Principles of cell signaling - UT Southwestern

More documents

Recommendations

Info

39057_ch14_cellbio.qxd 8/28/06 5:11 PM Page 626 gen. Small t displaces the regulatory subunit from PP2A and alters the activity and the subcellular localization of the phosphatase. In addition, natural toxins such as okadaic acid, calyculin, and microcystin inhibit PP2A and PP1 to varying extents both in vitro and in intact cells. Another major protein-serine/threonine phosphatase, called calcineurin (also known as phosphoprotein phosphatase 2B), is regulated by Ca2+-calmodulin (see 14.15 Ca2+ signaling serves diverse purposes in all eukaryotic cells) and plays essential roles in cardiac development and T cell activation, among other events. The major mechanism of action of the immunosuppressants cyclosporin and FK506 is to inhibit calcineurin. The protein tyrosine phosphatases (PTPs) are cysteine-dependent enzymes that utilize a conserved Cys-Xaa-Arg motif to hydrolyze phosphoester bonds in their substrates. The PTPs are encoded by over 100 genes in humans and are classified in four subfamilies: the phosphotyrosine-specific phosphatases, the Cdc25 phosphatases, the dual specificity phosphatases (DSPs), and the low molecular weight phosphatases. Thirty-eight of the PTPs are highly selective for phosphotyrosine residues within substrates. Some of the phosphotyrosine-selective phosphatases are transmembrane proteins, whereas others are membrane associated. The most obvious function of the PTPs is to reverse the functions of tyrosine kinases; however, some have primary functions in transducing tyrosine kinase signals. For example, the protein tyrosine phosphatase SHP2 (also known as SHPTP2), binds to certain tyrosine kinase receptors through its SH2 domain and is itself tyrosine phosphorylated, thereby creating a binding site for the SH2 domain-containing adaptor protein, Grb2, which leads to activation of Ras (see 14.32 MAPKs are central to many signaling pathways). The Cdc25 phosphatases recognize cyclindependent kinase (CDK) family members as substrates and play a critical role in increasing CDK activity at key junctures of the cell cycle (see Figure 14.39 and 11.4 The cell cycle is a cycle of CDK function). Similar to the dual specificity kinases, the dual specificity phosphatases are specific for a restricted number of substrates. A number of DSPs dephosphorylate MAPKs; these DSPs are called MAP kinase phosphatases, or MKPs. Several of these have been implicated in MAPK nuclear entry and exit. Some MKPs are encoded by early response genes, whose products are active near the initiation of the cell cycle (see 11.7 Entry into cell cycle and S phase is tightly regulated). Substrates of other PTP family members, such as the tumor suppressor PTEN, include phosphoinositides, which are phosphorylated derivatives of the glycerolipid phosphatidylinositol that serve as second messengers (see 14.16 Lipids and lipid-derived compounds are signaling molecules). Removal of the phosphate group inactivates the second messenger. It remains unclear whether members of this group work exclusively on phophoinositides or also on protein tyrosine phosphate. 14.28 Covalent modification by ubiquitin and ubiquitinlike proteins is another way of regulating protein function Key concepts • Ubiquitin and related small proteins, may be covalently attached to other proteins as a targeting signal. • Ubiquitin is recognized by diverse ubiquitin binding proteins. • Ubiquitination can cooperate with other covalent modifications. • Ubiquitination regulates signaling in addition to its role in protein degradation. An important mechanism for control of protein function is through covalent modification with small proteins of the ubiquitin family. Ubiquitin is one of a family of proteins referred to as ubiquitin-like (Ubl) proteins. Ubiquitin itself is highly conserved among species, suggesting the functional importance of all of its 76 residues. In addition to the long-established role of ubiquitin in initiating protein degradation, ubiquitin modification also has a variety of functions in signal transduction. Ubl proteins are conjugated to the substrate protein by an isopeptide bond between an amino group on the substrate, usually from a Lys side chain, and the C-terminal Gly residue of the processed Ubl protein. E1, E2, and E3 proteins are required to catalyze conjugation to Ubl proteins (see Biochem 4.3 Ubiquitin attachment to substrates requires multiple enzymes). Several Ubl proteins may be attached to one substrate, often by serial formation of a polyubiquitin chain. Mono- and polyubiquitination both change the 626 CHAPTER 14 Principles of cell signaling
