Regulation of the ubiquitin-mediated proteolytic pathway: role of the substrate alpha-NH2 group and of transfer RNA

The N-end rule pathway catalyzes a major fraction of the protein degradation in skeletal muscle

In skeletal muscle, overall protein degradation involves the ubiquitin-proteasome system. One property of a protein that leads to rapid ubiquitin-dependent degradation is the presence of a basic, acidic, or bulky hydrophobic residue at its N terminus. However, in normal cells, substrates for this N-end rule pathway, which involves ubiquitin carrier protein (E2) E214k and ubiquitin-protein ligase (E3) E3alpha, have remained unclear. Surprisingly, in soluble extracts of rabbit muscle, we found that competitive inhibitors of E3alpha markedly inhibited the 125I-ubiquitin conjugation and ATP-dependent degradation of endogenous proteins. These inhibitors appear to selectively inhibit E3alpha, since they blocked degradation of 125I-lysozyme, a model N-end rule substrate, but did not affect the degradation of proteins whose ubiquitination involved other E3s. The addition of several E2s or E3alpha to the muscle extracts stimulated overall proteolysis and ubiquitination, but only the stimulation by E3alpha or E214k was sensitive to these inhibitors. A similar general inhibition of ubiquitin conjugation to endogenous proteins was observed with a dominant negative inhibitor of E214k. Certain substrates of the N-end rule pathway are degraded after their tRNA-dependent arginylation. We found that adding RNase A to muscle extracts reduced the ATP-dependent proteolysis of endogenous proteins, and supplying tRNA partially restored this process. Finally, although in muscle extracts the N-end rule pathway catalyzes most ubiquitin conjugation, it makes only a minor contribution to overall protein ubiquitination in HeLa cell extracts.

A network theory of ageing: the interactions of defective mitochondria, aberrant proteins, free radicals and scavengers in the ageing process

Evolution theory indicates that ageing is caused by progressive accumulation of defects, since the evolutionary optimal level of maintenance is always below the minimum required for indefinite survival. Evolutionary theories also suggest that multiple processes are operating in parallel, but unfortunately they make no predictions about specific mechanisms. To understand and evaluate the many different mechanistic theories of ageing which have been proposed, it is therefore important to understand and study the network of maintenance processes which control cellular homeostasis. In this paper we describe a Network Theory of Ageing which integrates the contributions of defective mitochondria, aberrant proteins, and free radicals to the ageing process, and which includes the protective effects of antioxidant enzymes and proteolytic scavengers. The model simulations not only confirm and explain many experimental, age related findings like an increase in the fraction of inactive proteins, a significant rise in protein half-life, an increase in the amount of damaged mitochondria, and a drop in the energy generation per mitochondrion, but they also show interactions between the different theories which could not have been observed without the network approach. In some simulations, for example, the mechanism of the final breakdown seems to be a consequence of the cooperation of mitochondrial and cytoplasmic reactions, the mitochondria being responsible for a long term, gradual change which eventually triggers a short lived cytoplasmic error loop.

The culture of rat lenses in high sugar media: effect on mixed disulfide levels

Lens proteins are long lived proteins with those in the center of the lens predating the birth of the individual. As a result, they are subject to a host of modifications and damage through a variety of mechanisms. Two such modifications have been proposed as primary events which could cause conformational changes potentiating further modifications. These are non-enzymatic glycation and mixed disulfide formation. Human lenses accumulate protein-thiol mixed disulfides of three kinds throughout the lifespan. The presence of one of these, protein-glutathione (PSSG) mixed disulfide has been shown to be intimately involved in protein aggregation. We have utilized ex vivo lens culture and in vitro incubations of purified gamma-crystallin to evaluate the following hypotheses. A) Lenses cultured with a high sugar media will form higher mixed disulfide levels than controls; B) glycation of lens proteins will be dependent on initial mixed disulfide level. Xylose levels in the cultured lens rise rapidly (to 23 mM by 4 h), and the level of glycation after one week is elevated 6-7% over control values. Mixed disulfide levels are also substantially increased but not more than for lenses cultured in control media. gamma-Crystallin modified with 0, 1, or 5 equivalents of GSH was differentially glycated by radioactive fructose. The amount of fructose bound by the protein was found to be inversely related to the extent of mixed disulfide formation. These results indicate that 1) protein modification of one kind may influence further modifications of other types; 2) glycation of lens proteins has no effect on mixed disulfide formation in this system; 3) the sulfhydryl status of lens proteins can affect the potential for protein glycation.

