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The molecular architecture of human complement component C6
1Research Institute of Scripps Clinic, La Jolla, California 92037.
Insights
The molecular structure of human complement component C6 was detailed, revealing its primary and secondary structures, disulfide bonds, and glycoprotein nature. This provides insights into complement system function and related proteins.
Area of Science:
- Immunology
- Structural Biology
- Molecular Biology
Background:
- The human complement system is crucial for innate immunity.
- Terminal complement components (C5-C9) form the membrane attack complex.
- Understanding C6 structure is key to deciphering complement-mediated lysis.
Purpose of the Study:
- To elucidate the molecular architecture of human complement component C6.
- To determine the primary, secondary, and tertiary structures of C6.
- To compare C6 structure with other complement components and related proteins.
Main Methods:
- Complement component C6 cDNA sequencing from a human liver library.
- Circular dichroism spectroscopy for secondary structure analysis.
- Transmission electron microscopy for tertiary structure visualization.
Main Results:
- The primary structure of C6 comprises 913 amino acids, showing homology with C7-C9.
- C6 contains 32 disulfide bonds and exhibits homology with thrombospondin, LDL receptor, EGF, and complement factors H and I.
- Secondary structure analysis revealed 12% alpha-helix, 29% beta-sheet, and 21% beta-turns; tertiary structure visualized as a sickle shape (144 x 66 Å).
Conclusions:
- The study provides a comprehensive structural characterization of human complement component C6.
- C6's structural features suggest functional roles and evolutionary relationships within the complement cascade.
- Structural insights into C6 contribute to understanding the membrane attack complex and its implications in disease.
Abstract:
The molecular architecture of human complement component C6 was elucidated at several levels of structural organization. The entire primary structure of C6 was determined by sequencing C6 cDNA that was cloned from a human liver lambda gt11 library. The polypeptide chain of C6 contains 913 amino acids. The protein is homologous with the other terminal components of complement, C7-C9. Specifically, C6 has 29% of its residues identical with C7, and 55 of the 56 cysteines found in C7 match those in C6. The C6 polypeptide chain is cross-linked by 32 disulfide bonds, and most of the cysteines are located in short (34-77 amino acids) discrete segments that exhibit homology with a wide variety of other proteins such as thrombospondin, the low density lipoprotein receptor, epidermal growth factor, and complement factors H and I. C6 is a glycoprotein, and it has two oligosaccharide groups attached to asparagines located near the amino and the carboxyl termini of the molecule. The organization of secondary structural elements in C6 was elucidated using circular dichroism spectroscopy and an empirical method based on sequence analysis. C6 has an estimated 12% alpha-helix, but is comparatively richer in beta-sheet (29%) and beta-turns (21%). Most of the predicted alpha-helical structure resides in a portion of the polypeptide chain that is free of cysteine and which shares homology with C9 and perforin. The tertiary structure of the C6 molecule was visualized by transmission electron microscopy; it has a sickle shape with dimensions of 144 x 66 A. The combined results are discussed and comparisons made with the other late acting components of complement and perforin.