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The complete exon-intron structure of a human complement component C4A gene. DNA sequences, polymorphism, and linkage
1Department of Biochemistry, Oxford University, U.K.
Insights
The human complement component C4 gene structure, including its 41 exons and complex polymorphisms, was detailed. These variations likely explain diverse C4 functions and serologic differences.
Area of Science:
- Immunogenetics
- Molecular Biology
Background:
- The human complement component C4 (C4) genes, C4A and C4B, are located in the MHC class III region.
- C4 genes exhibit significant polymorphism in isotype, size, and copy number, contributing to their complexity.
Purpose of the Study:
- To determine the DNA sequence of the human C4A gene and analyze its exon-intron structure.
- To identify the location of functional and structural elements within specific exons.
- To investigate the evolutionary conservation of exon-intron structures with related genes.
Main Methods:
- DNA sequencing of the C4A gene.
- Analysis of exon-intron boundaries and encoded protein features.
- Comparative analysis with mouse C4, human C3, and rat alpha 2 macroglobulin gene structures.
- Detection of DNA polymorphisms using various techniques, including restriction fragment length polymorphism (RFLP).
Main Results:
- The DNA sequence of the C4A gene was determined, revealing 41 exons encoding a 1744 amino acid precursor protein.
- Specific functional sites, including the C4a active site, alpha-gamma-chain junction, tyrosine sulfation sites, and protease cleavage site, were mapped to individual exons.
- Exon-intron structures of human C4 show high similarity to mouse C4, human C3, and rat alpha 2 macroglobulin.
- Twenty polymorphic sites were detected in human C4, with a PvuII RFLP identified in the C4A coding region.
- The intergenic region between C4 and CYP21 is approximately 3028 bp.
Conclusions:
- The detailed C4A gene structure provides a basis for understanding its functional and structural diversity.
- Exon mapping of key functional sites elucidates structure-function relationships.
- The conserved exon-intron organization suggests a common evolutionary origin for these complement genes.
- Detected polymorphisms are likely responsible for observed functional, structural, and serologic variations in C4 allotypes.
Abstract:
The human complement component C4A and C4B genes are located within the class III region of the MHC. The polymorphic C4 genes are highly complex including variations in class (isotype), size, and number of genes. The DNA sequence for a C4A gene has been determined, except for a large intron of 6 to 7 kb long. The C4A gene consists of 41 exons encoding a transcript for a precursor protein of 1744 amino acid residues. Several structural and functional aspects of C4 have been located to individual exons. The active site of the anaphylatoxin C4a matches to a splice junction. Some unique properties of C4, such as, the alpha-gamma-chain junction, the tyrosine sulfation sites, and the post-secretory metalloprotease cleavage site, are encoded by a single exon. Comparison of human C4 with published data for mouse C4, human C3 and rat alpha 2 macroglobulin genes revealed that these evolutionary-related genes share very similar exon-intron structures. Altogether 20 polymorphic sites in human C4 have been detected by various techniques. Presumably, these polymorphic residues account for the functional, structural, and serologic variations observed among the various allotypes. A PvuII restriction length polymorphism has been detected within the region of DNA coding for C4a. The intergenic region between C4 and the neighboring 21-hydroxylase gene, CYP21, is approximately 3028 bp in size.