Sites within the complement C3b/C4b receptor important for the specificity of ligand binding

Area of Science:

• Immunology
• Molecular Biology
• Protein Science

Background:

• Cysteine-rich repeated units, known as short consensus repeats (SCRs), are fundamental building blocks in mammalian proteins, notably within the complement system.
• The human complement receptor 1 (CR1) protein features these SCR motifs, forming its extracellular domain.
• Previous research localized C4b and C3b binding domains to specific SCRs (SCR-1/2 for C4b, SCR-8/9 for C3b), despite sequence homology between SCR-1/8 and SCR-2/9.

Purpose of the Study:

• To investigate the necessity of one or two SCRs for effective ligand binding.
• To pinpoint specific amino acid sites within SCRs that dictate the specificity of C4b and iC3 binding.
• To elucidate the structural requirements for ligand recognition in the complement system.

Main Methods:

• Site-directed mutagenesis was employed on a truncated CR1 construct (CR1-4), which encompasses the first 8.5 SCRs.
• Amino acids within SCR-1 and SCR-2 of CR1-4 were systematically substituted with their homologous counterparts from SCR-8 and SCR-9.
• The binding affinities of wild-type and mutant CR1-4 proteins to C4b and iC3 were assessed.

Main Results:

• SCR-1 and SCR-2 were found to be essential for the C4b binding activity of CR1-4.
• Mutagenesis identified one amino acid in SCR-1 and three in SCR-2 critical for C4b binding.
• Introducing five amino acids from the C-terminus of SCR-9 into SCR-2 conferred iC3 binding, but only when SCR-1 was present, indicating a requirement for two contiguous SCRs for iC3 recognition.

Conclusions:

• Specific amino acid residues within SCR-1 and SCR-2 are crucial for C4b binding.
• Ligand specificity for C4b and iC3 is determined by distinct amino acid residues within SCRs.
• The binding of iC3 requires the presence of at least two contiguous SCRs, highlighting the importance of domain arrangement for complement protein function.