The major RNA in prosomes of HeLa cells and duck erythroblasts is tRNA(Lys,3)

Two-dimensional gel electrophoresis of HeLa cell prosomal RNAs, 3'-end labeled by RNA ligase, revealed one prominent spot. Determination of a partial sequence at the 3'-end indicated full homology to the 18 nucleotides at the 3'-end of tRNA(Lys,3) from rabbit, the bovine and the human species. An oligonucleotide complementary to the 3'-end of tRNA(Lys,3) hybridized on Northern blots with prosomal RNA from both HeLa cells and duck erythroblasts. In two-dimensional PAGE, the major pRNA of HeLa cells co-migrated with bovine tRNA(Lys,3). Reconstitution of the CCA 3'-end of RNA from both human and duck prosomes, by tRNA-nucleotidyl-transferase, confirmed the tRNA character of this type of RNA. Furthermore, it revealed at least one additional tRNA band about 85 nt long among the prosomal RNA from both species. Finally, confirming an original property of prosomal RNA, we show that in vitro synthesized tRNA(Lys,3) hybridizes stably to duck globin mRNA, and to poly(A)(+)- and poly(A)(-)-RNA from HeLa cells.

Organ-specific and non-organ-specific lymphocyte receptors for vascular endothelium

The recirculation of lymphocytes from blood to lymph and back to blood is necessary for the proper functioning of the immune system as it facilitates interactions between antigen-reactive clones of lymphocytes and antigen-presenting cells. The first step in the emigration of a blood-borne lymphocyte into either a secondary lymphoid organ or an inflammatory lesion is its adherence to vascular endothelial cells (EC) lining unique post-capillary venules known as high endothelial venules (HEV). Several groups have recently cloned and sequenced genes which may encode organ-specific lymphocyte receptors for the EC of such HEV. The extracellular portion of the putative murine lymphocyte homing receptor for peripheral lymph node HEV is composed of an N-terminal lectin-like domain, followed by an epidermal growth factor-like domain, and then two identical repeating domains which are homologous to a number of complement-binding proteins. A hydrophobic transmembrane domain and a cytoplasmic tail complete the structure. A very similar gene structure has been reported for a cytokine-inducible EC surface protein which is involved in neutrophil-EC adhesion in vitro. In marked contrast, the gene for a putative human lymphocyte homing receptor appears to belong to a gene family which encodes cell-surface molecules with receptor activity for extracellular matrix (ECM) proteins. Similarly, the cell-surface molecule which appears to be the murine lymphocyte receptor for Peyer's patch HEV is homologous, if not identical, to the human VLA-4 molecule, another receptor with binding activity for an ECM protein. It has also been demonstrated that lymphocyte function-associated antigen 1 (LFA-1) acts in a non-organ-specific manner to mediate lymphocyte-EC adhesion. Finally, other non-organ-specific lymphocyte adhesion molecules for EC may include CD4 and CD8 (which bind to class II and class I MHC antigens, respectively), and CD2 (which binds to LFA-3).

Lymphocyte adhesion to endothelial cells in vitro: models for the study of normal lymphocyte recirculation and lymphocyte emigration into chronic inflammatory lesions

Lymphocytes preferentially leave the bloodstream to enter secondary lymphoid organs or sites of inflammation by first adhering to, and then migrating through, the walls of specialized postcapillary venules known as high endothelial venules. To study the initial adhesion event between lymphocytes and endothelial cells, two different in vitro assays have been developed. In the first, lymphocytes are incubated on frozen sections of lymphoid organs or other tissues. Under certain conditions, lymphocytes specifically bind to the endothelial cells of high endothelial venules in such sections. The results of monoclonal antibody inhibition studies have suggested that endothelial cells at various lymphoid organs, and at certain inflammatory lesions, may express organ-specific ligands on their surface, which bind to corresponding organ-specific "homing receptors" on the lymphocytes. The second assay measures the adhesion of lymphocytes to confluent monolayers of viable, cultured endothelial cells. Because pretreatment of such cell monolayers with a number of cytokines produced at inflammatory sites stimulates an increase in the adhesiveness of the cells for lymphocytes, it has been suggested that such cytokine-induced increases in endothelial cell adhesiveness may be important in the induction of lymphocyte traffic into such lesions. Unlike the homing receptor type of binding described above, the cytokine-induced increase in lymphocyte-endothelial cell adhesion is presumably non-tissue-specific. Results of monoclonal antibody inhibition studies in this system have suggested that at least two ligand-receptor interactions may be involved. Monoclonal antibodies to the lymphocyte function-associated antigen-1 greatly inhibited the adhesion of T cells to untreated endothelial cells, but had little or no inhibitory effect on T-cell adhesion to cytokine-treated endothelial cells.

