Proteolysis of puromycin-peptides in rabbit reticulocytes: detection of a high molecular weight oligopeptide proteolytic substrate

Bovine lens epithelial cells have a ubiquitin-dependent proteolysis system

Lens cells must remove obsolete or damaged proteins produced during development, maturation and aging to maintain lens transparency. In reticulocytes removal of abnormal or obsolete proteins is thought to involve a ubiquitin-dependent proteolytic pathway. Two hallmarks of ubiquitin (Ub) dependent proteolysis have previously been demonstrated in lens cell or tissue supernatants: (1) the presence of ubiquitin conjugates, and (2) ATP-dependent proteolysis. Nevertheless, conclusive proof was lacking of a requirement for ubiquitination of substrate proteins for proteolysis. Here we show that in bovine lens epithelial cell (BLEC) supernatant, ATP-dependent proteolysis is also ubiquitin-dependent. Ubiquitin-activating enzyme (E1), the first enzyme in the cascade of ubiquitin ligation, was purified over 3000-fold from a rabbit reticulocyte lysate using Ubiquitin-Sepharose, and showed ATP-PPi exchange activity. Antiserum to E1 was prepared in goats and affinity-purified on Protein G-Sepharose. Western blot analysis revealed that both the goat antiserum and purified antibody (anti-E1(IgG)) recognize specifically E1. Anti-E1(IgG) inhibits 86% of the ATP-dependent degradation of labeled histone H2A in reticulocyte lysate and 75% of the ATP-dependent degradation in BLEC. Upon reconstitution with purified E1, 100% and 80% of proteolysis was restored in reticulocytes and BLEC supernatant, respectively. This confirms that there is a ubiquitin-dependent proteolytic system in lens.

Effects of aging in vitro on intracellular proteolysis in cultured rabbit lens epithelial cells in the presence and absence of serum

Alterations in proteolytic capabilities have been associated with abnormalities in the aged eye lens, but in vivo tests of this hypothesis have been difficult to pursue. To simulate aging, we cultured cells from an 8-yr-old rabbit to early (population-doubling level 20 to 30) and late (population-doubling level greater than 125) passage. Long-lived (t1/2 greater than 10 h) and short-lived (t1/2 less than 10 h) intracellular proteins were labeled with [3H]leucine, and the ability of the cells to mount a proteolytic response to the stress of serum withdrawal was determined. For early passage cells, the average t1/2 of long-lived proteins in the presence and absence of serum was 62 and 39 h, respectively. For late-passage cells, the average t1/2 of long-lived proteins in the presence and absence of serum was 58 and 43 h, respectively. The net increase in intracellular proteolysis in the absence of serum was 59 and 35% for early and late-passage cells, respectively. Thus, in vitro-aged rabbit lens epithelial cells amount only 60% the proteolytic response to serum removal shown in "younger" cells. The enhanced ability of early passage cells to respond to serum removal seems to involve lower homeostatic levels of proteolysis in the presence of serum and greater enhancement of proteolysis in the absence of serum. Less than 2% of the protein is in the pool of short-lived proteins. Rates of proteolysis of short-lived proteins in the presence and absence of serum were indistinguishable. With respect to basal proteolytic rates in the presence of serum and ability to mount a proteolytic response upon serum withdrawal, these rabbit lens epithelial cells are similar to bovine lens epithelial cells and fibroblasts.

Abnormal proteins of shortened length are preferentially degraded in the cytosol of cultured MRC5 fibroblasts

Puromycyl peptides were degraded in MRC5 fibroblasts more rapidly than normal proteins labelled for the corresponding length of time for both long and short labelling periods. The degradation of the puromycyl peptides occurred almost exclusively in the cytosol of the cells. Even when the half-lives of normal and puromycyl peptides were manipulated to be similar, proportionally more of the normal proteins were degraded in the lysosomes. The rapid degradation of the puromycyl peptides was not due to the inhibition of protein synthesis brought about by puromycin but was due to the structure of the substrates themselves. The degree and intracellular site of degradation of puromycyl peptides closely mimic those of abnormal (missense) proteins containing amino acid analogues.

Changes in proteinase and peptidase activities during reticulocyte maturation

The ability of rabbit reticulocytes to degrade puromycin-peptides and aminoethylcysteine-induced aberrant polypeptides decreased during cellular maturation. Cell-free studies indicate that the fall in proteolytic activity is not a consequence of accumulation of proteinase inhibitors or the conversion of all of the abnormal protein into undegradable forms. A decrease in peptidase activity using seven dipeptides and one tripeptide as substrates was also found to accompany reticulocyte maturation. Addition of the aminopeptidase B inhibitor bestatin to reticulocyte extracts did not inhibit the conversion of acid-precipitable puromycin-peptides to acid-soluble products; bestatin did induce the accumulation of very low molecular weight material (possibly di- or tripeptides) within the acid soluble fraction.