39057_ch14_cellbio.qxd 8/28/06 5:11 PM Page 627 protein’s behavior to induce downstream signals. Monoubiquitination is a significant regulatory modification in vesicular trafficking and DNA repair. For example, the monoubiquitinated form of the FANCD2 protein becomes associated with the repair protein BRCA1 at sites of DNA repair. Modification by the Ubl protein SUMO has roles in nuclear transport, transcription, and cell cycle progression. Polyubiquitin chains are formed when the Lys residues of ubiquitin itself, particularly K48 and K63, are ubiquitinated. Addition of polyubiquitin with a K48 linkage generally directs proteins to the proteasome for degradation, whereas conjugation to polyubiquitin chains with a K63 linkage promotes signal transmission, not proteolysis. Protein-bound ubiquitin is recognized by a variety of ubiquitin binding domains, including UIM (ubiquitin-interacting motif), UBA (ubiquitin association), and certain zinc finger domains. Such domains have the capacity to act as receptors for ubiquitin within modified proteins. Activation of the transcription factor NF-κB occurs by a mechanism dependent on modification both by the addition of Ubl proteins and phosphorylation. This fascinating example of regulation by ubiquitin is depicted in FIGURE 14.33. Prior to stimulation, NF-κB is retained in the cytoplasm in an inactive form by binding to its inhibitor, IκB. Phosphorylation of IκB by the IκB kinase (IKK) complex promotes its recognition by a multisubunit E3 ligase, which directs its ubiquitination and subsequent proteasomal degradation. Destruction of IκB allows NF-κB to move to the nucleus to mediate changes in transcription. IκB can be stabilized in response to certain signals through covalent attachment of the Ubl, SUMO. Sumoylation occurs on the same Lys residues that must be conjugated to ubiquitin to achieve IκB degradation. Thus, SUMO attachment stabilizes IκB and attenuates NF-κB action. This is one of numerous examples of crosstalk between Ubl conjugates. A key regulatory event in NF-κB signaling is activation of the IKK complex. IKK is itself regulated by ubiquitination and phosphorylation. The cytokine interleukin-1β (IL-1) causes association of adaptor proteins with its receptor to create a receptor activation complex. The interleukin-1β receptor activation complex recruits another adaptor complex containing TRAF6. A phosphorylation event releases a TRAF6 complex from the receptor activation complex into the cytoplasm. TRAF6 contains a RING domain, and is an E3 ubiquitin ligase that catalyzes formation of Modification with Ubl proteins plays multiple roles in IL-1β signaling TRAF6 TRAF6 TRAF6 IKK IL-1β TRAF6 TRAF6 K63 ubiquitination TRAF6 Nemo TAB2 IKK TAK1 Complex formation ATP ADP CYTOPLASM ADP ATP K63 polyubiquitin chains on the protein kinase TAK1. Polyubiquitinated TAK1 can then recruit TAB2 and TAB3, which are adaptor proteins with conserved zinc finger domains. These particular zinc finger domains bind to polyubiquitinated TAK1 and enhance its activity. TAK1, thus activated, phosphorylates and activates IKK, which then phosphorylates IκB, targeting it for degradation. Thus, ubiquitin-binding domains, such as the TAB2 and TAB3 zinc fingers, may selectively recognize K63 polyubiquitin chains to promote signal transmission. Naturally occurring small molecules may control ubiquitin ligase activity directly. Auxin (indole 3-acetic acid) is a plant hormone that regulates development by promoting the transcription of a large number of genes. Rather than K48 ubiquitination Degradation NUCLEUS DNA FIGURE 14.33 Activation of NF-B involves steps dependent on the interaction of proteins attached to ubiquitin through ubiquitin-binding proteins, competition by sumoylation, phosphorylation, and ubiquitin-mediated protein degradation. 14.28 Covalent modification by ubiquitin and ubiquitin-like proteins is another way of regulating protein function 627
Page 1 and 2: Ser Thr IRAK-M TRIF/ TICAM-1 SOCS1
Page 3 and 4: 39057_ch14_cellbio.qxd 8/28/06 5:11
Page 37: 39057_ch14_cellbio.qxd 8/28/06 5:11
Page 55: 39057_ch14_cellbio.qxd 8/28/06 5:11

Principles of cell signaling - UT Southwestern

Create successful ePaper yourself

Delete template?

Save as template?