Haya Herscovitz, Ph.D.

Research

Assembly and secretion of lipoproteins Chaperone-assisted folding of apolipoprotein B

Our research focuses on the mechanisms that regulate the assembly of triacylglycerol (TAG)-rich lipoproteins (such as very low density lipoproteins, VLDL) and their secretion from the liver into the plasma. One of the key players in this complex process is apolipoprotein (apo) B, a very large (4536 amino acids and molecular mass of about 550 kDa), hydrophobic glycoprotein that directs the assembly of VLDL. VLDL particles are remodeled in the plasma to give rise to low density lipoproteins (LDL), the major carriers of plasma cholesterol, and a major risk factor for the premature development of coronary heart disease. Our goal is to elucidate the molecular details of the folding of apoB into its mature, secretion-competent form. This might allow to develop means to modulate the secretion of VLDL and thereby, reduce plasma cholesterol levels.

Because apoB has unusual structural properties, as it is virtually water insoluble, and it requires association with lipids in order to attain its secretion-competent form, its maturation is more complex than typical secretory proteins. While typical nascent secretory proteins are translocated into the endoplasmic reticulum (ER) lumen where their folding and maturation take place (see left panel of the figure depicted below), apoB associates with the ER membrane co-translationally where its folding presumably begins (see right panel of figure). When it binds enough lipids to form a "primordial" particle, it is released into the ER lumen, where additional folding steps seem to occur, leading to the formation of nascent VLDL. Only a fraction of nascent apoB is secreted. The size of this fraction is dependent on lipid availability. The remaining fraction is targeted for degradation (see figure).

Folding of nascent proteins in the cell is a complex process that requires the assistance of molecular chaperones, which are highly conserved proteins found in all types of cells from bacteria to men. The primary role of molecular chaperones is to bind transiently to nascent polypeptides, prevent their aggregation, (which may otherwise occur in the cell due to exposed hydrophobic domains), and maintain them in conformations competent for efficient folding. Our objective is to identify and characterize molecular chaperones that assist in the folding of apoB.

Several chaperones have been identified by other investigators, including BiP, calnexin, Hsp70 and microsomal triglyceride transfer protein (MTP). MTP is unique to apoB and is critical for the binding of lipids to apoB. We have identified a number of additional chaperones that interact with apoB during its maturation.

Through the use of biochemical, immunological, cell and molecular biological techniques we hope to gain a better understanding of the mechanism by which molecular chaperones mediate folding of nascent proteins in general, and apoB in particular. Our long-term goal is to identify novel chaperones that may be unique to apoB and thus may serve as a target to modulate its secretion.

Figure 1. Translocation and folding of nascent secretory proteins.

Proteins synthesized on ribosomes bound to the ER membrane are translocated into the ER lumen through the translocation channel, "translocon" (composed primarily of sec61 complex). Molecular chaperones (e.g. BiP, PDI, calnexin, GRP94 etc,) associate with the nascent polypeptide co-translationally and are released upon completion of protein folding and the nascent protein is secreted (left panel). ApoB initially appears to follow the same pathway. However, as the nascent polypeptide chain is elongated, it appears to associate with the ER membrane with domains exposed to the cytosol. If lipids (primarily triacylglycerols, TAG) are available, recruitment of lipids begins and "primordial particles" form. These particles are subsequently released into the ER lumen and most chaperones presumably dissociate from the partially folded apoB. These particles are than transformed into larger nascent VLDL particles. However, if lipid availability is limited, misfolded apoB remains bound to some chaperones, which presumably target it for degradation via the proteasome, in a process mediated by Hsp70. Some degradation appears to occur in the ER lumen by an unknown protease.

Contact Us

Department of Physiology and Biophysics
Boston University School of Medicine
715 Albany Street
Boston MA 02118-2526

Phone: (617) 638-4012
Fax: (617) 638-4041
e-mail: haya@bu.edu