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Biochemical analysis of the rat MHC class I antigens RT1.Aa, RT1.Fa and Pa
1Department of Pathology, University of Pittsburgh School of Medicine, Pennsylvania 15261.
Insights
In rats, three major histocompatibility complex (MHC) class I antigens (Aa, Fa, and Pa) show high structural similarity, with unique glycosylation patterns. This rat-specific similarity differs significantly from mouse and human MHC class I diversity.
Area of Science:
- Immunogenetics
- Molecular immunology
- Rattus norvegicus research
Background:
- Rats possess multiple MHC class I loci, including the classical RT1-Aa, pregnancy-associated Pa, and RT1-Fa.
- These antigens play crucial roles in immune responses and transplantation.
- Previous studies identified these antigens but lacked detailed structural comparisons.
Purpose of the Study:
- To structurally compare the rat MHC class I antigens: RT1-Aa, Pa, and RT1-Fa.
- To investigate the glycosylation patterns and primary structures of these highly similar rat MHC molecules.
- To understand the evolutionary implications of rat MHC class I gene organization and expression.
Main Methods:
- Isolation of Aa, Fa, and Pa antigens using specific monoclonal antibodies.
- High-Performance Liquid Chromatography (HPLC) peptide mapping for primary structure analysis.
- Isoelectric focusing (pI) to determine charge heterogeneity.
- Analysis of N-linked glycosylation sites.
Main Results:
- Aa, Fa, and Pa antigens exhibit remarkable similarity in primary structure, differing mainly in glutamic/aspartic acid residues.
- All three antigens share two identical N-linked glycans; Fa possesses an additional glycan.
- Rat classical RT1-Aa and Pa antigens lack glycosylation in the second domain, unlike mouse and human counterparts.
- The high structural similarity among rat antigens encoded by different class I genes is unique compared to mouse and human.
Conclusions:
- Rat MHC class I antigens Aa, Fa, and Pa are highly conserved at the primary structure level.
- Unique glycosylation patterns, particularly the lack of second domain glycosylation in RT1-Aa, distinguish rat MHC class I.
- The extensive similarity among rat MHC class I molecules suggests a unique evolutionary trajectory compared to other species.
Abstract:
In DA strain rats, there are two other MHC class I loci (Pa and RT1.Fa) in the vicinity of the classical class I locus RT1.Aa. The Pa antigen is the pregnancy-associated antigen, and it was detected by antibodies elicited in WF females pregnant by DA males without any other immunization. The Fa antigen was detected by a monoclonal antibody raised by alloimmunization. In the present work, the Aa, Fa and the Pa antigens have been compared by HPLC peptide mapping and by isoelectric focusing after their isolation by appropriate monoclonal antibodies. All the three antigens are identical in primary structure with respect to lysine, methionine, asparagine and the aromatic amino acid residues, but they differ from one another with respect to glutamic acid and/or aspartic acid residues. The pI values of the antigens differ slightly. All three antigens have two identical N-linked glycans, but the Fa antigen has an additional N-linked glycan. Based on the available amino acid sequence of the Pa antigen, it can be concluded that both Aa and Pa antigens are devoid of glycosylation in the second domain. This lack of glycosylation of the classical antigen Aa is unique for the rat, since classical class I antigens of the mouse show glycosylation in the first and second, and sometimes in the third domain, and those in the human, in the first domain only. The high degree of similarity among the Aa, Fa, and Pa molecules that this study indicates is also unique for the rat, since antigens encoded by different class I genes of the same haplotype are quite disparate in the mouse and human.