Primary structure of the fifth component of murine complement

Area of Science:

• Molecular Biology
• Immunology
• Genetics

Background:

• Complement component 5 (C5) is a crucial protein in the complement system.
• Understanding the structure of murine C5 is essential for comparative immunology and molecular studies.

Purpose of the Study:

• To construct and characterize a complementary DNA (cDNA) clone encoding murine C5.
• To determine the complete amino acid sequence of murine C5.
• To compare the murine C5 sequence with homologous proteins like human C5, C3, and C4.

Main Methods:

• Construction of a size-selected poly(A+) liver RNA cDNA library from B10.WR mice.
• Screening of 150,000 recombinants using a human C5 cDNA probe.
• Dideoxy sequencing of a 5.2 kilobase pair cDNA insert.
• Bioinformatic analysis for open reading frame, amino acid sequence, and homology.

Main Results:

• A full-length murine C5 cDNA clone was isolated and sequenced.
• The sequence encodes a 1640-amino acid protein with a 4920 base pair open reading frame.
• High homology was observed between murine C5 and human C5, particularly in the C5a region.
• A beta alpha-chain orientation for the promolecule was identified, with a basic sequence upstream of C5a.
• Murine C5 lacks the cysteine and glutamine residues forming the thiol ester bond found in C3 and C4, instead having serine and alanine.

Conclusions:

• The complete sequence of murine C5 provides a detailed molecular blueprint.
• The identified promolecule structure and chain orientation offer insights into C5 processing.
• The absence of a thiol ester bond in C5 distinguishes its function from C3 and C4 within the complement cascade.