Unassembled subunits of several membrane-associated multimeric proteins as well as several mutant secretory proteins are retained within the endoplasmic reticulum rather than secreted from cells. It is now apparent that the cell has developed a mechanism for identifying and subsequently removing these retained proteins from an early compartment of the secretory pathway. Greater than 85% of the transport-impaired PiZ variant of the hepatic secretory protein alpha-l-antitrypsin is not secreted, but rather, is retained within the cell and undergoes degradation by the aforementioned pathway. Protein degradation via this pathway occurs within a pre-Golgi subcellular compartment that is apparently not associated with lysosomes. Attachment of the tetrapeptide sequence KDEL (a signal for the re-cycling of soluble luminal proteins from a putative post-ER compartment back to the endoplasmic reticulum), but not KDAS, to the carboxyl terminus of a genetically-engineered truncated PiZ variant abolishes the degradation of the retained protein. Although this is evidence that degradation of the PiZ variant occurs following its exit from the endoplasmic reticulum, the exact subcellular location of the pre-Golgi degradative compartment is unknown. Interestingly, although the naturallyoccurring retained PiZ variant undergoes intracellular degradation, a small fraction of the retained protein escapes the degradative pathway and accumulates as an insoluble aggregate within reminant vesicles of the hepatic rough endoplasmic reticulum. The intrahepatic accumulation of the insoluble protein can act as an etiologic agent for the development of liver disease in transgenic mice. Furthermore, because of the physiological role of alpha-antitrypsin, its severe reduction in plasma can predispose individuals toward the development of pulmonary emphysema. The immediate goal within the context of this proposal is to incorporate the use of stably-transfected cells that express natural and genetically-engineered human alpha-l-antitrypsin variants for the molecular, biochemical and ultrastructural analysis of this degradative pathway. Results from this research endeavor will enhance our understanding of the regulation of protein """"""""trafficking"""""""" within the endoplasmic reticulum, as well as to define the pathophysiology of liver disease that is associated with the intrahepatic accumulation of the PiZ variant.
| Le, A; Steiner, J L; Ferrell, G A et al. (1994) Association between calnexin and a secretion-incompetent variant of human alpha 1-antitrypsin. J Biol Chem 269:7514-9 |
| Le, A; Ferrell, G A; Dishon, D S et al. (1992) Soluble aggregates of the human PiZ alpha 1-antitrypsin variant are degraded within the endoplasmic reticulum by a mechanism sensitive to inhibitors of protein synthesis. J Biol Chem 267:1072-80 |
