Plasmalipoproteins are large complexes of lipids and proteins involved in thetransport of lipids across different tissues and in lipid metabolism (Mahley et al., 1984).
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) (Kwan et al.,2007). Apolipoproteins form the protein portion of plasma lipoproteins (Mahley et al., 1984). Several apolipoproteinshave been identified in humans (e.g.
Apo A (I, II, IV); Apo B (48, 100); Apo C (I, II, III, IV); Apo D; ApoE; Apo F; Apo L (1-6); Apo M), and they associate with different typesof 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).
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. For example, the apolipoprotein L family are highlyexpressed in the placenta and may play a role in exchange and transport oflipids across this and other tissues, however, their main functions have notbeen fully determined (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 determine how haveapolipoproteins A-I, A-IV and E evolved in different species, and to compare commonstructural features of Apo A-I, A-IV, E and L1. The hypothesis behind this workis that apolipoproteins are highly conserved in different taxa 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. Evidencefrom previous studies suggest that the exchangeable apolipoproteins haveevolved from a common ancestral genes by means of gene duplication (Segrest et al.
1992), and that apolipoproteinshave undergone speciation events early in the evolution of vertebrates (Babinet al. 1997).