Invertebrate Lipoproteins ID 519

Lipoproteins mediate the transport of lipophilic compounds in a hydrophilic environment. Vertebrates on the one hand posses different types of lipoproteins with various functions (Yokoyama 1998; Rhainds et al. 1999; Khovidhunkit et al. 2004); in insect hemolymph on the other hand, only a single type of lipoprotein - lipophorin - is found (see Rodenburg et al. 2005, for a review). The lipoproteins of insects have received most attention among invertebrates due to their high lipid mobilization rates during flight. Lipophorins can be classified as low-density (LDLp), high-density (HDLp) and very high-density lipophorins, depending on their lipid loading

In crustaceans, lipoproteins are also major components involved in blood clotting, indicating their role in defense reactions in addition to their lipid transport functions. Similarly, insect lipohorins were also shown to be directly linked to immune responses and blood clotting. As in crustaceans, the immune reactive lipoproteins carry a β-1,3-glucan binding site and even vertebrate lipoproteins have retained a glucan binding site by which they bind to proteoglycans .

In contrast to vertebrate lipoproteins that transport different lipid classes, diacylglycerol (DAG) is the predominant core neutral lipid of the lipophorins, with lower amounts of sterols, hydrocarbons, carotenoids and other acylglycerols, which are surrounded by a monolayer of phospholipids (Ryan et al. 2000). In crustaceans, likewise, the predominating lipid species are phospholipids (especially phosphatidylcholine) and, to a lesser content, diacylglycerols and steroids in the neutral lipid core of the lipoproteins . In mollusks, the predominating lipids are also phospholipids and to a lesser extent sterols and triacylglycerols.

In addition to their differences in lipid content, the structure of lipoproteins is also different. Most lipoproteins, with exception of immature vertebrate HDL, seem to be globular particles, while in crustaceans, examples of mature discoidal lipoproteins have also been described (Lee et al. 1978; Spaziani et al. 1986; Ryan et al. 1990; Hevonoja et al. 2000; Koppaka 2001). Here, we report another example of a mature annelid discoidal lipoprotein.

Invertebrates other than insects and crustaceans have been poorly studied with respect to lipoprotein structure and function, however, the fact that many lipoproteins are highly conserved throughout evolution (van Hoof et al. 2002), make them an interesting group of proteins, especially in light of their proposed common evolutionary ancestors, the female specific lipoproteins, the vitellogenins and the clotting proteins found in crustaceans.

Since both vitellogenins and lipoproteins have evolved from a common ancestor

, the study of lipoproteins in lower groups is of interest, particularly in terms of the comparison of lipid transport and function of these apparently ancient molecules. Comparison of these two types of lipoproteins may provide insights into the evolution of the functions of these two groups of lipoproteins and lead to further understanding of the general principles of lipid transport.

Recently, we have found another discoidal lipoprotein in the hemolymph of a crayfish, Astacus leptodactylus, which was not present in a congener, A. astacus.

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