MECHANISM OF INSERTION OF TAIL-ANCHORED PROTEINS INTO THE ENDOPLASMIC RETICULUM MEMBRANE: CHARACTERIZATION OF A NOVEL TRANSLOCATION SYSTEM AND ITS IMPLICATIONS FOR ER PHYSIOLOGY AND PATHOLOGY

The Endoplasmic Reticulum (ER) is a membrane-delimited intracellular compartment which plays fundamental roles in the comunication between the inside of cells and the outside world. Embedded in its membrane are many important proteins that play key regulatory roles. Relevant to this project are 3 ER proteins: cytochrome b(5), protein tyrosine phosphatase 1B (PTP-1B), and ubiquitin conjugating enzyme 6 (UBC6). The first has such fundamental roles in lipid metabolism that loss of its function is probably not compatible with life; the second removes phosphate residues from a class of phosphorylated receptor proteins, among which the insulin receptor, causing their inactivation; thus PTP-1B is involved in insulin-resistant diabetes and is envisaged as a plausible drug target for the therapy of this disease; the third, UBC6, is instead involved in the degradation of membrane proteins. This process is very important, because within the cell the levels of each single protein must be kept at appropriate levels by a balance of synthesis and degradation. UBC6 plays a role in the degradation of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR), and may thus have a role in the pathogenesis of Cystic Fibrosis. The 3 proteins studied in this project have in common their particular disposition in the membrane, because of which they are called “tail-anchored”: they consist of a cytoplasmic portion, containing the enzymatic active site, that is anchored to the ER lipid bilayer by a short, carboxy-terminal, hydrophobic sequence. Although much detailed knowledge is available on how the ER membrane inserts most of its proteins, surprizingly little is known about the insertion process of tail-anchored proteins. Here, we propose to study the mechanism of insertion and of degradation of this class of proteins, and by so doing to acquire information on a basic function of the ER and on the biology of proteins that play a role in the pathogenesis of genetic diseases.