Plasmalipoproteins are large complexes of lipids and proteins involved in thetransport of lipids across different tissues and in lipid metabolism (Kwan et al., 2007). Depending on the densityby which they are separated following ultracentrifugation, plasma lipoproteinsare classified as: chylomicrons; very low density lipoproteins (VLDL); lowdensity lipoproteins (LDL); intermediate density lipoproteins (IDL); highdensity lipoproteins (HDL). Apolipoproteins form the protein portion of plasmalipoproteins (Mahley et al., 1984).Several apolipoproteins have been identified in humans (e.g. Apo A (I, II, IV); Apo B (48, 100);Apo C (I, II, III, IV); Apo D; Apo E; Apo F; Apo L (1-6); Apo M), andthey associate with different types of lipoproteins (table 1).
Apolipoproteins play severalroles in lipid metabolism and are essential in regulating the function andstructure of lipoproteins. They are involved in the transport lipids(cholesterol, phospholipids, triglycerides) across different tissues, lipiduptake by binding to specific cell surface receptors, and activation of enzymesof lipid metabolism (Mahley et al.,1984). For example, Apo A-I is an activator of the enzyme lecithin:cholesterolacyltransferase (LCAT), which esterifies free cholesterol in reversecholesterol transport to the liver, while Apo B-100 allows LDL particles to berecognised and taken up by the LDL-receptor on hepatocytes (Kwan et al., 2007). Other apolipoproteinshave less clear functions. The apolipoprotein L family are highly expressed inthe placenta and may play a role in exchange and transport of lipids acrossthis and other tissues, however, their main functions have not been fullydetermined (Page et al.
, 2001). Apo A-I, A-IV and E shareboth structural and physiochemical similarities. They are members of the exchangeableapolipoproteins, which have the ability to move and transfer betweenlipoprotein particles (Segrest et al., 1992).The aim of this project will be to identify apolipoprotein sequences and to compare common structuralfeatures of Apo A-I, A-IV, E and L1, and to determine how have apolipoproteins A-I,A-IV and E evolved in different species.
The hypothesis behind this work isthat apolipoproteins are highly conserved in different species due to theessential functions they carry out in lipid transport and metabolism, and thattheir evolution follows the evolutionary path expected from the tree of life.