Related Experiment Videos
Dominant effects of mutations in the collagenous domain of mannose-binding protein
1Department of Biochemistry, Glycobiology Institute, University of Oxford, Oxford, United Kingdom. rwallis@glycob.ox.ac.uk
Abstract:
Individuals heterozygous for mutant alleles encoding serum mannose-binding protein (MBP, also known as mannose-binding lectin) show increased susceptibility to infections caused by a wide range of pathogenic microorganisms. To investigate the molecular defects associated with heterozygosity, wild-type rat serum MBP polypeptides (MBP-A: 56% identical in sequence to human MBP) and rat MBP polypeptides containing mutations associated with human immunodeficiency have been coexpressed using a well-characterized mammalian expression system. The resulting proteins are secreted almost exclusively as heterooligomers that are defective in activating the complement cascade. Functional defects are caused by structural changes to the N-terminal collagenous and cysteine-rich domains of MBP, disrupting interactions with associated serine proteases. The dominant effects of the mutations demonstrate how the presence of a single mutant allele gives rise to the molecular defects that lead to the disease phenotype in heterozygous individuals.
Insights
Individuals with one mutated mannose-binding protein (MBP) allele have defective MBP, increasing infection susceptibility. This occurs due to structural changes disrupting complement activation, even with one functional MBP allele.
Area of Science:
- Immunology
- Molecular Biology
- Genetics
Background:
- Mannose-binding protein (MBP) deficiency, also known as mannose-binding lectin (MBL) deficiency, is linked to increased susceptibility to infections.
- Heterozygosity for mutant MBP alleles is common and associated with impaired immune responses.
Purpose of the Study:
- To investigate the molecular basis of impaired complement activation in heterozygous individuals with mutant MBP alleles.
- To elucidate how structural defects in MBP lead to functional deficiencies.
Main Methods:
- Coexpression of wild-type rat MBP and mutant rat MBP in a mammalian expression system.
- Analysis of secreted MBP heterooligomer formation and complement activation.
- Structural analysis of MBP domains to identify mutation-induced changes.
Main Results:
- Coexpressed MBP formed heterooligomers, with mutant and wild-type subunits present.
- These heterooligomers exhibited defective complement cascade activation.
- Structural alterations in the N-terminal collagenous and cysteine-rich domains of MBP were identified, impairing serine protease interactions.
Conclusions:
- The presence of a single mutant MBP allele leads to defective MBP heterooligomers.
- Structural defects in MBP disrupt complement activation, explaining the increased infection susceptibility in heterozygous individuals.
- This highlights the dominant effect of mutant alleles in disease pathogenesis.