) the proteolytic cleavage of Ii to p25; (3) the proteolytic cleavage of the class II MHC alpha and beta chains, and (4) the phosphorylation of Iiand the alpha and beta chains.
In exploring the hypothesis that amphipathic alpha helical peptides digested from foreign antigen, bind to the class II MHC desetope, to be presented to T cell receptors, we found such an extended, amphipathic helix in Ii (Phe146-Val164). A hypothesis was developed that this amphipathic alpha helix of Ii bound to the desetope of class II MHC molecules, and remained there from time of synthesis until catalyzing the charging of the desetope with a foreign peptide. This region of Iicould then be considered to be the prototypic T cell-presented peptide and the "strip-of-helix" algorithm was developed to search the sequences of proteins for similar amphipathic alpha helices. Such peptides might bind to the class II MHC desetope and have a high probability to be presented to the T cell.
The strip-of-helix algorithm calculated the mean hydrophobicity (from Kyte-Doolittle values; Kyte and Doolittle, 1982) of sets of amino acids in axial strips down sides of helices for 3 to 6 turns, at positions n, n+4, n+7, n+11, n+14, and n+18. Peptides correlating well with T cell responsiveness had: (1) 12 to 19 amino acids (4-6 turns of an alpha helix), (2) a strip with highly hydrophobic residues, (3) adjacent, moderately hydrophilic strips, and (4) no prolines to break the helix. This algorithm predicted 10 of 12 T cell-presented peptides in 7 well-studied proteins.
In a study of the post-translational modifications of Ii, an early proteolytic pathway of the destruction of Ii, resulting in the generation of p25, was described. This 25,000 dalton protein, seen in immunoprecipitates with antibodies to class II MHC molecules or to Ii, was shown to be a C-termina1 fragment of a high mannose form of Ii. The evidence for this conclusion includes the following results. [35S]methionine-1abe1ed Ii and associated molecules were immunoprecipitated, denatured, resolubi1ized and subjected to a second immunoprecipitation with various antibodies. Two antisera to C-termina1 peptides of Ii (183-193 and 192-211), but not an antiserum to an N-termina1 peptide (12-28), immunoprecipitated p25. A monoclonal antibody (mAb) to Ii immunoprecipitated [35S]methionine-1abe1ed p25 but not [35S]cysteine-1abe1ed p25, consistent with the loss of a portion of Ii containing the only cysteine in Ii, Cys28. [35S]methionine pulse-chase labeling demonstrated the maximal appearance of p25 at 20-40 min chase times. p25 molecules were reduced to about 10.5 kD by treatment with endoglycosidases F and H. p25 was, therefore, generated from a high mannose form of Ii in the ER or cis-Golgi. This finding could either implicate that site for class II MHC desetope charging with foreign peptides or reflect a mechanism for degradation of "excess" Ii molecules in the ER. Digestion of class II MHC antigen-Ii complexes with various proteases yielded fragments, migrating at and near p25 in 2-D electrophoretic gels, which were relatively resistant to further digestion. This observation was consistent with the presence of relatively protease-resistant secondary structures (domains) and a relatively protease-sensitive (IgG hinge-like) region in Iinear its insertion into the membrane.
In a study of the post-translational modifications of the class II MHC alpha and beta chains, well conserved pairs of basic amino acids in the sequences of these molecules were observed. It was hypothesized these could be sites for proteolytic cleavage, as precedented in other systems (i.e.proinsulin processing). These potential cleavage sites fall in significant locations with respect to the deduced structure of the class II MHC desetope, supporting the hypothesis that these cleavages might either aid or destroy antigen presenting functions. To test this hypothesis we looked for remnant polypeptides of the alpha and beta chains. Polypeptides were observed in gels of immunoprecipitated class II MHC complexes. To identify if such polypeptides were derived from the alpha and beta chains, immunoblotting to electrotransferred polypeptides was attempted, with antisera made to synthesized peptides that mimicked eight regions of the alpha and beta chains. These antisera were produced and characterized by dot blotting, ELISA, western blotting, and immunoprecipitation of native and denatured material. One antiserum, to an alpha chain peptide (77-88), blotted to a polypeptide immunoprecipitated by anti-class II MHC antiserum. This observation supported the hypothesis that the alpha and beta chains undergo proteolytic cleavages, possibly in the control of antigen presentation.
It was also demonstrated that Ii and the alpha and beta chains can be phosphorylated under varying culture conditions, but this project was not pursued.
Thomas, LJ. Proteolytic Cleavages of Molecules Involved in Antigen Processing and Presentation: A Thesis. (1989). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 97. DOI: 10.13028/4wy9-3a44. https://escholarship.umassmed.edu/gsbs_diss/97
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