Cellular protein maturation and degradation

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DESCRIPTION (provided by applicant): The overall goal of this proposal is to elucidate the role of N-linked glycans in the maturation and quality control of proteins that traverse the secretory pathway. Carbohydrates play a pivotal role in maturation and degradation of proteins in the cell. The addition of the large hydrophilic and flexible structures to the polypeptide backbone modifies the solubility and stability of proteins. In recent years, N-linked glycans have emerged as the lumenal tags within the secretory pathway that direct the folding, quality control and degradation of nascent glycoproteins by providing attachment sites for resident ER proteins possessing carbohydrate-binding properties. We hypothesize that the position of N-linked glycans on a protein plays an important role in the efficient maturation of glycoproteins in the cell by mediating the timing and location of interactions between the nascent chain and resident ER proteins that assist in the maturation process. Resident ER proteins can protect problematic regions of a maturing protein from premature folding or aggregation, aid in the recruitment of folding catalysts, or the joining of distal domains. Therefore, by controlling the binding of the lectin chaperones calnexin and calreticulin, and the subsequent recruitment of their associated oxidoreductase ERp57, glycans can direct the co- and post-translational maturation of glycoproteins. In addition, N-linked glycans also act as quality control sorting signals within the ER- a type of 'lumenal ubiquitin' that targets aberrant proteins for ER associated-protein degradation (ERAD). Thus, enzymes that regulate the structures of glycans and carbohydrate-binding proteins that recognize specific glycan structures play key roles in directing the protein quality control traffic in the secretory pathway. The specific aims of this proposal are: (l) to investigate the interplay between glycans and problematic folding regions focusing on the relationship between glycans and vulnerable Cys residues during the folding process; (2) to elucidate the mechanisms by which the lectin ER chaperones calnexin and calreticulin, and their associated foldase ERp57 aid in the maturation of glycoproteins; (3) to characterize the structure and ER maturation pathway of tyrosinase, a model ERAD substrate whose misfolding is associated with albinism and melanoma, by using novel cell biological approaches; (4) to uncover the mechanism by which N-linked glycans direct proteins to the degradative pathway; and (5) to determine the role of he ER resident and putative quality control receptor EDEM in the degradation of aberrant glycoproteins. These studies will provide insights into how proteins are able to fold with high efficiencies in the cell and how the cell degrades aberrant proteins associated with disease states.