One of the major ways of degrading cytosolic and nuclear proteins involves the covalent attachment of ubiquitin residues to these proteins. Although ubiquitination of proteins is frequently linked to the degradation of these proteins in proteasomes, proteins can be ubiquitinated in a number of ways, each form of ubiquitination serving a different function. In the context of signal transduction, two distinct types of ubiquitination mediate signal attenuation on the one hand and signal generation on the other.
Ubiquitin is a 76-amino acid protein that is activated in an ATP-dependent fashion by an E1 enzyme, then transferred to an E2 enzyme, which then covalently attaches the activated ubiquitin to lysine residues on specific substrates that are recognized by specific E3 ubiquitin ligases. In many cases, after the C terminus of a ubiquitin moiety is covalently linked to a lysine residue on a target protein, the C-terminal ends of subsequent ubiquitin moieties may be covalently attached to lysine residues on the preceding ubiquitin to generate a polyubiquitin chain. The shape of the polyubiquitin chain is different depending on which specific lysine residue on the preceding ubiquitin molecule in the chain is the site for covalent binding of the next ubiquitin molecule, and this has important functional con sequences. If lysine in position 48 of the first ubiquitin moiety forms an isopeptide bond with the C terminus of the next ubiquitin and so on, a lysine-48 type of ubiquitin chain will be generated that can be recognized by the proteasomal cap, and the ubiquitinated protein will be targeted for degradation in the proteasome. Some E3 ligases generate a different type of polyubiquitin chain, which does not target proteins for degradation but instead generates a structure for latching the marked proteins onto other specific proteins; this is important in NF-κB signaling.
Several E3 ligases are found in T cells, some of which are involved in signal transduction (such as in NF-κB signaling) and others in signal attenuation. The prototype of E3 ligases involved in terminating T-cell responses is CBL-b, but several others serve similar functions. Recruitment of CBL-b to the TCR complex and associated adaptor proteins leads to the monoubiquitination, endocytosis, and lysosomal degradation of the TCR complex, and this may be a mechanism for the attenuation of TCR signaling (Fig. 1). CD28 signals block the inhibitory activity of CBL-b, and this is one mechanism by which costimulation augments TCR signals. In knockout mice lacking CBL-b, the T cells respond to antigens even without CD28-mediated costimulation and produce abnormally high amounts of IL-2. These mice develop autoimmunity as a result of the excessive activation of their T cells. There is some evidence that antigens that shut off immune responses (so-called tolerogenic antigens, such as self antigens) activate ubiquitin ligases in T cells that degrade essential signaling proteins, and this is a mechanism of antigen-induced unresponsiveness called anergy.
Fig1. Role of the ubiquitin (Ub) ligase CBLb in terminating T-cell responses. CBLb is recruited to the TCR complex, where it facilitates the monoubiquitination of CD3, ZAP70, and other proteins of the TCR com plex. These proteins are targeted for proteolytic degradation in lysosomes and other organelles (not shown).
