Intracellular degradation of the transport-impaired human PiZ alpha 1-antitrypsin variant. Biochemical mapping of the degradative event among compartments of the secretory pathway

The naturally occurring PiZ and Pi NullHong Kong variants of the human secretory protein alpha 1-antitrypsin (AAT) are retained within an early compartment of the secretory pathway. Intracellular degradation of these transport-impaired secretory proteins is initiated 30-45 min following their synthesis and translocation into the endoplasmic reticulum (ER). Interestingly, the overall rate of degradation of the retained mutant protein is significantly accelerated when all subcellular compartments are buffered at pH 6. In contrast, degradation is virtually abolished when intravesicular compartments are buffered at pH 8. However, despite this pH sensitivity neither lysosomotrophic amines, leupeptin, or leucine methyl ester have an apparent effect on the intracellular removal of the PiZ variant. The inability of a variety of inhibitors of ER-to-Golgi protein trafficking to hinder the degradative process suggests that degradation of the PiZ variant occurs prior to its delivery to the Golgi complex. To biochemically map the subcellular site of the degradation of the retained mutant protein, a recombinant truncated PiZ variant containing the tetrapeptide KDEL at its carboxyl terminus (a signal for sorting luminal proteins from a post-ER compartment back to the ER) was expressed in cells. Attachment of this ER-recycling signal to the recombinant protein prevented its intracellular degradation. These findings indicate that degradation of the PiZ variant occurs following its export from the ER.