INTERESTS Teaching: Molecular Biology; Pathobiology of Atherosclerosis; Pathologic Biochemistry; Disturbances in Lipoprotein Metabolism Research: Secretory Protein Biogenesis; Lipoprotein Assembly Current Project: Lipoproteins, Protein folding, Intracellular protein transport
Apolipoprotein B (apoB) is the major protein component of very low density lipoprotein (VLDL) and an important factor in the pathogenesis of coronary artery disease. Our laboratory is studying the way in which apoB undergoes its folding and assembly with lipids in the hepatic secretory pathway and how these steps may be controlled to achieve the regulation of VLDL secretion. Our initial studies have focused on the mechanism by which apoB undergoes its translocation across the endoplasmic reticulum (ER) membrane, and whether this process is responsible for regulating the entry of apoB into the hepatic secretory pathway. More recent studies have focused on the function of the amino terminal globular domain of apoB. This specialized domain has limited lipid-binding properties, yet is essential for initiating the assembly of VLDL. We are investigating how this initiation step is achieved and identifying the ER-localized chaperones and assembly factors that facilitate this process.
In addition to our studies on VLDL assembly, we continue to characterize resident proteins of the ER that are responsible for various stages of secretory protein maturation. The signal peptidase complex is necessary for the proteolytic processing of secretory precursor proteins and may be responsible for other important aspects of ER function. Studies are underway to define the structure and function of the individual subunits that compose the signal peptidase complex. We also intend to use the signal peptidase complex as a model system to understand the biogenesis of the rough ER membrane.
FIGURE LEGEND: Assembly and degradation of apoB-containing lipoproteins. The apoB signal peptide (SP) targets apoB-synthesizing ribosomes to the ER membrane (step 1). Translation of the amino-terminal ~25% of apoB in combination with MTP creates a small dense emulsion particle (step 2) that continues to be enlarged during translation. Upon release from the ribosome (step 3) the lipoprotein precursor fuses with TG droplets originating in the smooth ER (step 4). This fusion reaction gives rise to mature VLDL in the liver or chylomicrons in the intestine. In the absence of MTP, apoB is translated without undergoing lipidation (step 2a). This population is rapidly degraded, sometimes prior to the completion of translation, by a process of retrograde translocation from the ER lumen to the cytosol coupled to addition of multiubiquitin chains and proteasomal degradation (step 3a). Under conditions of limiting MTP or lipid, aberrant, under-lipidated particles are generated that are too small to accommodate the proper folding of apoB (step 2b). The unfolded domains of apoB may interact with the inner leaflet of the ER membrane. These particles can be lipidated posttranslationally and released from the ER membrane (3b) or targeted for proteasomal degradation (step 3c). Both the co- and posttranslational proteasomal degradation of apoB is dependent upon Hsp70. |
Recent Publications (selected):
Davidson, N.O., and Shelness, G.S. (2000) Apolipoprotein B: RNA editing, lipoprotein assembly, and presecretory degradation. Annu. Rev. Nutr. 20:169-193
Joyce, C., Shelness, G.S., Anderson, R.A., and Rudel. L.L. (2000) ACAT1 and ACAT2 membrane topology segregates a serine residue essential for activity to the opposite side of the endoplasmic reticulum membrane. Mol.Biol.Cell. 11: 3675-3687
Rudel, L., and Shelness, G. (2000) Cholesterol esters and atherosclerosis - a game of ACAT and mouse. Nature Med .6:1313-1314.
Reagan, J. W., Jr., Hubbert, M. L., and Shelness, G. S. (2000) Posttranslational regulation of acid sphingomyelinase in Niemann-Pick type C1 fibroblasts and free cholesterol-enriched Chinese hamster ovary cells. J. Biol. Chem. 275:38104-38110
Shelness, G.S., and Sellers, J.A. (2001) Very Low Density Lipoprotein Assembly and Secretion. Curr. Opin. Lipidol. 12,151-157.
DeLozier, D.A., Parks, J.S., and Shelness, G.S. (2001) Vesicle binding properties of wild type and cysteine mutant forms of the a1 domain of apolipoprotein B. J. Lipid Res. 42:399-406.
Sellers, J. A., and Shelness, G. S. (2001) The lipoprotein assembly capacity of the mammary tumor-derived cell line, C127, is due to the expression of functional microsomal triglyceride transfer protein. J. Lipid Res. 42, 1897-1904
Chisholm, J. W., Burleson, E. R., Shelness, G. S. and Parks J. S. (2002) Apo A-I secretion from HepG2 cells: Evidence for the secretion of both lipid-poor apo A-I and intracellularly assembled nascent HDL. J. Lipid Res.43:36-44
Sellers, J.A., Hou, L., Athar, H., Husssain, M.M., and Shelness. G.S. (2003) A Drosophila Microsomal Triglyceride Transfer Protein Homolog Promotes the Assembly and Secretion of Human Apolipoprotein B. Implications for Human and Insect Lipid Transport and Metabolism. J. Biol. Chem. 278:20367-20373
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