Molecular cloning of human complement component Cls
Cls cDNA clones, which together contained the entire coding region of the protein, were isolated from two human-liver cDNA libraries. The initial Cls clones were identified using a synthetic oligonucleotide probe which corresponded to a region of low degeneracy near the C-terminus of the Cls catalytic chain. Fragments of the Cls cDNA were used to screen a cosmid library in an attempt to isolate the Cls gene, but this proved unsuccessful and no positive clones were isolated. The complete primary sequence of Cls revealed that the homology between the Cls and Clr catalytic chains also extends throughout their non-catalytic chains. Like Clr, Cls can be divided into six structurally independent domains of which the sixth represents the catalytic B chain. Domains I and III in the A chain of Cls are internally homologous, as are domains IV and V. The latter domains are homologous to the internally repeating 60-residue sequences found in Factor B, C2 and other proteins. Domain II of Cls is similar to the 40-residue repeat sequences found in epidermal growth factor precursor and many of the vitamin K-dependent proteins. The assignment of these domains to the different regions of Cls tertiary structure has still to be achieved, but studies in this area should be facilitated now that the complete primary sequence for the Cls zymogen is available.