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Molecular cloning and characterization of MACIF, an inhibitor of membrane channel formation of complement
Abstract:
Human erythrocytes contain a membrane protein, MACIF, which inhibits the formation of a membrane attack complex (MAC) of complement. We have cloned and sequenced the complementary DNA of MACIF messenger RNA. The amino acid sequence predicted from its nucleotide sequence consists of 128 amino acids. The amino-terminal 25 residues may correspond to a signal peptide. The carboxy-terminal sequence confirmed that MACIF is a glycosylphosphatidylinositol (GPI)-anchored protein. The amino acid sequence of MACIF was partially determined by established techniques for protein chemistry and the resultant sequence was consistent with that predicted from the nucleotide sequence. The results of sequence analyses also suggested that asparagine at the 18th position was N-glycosylated. When mRNA obtained from the MACIF cDNA clone with SP6 RNA polymerase was microinjected into Xenopus oocytes, the oocytes synthesized a product which exhibited MACIF activity and reacted with anti-MACIF antibody. Comparison of the predicted sequence revealed significant homology with mouse Ly-6 antigens.
Insights
Researchers identified and sequenced the MACIF protein in human erythrocytes, crucial for inhibiting complement membrane attack complex formation. This study elucidates its structure and function, revealing its role as a glycosylphosphatidylinositol-anchored protein.
Area of Science:
- Molecular Biology
- Immunology
- Biochemistry
Background:
- Human erythrocytes possess a membrane protein known as MACIF (Membrane Attack Complex Inhibitory Factor).
- MACIF plays a critical role in regulating the complement system by inhibiting the formation of the membrane attack complex (MAC).
Purpose of the Study:
- To clone and sequence the complementary DNA (cDNA) of MACIF messenger RNA (mRNA).
- To elucidate the predicted amino acid sequence, identify post-translational modifications, and confirm the protein's anchoring mechanism.
- To functionally validate the cloned MACIF by assessing its activity after expression in Xenopus oocytes.
Main Methods:
- Complementary DNA (cDNA) cloning and sequencing of MACIF mRNA.
- Bioinformatic analysis to predict amino acid sequence and identify signal peptides and glycosylation sites.
- Protein chemistry techniques for partial amino acid sequencing.
- Messenger RNA (mRNA) synthesis from cDNA clone using SP6 RNA polymerase.
- Microinjection of mRNA into Xenopus oocytes for protein expression and functional assays.
- Immunological detection using anti-MACIF antibody.
Main Results:
- The nucleotide sequence predicted a 128-amino acid protein, with a potential signal peptide at the N-terminus.
- Sequence analysis confirmed MACIF as a glycosylphosphatidylinositol (GPI)-anchored protein.
- Partial protein sequencing was consistent with the predicted amino acid sequence.
- Sequence analysis suggested N-glycosylation at asparagine residue 18.
- Expressed MACIF in Xenopus oocytes exhibited inhibitory activity against MAC formation and reacted with anti-MACIF antibody.
- Significant homology was observed between MACIF and mouse Ly-6 antigens.
Conclusions:
- The study successfully cloned and sequenced MACIF, providing a detailed molecular understanding of this complement regulatory protein.
- The findings confirm MACIF's structure as a GPI-anchored protein and demonstrate its functional activity upon expression.
- The homology to mouse Ly-6 antigens suggests potential conserved functions or evolutionary relationships within this protein family.