Molecular cloning and sequence analysis of the cDNA encoding human apolipoprotein H (beta 2-glycoprotein I)

Apolipoprotein H, also known as beta-2-glycoprotein I, was purified from human serum, and antiserum produced to denatured apolipoprotein H detected a cDNA clone from a lambda gt11 library derived from human liver. This cDNA coded for the complete sequence of the mature protein. The cDNA insert, along with a polymerase chain reaction product which extended the 5' end of the message, were subcloned and both strands were sequenced. The apolipoprotein H precursor was found to code for 345 amino acids, 326 of which appear in the mature protein. The deduced amino acid sequence of human apolipoprotein H differs from its rat homologue by the presence of a 48-amino acid stretch which is absent from the rat protein. The remainder of the proteins share a greater than 80% similarity. The amino acid sequence of apolipoprotein H consists largely of repeated units approximately 60 amino acids in length. These repeats are comparable to "sushi structures" found in a large number of diverse proteins, including complement components, receptors and regulators of complement activation, serum proteins, membrane-associated adhesion proteins, and other structural and catalytic proteins. Apolipoprotein H was shown to be transcribed by human hepatoma cell lines Hep 3B and Hep G2, and rat liver by detection of mRNA using northern blot analysis.