Fig. 7.6 Signaling must be turned off as well as turned on. The inability to terminate a signaling pathway can result in serious diseases such as autoimmunity or cancer. as a significant proportion of signaling events depend on protein phosphorylation, protein phosphatases, such as Shp, have an important part in shutting down signaling pathways (left panel). another common mechanism for terminating signaling is regulated protein degradation (center and right panels). phosphorylated proteins recruit ubiquitin ligases, such as cbl, that add the small protein ubiquitin to proteins, thus targeting them for degradation. cytoplasmic proteins are targeted for destruction in the proteasome by the addition of polyubiquitin chains, linked through lysine 48 (K48) of ubiquitin (center panel). membrane receptors that become ubiquitinated by individual ubiquitin molecules or di-ubiquitin are internalized and transported to the lysosome for destruction (right panel).
First, an E1 ubiquitin-activating enzyme promotes attachment of ubiquitin to an E2 ubiquitin-conjugating enzyme. The ubiquitin is then transferred to the protein substrate by an enzyme known as an E3 ubiquitin ligase. Ubiquitin ligases can continue to add ubiquitin molecules to form polyubiquitin. Importantly, different ubiquitin ligases add the carboxy terminus of one ubiquitin molecule to different lysine residues of the conjugated ubiquitin, typically either lysine 48 (K48) or lysine 63 (K63). These different forms of polyubiquitin produce divergent outcomes for signaling pathways.
When polyubiquitin chains are formed using K48 linkages, the outcome is to target the protein for degradation by the proteasome (see Fig 7.6). An important ubiquitin ligase of this kind in lymphocytes is Cbl, which selects its targets via its SH2 domain. Cbl can thus bind to specific tyrosine-phosphorylated targets, causing them to become ubiquitinated via K48 linkages. Proteins that recognize this form of polyubiquitin then target the ubiquitinated proteins to degradative pathways via the proteasome. Membrane proteins such as receptors can be tagged by single ubiquitin molecules or by di-ubiquitin. These are not recognized by the proteasome, but instead, are recognized by specific ubiquitin-binding proteins that target proteins for degradation in lysosomes (see Fig. 7.6). Thus, ubiquitination of proteins can inhibit signaling. Unlike phosphatases, where the mechanism of inhibition is reversible, inhibition by ubiquitin-mediated protein degradation is a more permanent means of terminating signaling.
Ubiquitination can also be used to activate signaling pathways. We have already discussed this aspect in Section 3-7 in connection with the NFκB signaling pathway from TLRs. There, the ubiquitin ligase TRAF-6 produces K63-linked polyubiquitin chains on TRAF-6 and NEMO. In lymphocytes, K63-linked polyubiquitination is a key step in signaling through tumor necrosis factor (TNF) receptor family members, as will be discussed in Section 7-23 (and Fig. 7.31). This form of polyubiquitin is recognized by specific domains in signaling proteins that recruit additional signaling molecules to the pathway (see Fig. 3.15).
7-6 The activation of some receptors generates small-molecule second messengers.