Secretory protein folding and the quality control of misfolded glycoproteins

The Endoplasmic Reticulum (ER) provides a special environment that caters to the maturation of secretory and membrane proteins. The folding of these proteins into their native conformations within the ER lumen is a process of paramount importance but is poorly understood. The identification of factors that constitute the folding machinery in the ER therefore, is an important step towards understanding secretory protein folding. Here we employ a powerful genetic screen that allows the isolation of mutants defective in various secretory pathway functions. A parallel effort was made to optimize existing methods to assay protein folding and also to develop new tools that are greatly needed to better study protein folding in vivo. Using these methods, we have identified several genes that may play a role in protein folding. In addition, we describe the identification and characterization of a novel gene, undiscovered at the time of discovery that appears to play a role in the folding of some secretory proteins.;Incorrect folding leads to aberrant conformations of proteins. Such aberrantly folded proteins are known to be inherently toxic and their accumulation in cells can be detrimental. In the (ER), a quality control system exists to detect, and ultimately degrade terminally misfolded proteins. While much is known about the later events of this ER associated protein degradation (ERAD), it is not clear how the quality control machinery differentiates aberrantly folded proteins from actively folding or properly folded ones, and selectively degrades them. Past studies have shown that specific N-linked glycan structures function as signals for the targeting of misfolded glycoproteins to ERAD via a lectin targeting machinery. Recently it was discovered by our group that, of the many glycans that may be present on a protein, only certain glycans function as ERAD targeting signals, suggesting the existence of substrate specific ERAD signals.;Here we show that specific regions within the polypeptide chain proximal to an ERAD glycan act in conjunction with it to target a model misfolded glycoprotein substrate for degradation. Furthermore, we show that these protein-glycan determinants can be moved out of their context within the protein and still retain their functionality.