Lipid-binding proteins extracellular

Apo A1 recycles extracellularly between lipid-poor (pre-beta-migrating) and lipid-rich (spheroidal) species (Fig. 7). Each spheroidal apo A1 particle transports multiple lipid loads from peripheral tissues to the liver. In contrast other lipid-binding proteins (e.g., apo B and apo E) are cleared by hepatocytes as part of whole lipoprotein particles. 

The Role of Myosins in Cell Locomotion The Role of Actin-Binding Proteins in Cell Locomotion The Transduction of Extracellular Motility Signals to the Cytoskeleton Lipid Flow and Cell Locomotion The Role of Cell Locomotion in Metastasis Intracellular Motility Microtubule-Based Intracellular Motility... 

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). 

Most important for the regulation of the membrane architecture are membrane potential, intracellular Ca2+ concentration, pH, changes in lipid composition due to the action of phospholipases and cell-cell coupling as well as the coupling of the membrane to the cytoskeleton and the extracellular matrix. Membrane architecture is additionally modulated by ions, lipo- and amphiphilic hormones, metabolites, drugs, lipid-binding peptide hormones, and amphitropic proteins [44]. 

Lipids, by virtue of their immiscibility with aqueous solutions, depend on protein carriers for transport in the bloodstream and extracellular fluids. Fat-soluble vitamins and free fatty acids are transported as noncovalent complexes. Vitamin A is carried by retinol-binding protein and free fatty acids on plasma albumin. However, the bulk of the body s lipid transport occurs in elaborate molecular complexes called lipoproteins. 

The retinoids comprise a family of polyisoprenoid lipids that inclndes vitamin A (retinol) and stracturaUy related componnds. The biological activity of retinoids can be mod-ihed, for example, by changes in the molecules state of oxidation and cis/trans isomerization. Their activity is also dependent on the levels of specific types of retinoid-binding proteins that exist in extracellular, cytosolic, and nuclear compartments. The role of retinoids in gene expression represents an important biological function for this family of molecules. Retinoid-dependent modulation of gene expression is critical for normal cell and tissue function in mature as well as developing animals. 

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