The ultimate mechanism that cells use to ensure the quality of intracellular proteins is the selective destruction of misfolded or damaged polypeptides. In eukaryotic cells, the large ATP-dependent proteolytic machine, the 26S proteasome, prevents the accumulation of non-functional, potentially toxic proteins. This process is of particular importance in protecting cells against harsh conditions (for example, heat shock or oxidative stress) and in a variety of diseases (for example, cystic fibrosis and the major neurodegenerative diseases). A full understanding of the pathogenesis of the protein-folding diseases will require greater knowledge of how misfolded proteins are recognized and selectively degraded.

Protein degradation and protection against misfolded or damaged proteins

Cellular defenses against unfolded proteins: a cell biologist thinks about neurodegenerative diseases.

Formation of nucleoplasmic protein aggregates impairs nuclear function in response to SiO2 nanoparticles.

Oxygen radicals stimulate intracellular proteolysis and lipid peroxidation by independent mechanisms in erythrocytes.

Immunochemical analysis of the turnover of ubiquitin-protein conjugates in intact cells. Relationship to the breakdown of abnormal proteins.

Properties of abnormal proteins degraded rapidly in reticulocytes. Intracellular aggregation of the globin molecules prior to hydrolysis.

Publisher Summary This chapter focuses on the selective degradation of abnormal proteins in animal and bacterial cells. The rates of degradation of individual proteins within cells differ markedly, and this diversity in protein half-lives results, in large part, from the differences in protein conformations. The strongest evidence that protein structure determines half-lives comes from the study of intracellular proteins with highly abnormal conformations. Extensive studies have also been carried out by many investigators on proteins of normal size with highly abnormal conformations. Although, normal tetrameric hemoglobin undergoes little if any degradation, several abnormal human hemoglobins resulting from missense mutations and even the excessive α-chains produced in patients with β-thalassemia appear to be rapidly degraded in reticulocytes. The association of the analog-containing proteins into the rapidly sedimenting structures appears to be closely linked to, perhaps even essential for, the degradative process.

Selective degradation of abnormal proteins in animal and bacterial cells

Catalytic and conformational properties of cross-linked derivatives of penicillinase.

Penicillinase (beta-lactamase I, EC 3.5.2.6) secreted by Bacillus cereus, strain 569/H, was covalently attached to aminoethyl cellulose via glutaraldehyde. The immobilized derivative shows increased thermostability and decreased susceptibility to conformational changes induced by certain substrates of penicillinase. The decline in the rate of hydrolysis of such substrates was consequently suppressed by immobilization. A marked increase in Km was observed with all substrates except for the unsubstituted 6-aminopenicillanic acid. The altered properties of the new derivative are attributed to the constraint imposed by immobilization on the conformational flexibility of the enzyme molecule. Thus, apart from obvious technological interest, immobilized penicillinase provides a useful model for the study of the role of flexibility in the function of an enzyme.

Preparation and properties of an immobilized derivative of penicillinase.

Exopenicillinase of Bacillus cereus 569/H was cross-linked with toluene 2,4-diisocyanate in the presence of cephalothin, cloxacillin or no substrate. The derivatives show significant differences in susceptibility to inactivation by heat, urea, iodination or proteolysis. Such differences can be predicted from the contrasting effects of these substrates on the conformation of the enzyme.

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Yoel Klemes

Papers

Properties of abnormal proteins degraded rapidly in reticulocytes. Intracellular aggregation of the globin molecules prior to hydrolysis.

Selective degradation of abnormal proteins in animal and bacterial cells

Catalytic and conformational properties of cross-linked derivatives of penicillinase.

Preparation and properties of an immobilized derivative of penicillinase.

Cross-linking preserves conformational changes induced in penicillinase by its substrates.