High-molecular-mass proteinases in rabbit reticulocytes: the multicatalytic proteinase is an ATP-independent enzyme and ATP-activated proteolysis is in part associated with a cysteine proteinase complexed to alpha 1-macroglobulin

We have investigated the proteolytic degradation of [14C]methylcasein and 125I-labeled bovine serum albumin at pH 7.8 and 37 degrees C by lysates of rabbit reticulocytes purified from rabbit blood by two different procedures. (I) Lysates obtained from reticulocytes after removal of plasma and buffy coat as well as after washing of cells, degraded casein and albumin, and released from the two substrates 1.3%/h and 0.4%/h, respectively, of acid-soluble radioactivity. The activity towards both substrates was stimulated about 4-fold by ATP/Mg2+. Chromatography of whole blood on a column of cellulose prior to washing and lysis of cells had profound but differential effects on these activities in that stimulation of casein-degradation by ATP/Mg2+ was almost completely lost, whereas degradation of albumin, albeit at a low rate, was measurable in the presence of ATP/Mg2+ only. (II) Degradation of casein by these lysates is largely inhibited by a monospecific antibody against rabbit multicatalytic proteinase, whereas digestion of albumin is not affected by the antibody, either in the presence or absence of ATP/Mg2+. The latter activity is partially inhibited by a specific antibody against rabbit alpha 1-macroglobulin. (III) The immunoreactive amount of multicatalytic proteinase is about 1.2 micrograms per mg of lysate protein and almost identical in the two lysates. In contrast, the immunologically detectable levels of alpha 1-macroglobulin vary and are much lower in reticulocyte-lysates following chromatography on cellulose than in lysates from washed reticulocytes. (IV) Caseinolytic activity of multicatalytic proteinase, purified from rabbit reticulocyte lysate, is not activated by ATP/Mg2+ and the enzyme is proteolytically inactive towards albumin. On the other hand, a complex consisting of the proteinase inhibitor alpha 1-macroglobulin and the cysteine proteinase, cathepsin B, does degrade both substrates at pH 7.8, in an ATP/Mg2+-activated fashion. From these results it is concluded that the multicatalytic proteinase is an ATP-independent enzyme and a cellular constituent of rabbit reticulocytes whereas the activity stimulated by ATP/Mg2+ appears to be associated, at least in part, with a cysteine proteinase complexed to alpha 1-macroglobulin.

IL1/ETAF activity and undetectable IL1 beta mRNA and minimal IL1 alpha mRNA levels in normal adult heat-separated epidermis

Interleukin 1 (IL 1), a cytokine expressed from a number of cell types, plays a vital part in immune and inflammatory processes. Epidermal IL1-like activity, also known as epidermal cell thymocyte-activating factor (ETAF), is associated with whole skin, epidermis, Langerhans cells and cultured keratinocytes and keratinocyte cell lines. We show that preparations of heat-separated epidermis possess abundant IL1/ETAF activity as indicated by PGE/collagenase induction in cultured synovial cells and proliferation of mouse thymocytes. However, using cDNA probes to interleukin-1 alpha and beta we show that freshly isolated heat-separated epidermis from healthy adult donors contains only a slight detectable level of IL1 alpha mRNA by dot blot analysis and no IL1 beta and IL1 alpha mRNA by Northern blot analysis. The implications of these observations will be discussed.

The 'molten globule' state is involved in the translocation of proteins across membranes?

Strong evidence exists that the translocation of proteins across a variety of membranes involves a non-native or denatured conformational states. On the other hand a compact state having secondary but not rigid tertiary structure and called the 'molten globule' state has been identified as being stable under mild denaturing conditions. A similar state has been shown to accumulate on the folding pathway of globular proteins. These states are compact though sufficiently expanded to include water, and they are internally mobile. It is proposed that these molten globule states may be suitable candidates for protein translocation across biological membranes.