Degradation of surface-labeled hepatoma membrane polypeptides: effect of inhibitors

N-terminal truncation of the scrapie-associated form of PrP by lysosomal protease(s): implications regarding the site of conversion of PrP to the protease-resistant state

Scrapie and related transmissible spongiform encephalopathies result in the accumulation of a protease-resistant form of an endogenous brain protein called PrP. As an approach to understanding the scrapie-associated modification of PrP, we have studied the processing and sedimentation properties of protease-resistant PrP (PrP-res) in scrapie-infected mouse neuroblastoma cells. Like brain-derived PrP-res, the neuroblastoma cell PrP-res aggregated in detergent lysates, providing evidence that the tendency to aggregate is an intrinsic property of PrP-res and not merely a secondary consequence of degenerative brain pathology. The PrP-res species had lower apparent molecular masses than the normal, protease-sensitive PrP species and were not affected by moderate treatments with proteinase K. This suggested that the PrP-res species were partially proteolyzed by the neuroblastoma cells. Immunoblot analysis of PrP-res with a panel of monospecific anti-PrP peptide sera confirmed that the PrP-res species were quantitatively truncated at the N terminus. The metabolic labeling of PrP-res in serum-free medium did not prevent the proteolysis of PrP-res, showing that the protease(s) involved was cellular rather than serum-derived. The PrP-res truncation was inhibited in intact cells by leupeptin and NH4Cl. This provided evidence that a lysosomal protease(s) was involved, and therefore, that PrP-res was translocated to lysosomes. When considered with other studies, these results imply that the conversion of PrP to the protease-resistant state occurs in the plasma membrane or along an endocytic pathway before PrP-res is exposed to endosomal and lysosomal proteases.

Turnover and compartmentation of gp70/p15 E in psi 2 cells

The distribution of Moloney murine leukemia virus gp70/p15 E between cell surface and intracellular compartments and the kinetics of transfer between these compartments was examined in psi 2 cells. A novel biotin derivatization and recovery assay was used to quantitate pulse-labeled protein accessible at 4 degrees C (cell surface), 18 degrees C (cell surface and an intracellular compartment), or inaccessible at any temperature. Cell surface (4 degrees C) gp70 and p15 E turn over more rapidly than intracellular pools of these proteins. The decrease in cell surface gp70 and p15 E after one hour of chase is accounted for by an increase in that which is inaccessible to biotinyl reagent.

Enhanced degradation of oxidized glutamine synthetase in vitro and after microinjection into hepatoma cells

Mixed-function oxidation of Escherichia coli glutamine synthetase has previously been suggested to mark the enzyme for intracellular degradation, and in vitro studies have demonstrated that oxidation renders the enzyme susceptible to proteolytic attack. In this study, the susceptibility of glutamine synthetase to degradation by purified proteases has been compared with the rate of degradation after microinjection into hepatoma cells. Upon exposure to an ascorbate mixed-function oxidation system the enzyme rapidly loses most of its activity, but further oxidation is required to cause susceptibility to extensive proteolytic attack either by a high-molecular-weight liver cysteine proteinase or by trypsin. The rate of degradation of biosynthetically 14C-labeled native and oxidized glutamine synthetase preparations after injection into hepatoma cells parallels their susceptibility to proteolysis in vitro. Native enzyme preparations and enzyme oxidatively inactivated, but not susceptible to extensive degradation by purified proteases, had similar intracellular half-lives; however, oxidized enzyme preparations that were susceptible to proteolytic breakdown in vitro were degraded almost ten times faster than the native enzyme within the growing hepatoma cells. These results suggest that the same features of the oxidized enzyme that render it susceptible to proteolysis in vitro are also recognized by the intracellular degradation system. In addition, they show that loss of enzyme activity does not necessarily imply decreased metabolic stability.

Sendai-viral HN and F glycoproteins as probes of plasma-membrane protein catabolism in HTC cells. Studies with fusogenic reconstituted Sendai-viral envelopes

Reconstituted Sendai-viral envelopes (RSVE) were produced by the method of Vainstein, Hershkovitz, Israel & Loyter [(1984) Biochim. Biophys. Acta 773, 181-188]. RSVE are fusogenic unilamellar vesicles containing two transmembrane glycoproteins: the HN (haemagglutinin-neuraminidase) protein and the F (fusion) factor. The fate of the viral proteins after fusion-mediated transplantation of RSVE into hepatoma (HTC) cell plasma membranes was studied to probe plasma-membrane protein degradation. Both protein species are degraded at similar, relatively slow, rates (t1/2 = 67 h) in HTC cells fused with RSVE in suspension. Even slower degradation rates for HN and F proteins (t1/2 = 93 h) were measured when RSVE were fused with HTC cells in monolayer. Lysosomal degradation of the transplanted viral proteins is strongly implicated by the finding that degradation of HN and F proteins is sensitive to inhibition by 10 mM-NH4Cl (81%) and by 50 micrograms of leupeptin/ml (70%).

Degradation of exogenous membrane proteins implanted into the plasma membrane of cultured hepatoma cells

The degradation of radiolabeled red cell band 3 and Sendai envelope proteins was studied after band 3 virosomes were fused with hepatoma cells as previously described (Hare, J E & Huston, M, Exp cell res 161 (1986) 317) [26]. 125I-band 3 (T1/2 = 13-14 h), Sendai HN (T1/2 = 37-40 h), and F (T1/2 = 21-23 h) envelope proteins were degraded by an apparent first-order process that was greater than 90% sensitive to 20 mM NH4Cl. 125I-Sendai envelope proteins were degraded at approximately similar rates when hepatoma cells were fused with intact virus, isolated viral membrane, or band 3 virosomes. There thus appears to be distinct heterogeneity among the degradation rates of implanted polypeptides dependent on structural aspects of each. To identify the subcellular site of membrane protein degradation, band 3 was labeled with membrane impermeant [14C]sucrose and implanted into hepatoma plasma membranes. After replating, trichloroacetic acid (TCA)-soluble label was found to accumulate in the lysosomal compartment of fractionated cells. The results identify the lysosome as the ultimate site of plasma membrane protein degradation, but suggest that plasma membrane proteins are selectively rather than non-selectively delivered to this compartment.