Lipid-binding proteins function

Of all the novel protein types found in nematodes, only two have had biochemical activites ascribed, and these both happen to be lipid-binding proteins (LBPs). This chapter will focus on these, plus those that are structurally similar to those of vertebrates but appear to have nematode-specific modifications to their structures and functions. 

R5. Ritter, M. C., and Dempsey, M, E., Squalene and sterol carrier protein. III. Structural properties, lipid binding, and functional role in cholesterol biosynthesis. Proc. Nat. Acad. Sd. U.S. 70, 265-269 (1973). 

The blue netlike surface in Plate 2 is also a contour map of a three-dimensional function. It represents a surface on which the electron density p(x,y,z) of adipocyte lipid binding protein (ALBP) is constant. Imagine that the net encloses 98% (or some specified value) of the protein s electron density, and so the net is in essence an image of the protein s surface. 

Bussell R, Eliezer D (2003) A structural and functional role for 11-mer repeats in alpha-synuclein and other exchangeable lipid binding proteins. J Mol Biol 329 763-778... 

The other major class of extracellular LBPs of mammals is the lipocalins (Flower, 1996). These are approximately 20 kDa, P-sheet-rich proteins, performing functions such as the transport of retinol in plasma or milk, the capture of odorants in olfaction, invertebrate coloration, dispersal of pheromones, and solubilizing the lipids in tears (Flower, 1996). The retinol-binding protein (RBP) of human plasma is found in association with a larger protein, transthyretin, the complex being larger than the kidney threshold and thus not excreted, although the RBP itself may dissociate from the complex to interact with cell surface receptors in the delivery of retinol (Papiz et al., 1986 Sundaram et al., 1998). 

Both the heat-stable SCP protein of Ritter and Dempsey (R2) and the heat-labile SCP protein of Scallen et al. (S2) bind other lipids (e.g., phospholipids and fatty acids) in addition to water-insoluble cholesterol and its precursors (R2, R3, R5, S2). In view of this apparent lack of specificity, Ritter and Dempsey (R5) have suggested that the carrier protein may be more generally called lipid carrier protein (LCP), although its binding to squalene and sterol carrier protein may more directly refiect its functional role in cholesterol biosynthesis. Obviously, more work is needed to clearly define both structural role and functional properties of this protein or proteins. 

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