Lipoproteins in pinnipeds: analysis of a high molecular weight form of apolipoprotein E

We have examined the apolipoprotein content of the lipoproteins of several marine mammals by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Their apolipoprotein (apo) B-100, apoB-48, and apoA-I migrated to virtually the same position as the comparable human apolipoproteins. In cetaceans (bottlenose dolphins and killer whales), the molecular mass of the apoE was identical to that of human apoE (35 kDa). In contrast, in the lipoproteins of pinnipeds (harbor seals, sea lions, and walrus) there was no protein comparable in size to human apoE; however, there were two proteins in the 40- to 44-kDa range. The protein with the higher apparent molecular weight (44 kDa) was apoA-IV, as determined by NH2-terminal amino acid sequencing. Sequencing of the NH2-terminal 15 amino acids of the lower molecular weight protein (40-42 kDa) revealed no obvious homology with human apoE. However, a human apoE-specific monoclonal antibody, 1D7, bound to the 40- to 42-kDa protein, allowing us to identify that protein as apoE. Sequencing of sea lion apoE cDNA clones demonstrated that sea lion apoE is 311 amino acids in length, 12 residues longer than human apoE. All 12 additional residues are located in the NH2-terminal 31 amino acids, a region that has extremely low homology with the NH2-terminal portion of human apoE. The remainder of the sea lion apoE sequence is 74% homologous to human apoE. The sea lion apoE cDNA was expressed in Chinese hamster ovary (CHO) cells as well as CHO ldlD cells, a cell line that is deficient in O-glycosylation of proteins. The size of the sea lion apoE secreted by these two cell lines, compared with the apoE in sea lion plasma, indicated that the predominant form of apoE in sea lion plasma must be posttranslationally modified. Thus, our studies have demonstrated that the higher apparent molecular weight of pinniped (sea lion) apoE is due to a longer polypeptide chain as well as posttranslational modification of the protein.