Degradation of activated protein kinases by ubiquitination

Abstract

Protein kinases are important regulators of intracellular signal transduction pathways and play critical roles in diverse cellular functions. Once a protein kinase is activated, its activity is subsequently downregulated through a variety of mechanisms. Accumulating evidence indicates that the activation of protein kinases commonly initiates their downregulation via the ubiquitin/proteasome pathway. Failure to regulate protein kinase activity or expression levels can cause human diseases.

Figure 1

Conventional (ERK1/2, JNK, and p38 MAP kinase) and ERK5 MAP kinase signaling pathways in mammalian cells and in yeast. Only representative signaling molecules and one of the multiple MAPK pathways for budding or fission yeast are shown. Solid arrows: main pathways; dashed arrows: branched pathways and indirect activation.

Figure 2

Cbl protein family in mammals [c-Cbl, Cbl-b, and Cbl-3 (also known as Cbl-c)]. The Cbl proteins contain, from N to C terminus, a TKB domain, a linker region (L), RING finger domain (RF), Pro-rich regions, poly-Pro-Arg motif (PR), and UBA domain. The TKB domain consists of a four-helix bundle (4H), EF hand (EF), and a variant Src homology region 2 (SH2) domain. Cbl-3 lacks the PR motif and UBA domain.

Figure 3

Function and regulation of the APC/C E3 ligase during somatic cell cycle. At the G1-S transition, accumulated cyclin A and B dependent on the inhibition of APC/C^Cdc20 by APC/C^Cdh1 will activate CDKs and promote entry into S phase. In coordination with increased expression of Emi1 regulated by E2F and Evi5, activated CDKs phosphorylate Cdh1, inhibit APC/C^Cdh1, and cause subsequent accumulation of Plk1. During prometaphase and metaphase, APC/C^Cdc20 is activated by combinatorial factors including downregulation of APC/C^Cdh1, CDK1-dependent APC/C^Cdc20 phosphorylation, dissociation of MCC from APC/C^Cdc20, degradation of Bub1 and Aurora kinases by APC/C^Cdh1 in G1 phase, and proteolysis of Emil by Plk1-dependent phosphorylation. Further enhanced activation of APC/C^Cdc20 can be achieved by degradation of CDK1 inhibitor p21^CIP1 by APC/C^Cdc20. Activated APC/C^Cdc20 initiates the degradation of cyclin A in prometaphase and cyclin B in metaphase and is involved in destruction of securin, which, in turn, reduces CDK1 activity and results in the dephosphorylation of Cdh1, in conjunction with Cdc14 phosphatase dephosphorylation, and activation of APC/C^Cdh1. Dephosphorylation of separase together with the APC/C^Cdh1-dependent proteolysis of securin (Pds1/Cut2) results in the activation of separase, cleavage of cohesin, and anaphase onset. Activated APC/C^Cdh1 orchestrates the degradation of a wide spectrum of substrates, facilitating mitotic exit and cytokinesis.

Figure 4

An EGFR internalization model. Homodimerization and autophosphorylation of EGFR induced by EGF binding recruits and interacts with Cbl/CIN85 complex through the variant SH2 domain of Cbls. Cbls mediate the ubiquitination of EGFR and monoubiquitination of CIN85. EGFR/Cbl/CIN85 with other monoubiquitinated endocytic proteins such as Eps15, Eps15R, epsins and Hrs travel from early endosomes to late endosomes and MVBs. Subsequent invagination of cargo proteins into the lumenal vesicles of MVBs targets MVBs for fusion with lysosomes and then final degradation of EGFR. A certain proportion of EGFR will be deubiquitinated by DUBs and recycled back to the cell membrane.

Figure 5

A model of ESCRT functioning in EGFR sorting. The UBC-like domain of Vps23, which is a component of ESCRT-I, likely binds to multiubiquitinated EGFR at the membrane of MVBs. ESCRT-II, acting downstream of ESCRT-I, is recruited, and then putatively directs the ESCRT-III complex (which is composed of Vps20, Snf7, Vps2, and Vps24) to the appropriate MVB membrane. ESCRT-III-recruited DUBs remove Ub from cargo proteins, resulting in the invagination of cargo proteins into MVB lumenal vesicles. Vps4 is required for the disassembly and release of the entire MVB sorting machinery, which allows the ESCRT machinery to recycle back into the cytoplasm for further rounds of MVB sorting (170).