Subcellular distribution of abnormal proteins in rabbit reticulocytes. Effects of cellular maturation, phenylhydrazine and inhibitors of ATP synthesis

Inhibitors of ATP synthesis (cyanide, dinitrophenol, fluoride) inhibited proteolysis of pulse-labelled abnormal proteins in rabbit reticulocytes and caused an accumulation of the aberrant polypeptides in the cell debris fraction in a manner analogous to phenylhydrazine-induced Heinz bodies. When the reticulocytes were separated into age-groups by sedimentation through discontinuous gradients of bovine serum albumin, the ability of the cells to degrade puromycin peptides decreased with increasing cellular maturity and, in the more mature cells, up to 40% of the labelled abnormal polypeptide remained associated with the cell debris fraction at the end of the chase period. It is suggested that the association of the abnormal polypeptide with the cell debris fraction is a consequence of a maturation-induced loss, or an inhibitor-induced inactivation of the cellular proteolytic activity.

Effects of ATP and cell development on the metabolism of high molecular weight aggregates of abnormal proteins in rabbit reticulocytes and cell-free extracts

Abnormal proteins synthesized in rabbit reticulocytes in response to (i) the lysine analogue aminoethylcysteine and (ii) puromycin, form high molecular weight aggregates prior to degradation. Inhibitors of ATP synthesis partially inhibit catabolism of the aminoethylcysteine-induced abnormal protein; degradation of puromycin peptides synthesized after incubation with 25 micrograms/ml puromycin was not inhibited. Catabolism of the analogue-induced high molecular weight aggregate of abnormal protein in cell-free extracts was markedly stimulated by ATP, whereas proteolysis of the aggregated puromycin-peptides was ATP-independent. The ability of the reticulocytes to degrade the puromycin-peptide aggregates was found to decrease with cellular maturity. It is suggested that the energy-dependency for proteolysis is in some way related to the chain length of the abnormal protein synthesized.

ATP-independent proteolysis of globin cyanogen bromide peptides in rabbit reticulocyte cell-free extracts

The catabolism of two rabbit globin cyanogen bromide peptides in cell-free extracts of ATP-depleted rabbit reticulocytes has been studied. Proteolysis of the peptides (3533 and 5957 molecular weight) proceeded rapidly in the absence of ATP, had a pH optimum of approximately 7.8, and was inhibited by omicron-phenanthroline, N-ethylmaleimide, cystamine zinc and cobalt ions, and puromycin peptide high-molecular-weight aggregates. Proteolysis of puromycin peptides was inhibited by the rabbit globin cyanogen bromide peptides. The ability of cell-free extracts of degrade the globin cyanogen bromide peptides decreased with the reticulocyte maturity. Blocking the globin cyanogen bromide peptide amino groups by succinic and maleic anhydride treatment decreased susceptibility to degradation. It is suggested that the globin cyanogen bromide peptides might provide model substrates, replacing puromycin peptides, for the investigation of ATP-independent proteolytic events.

Hemin inhibits ATP-dependent ubiquitin-dependent proteolysis: role of hemin in regulating ubiquitin conjugate degradation

Hemin has been shown to inhibit specifically the energy-dependent degradation of normal and abnormal proteins in reticulocytes [Etlinger, J. D. & Goldberg, A. L. (1980) J. Biol. Chem. 255, 4563-4568]. The present work demonstrates that the action of hemin involves the multi-enzyme ATP-dependent ubiquitin-dependent proteolytic system exclusively. At a concentration of approximately 25 microM, hemin produces 50% inhibition of the degradation of 125I-labeled bovine serum albumin by this pathway. Hemin has no effect on the basal rate of proteolysis in the absence of either ATP or ubiquitin. At a concentration of hemin that gives complete inhibition of proteolysis, ATP-dependent formation of ubiquitin conjugates continues at about 50% of the control rate but the degradation of these ubiquitin conjugates is completely blocked. Inhibition of overall proteolysis and conjugate degradation are sensitive to hemin concentration to exactly the same extent. Hemin inhibition of conjugate breakdown results in the accumulation of higher molecular weight conjugates that are lost when hemin is removed by dilution. A model is proposed in which hemin acts as a negative allosteric effector in the initial step of a sequential degradative path by which intact ubiquitin conjugates are first cleaved to ubiquitin-associated fragments.