The Physiological Function Of The Translocation Partners

Abl is a 145-kd nonreceptor tyrosine kinase with multiple and complex functions. Two isoforms (Abl-A and Abl-B) exist that differ in their N-terminal regions, depending on the usage of exon Ia or exon Ib (Figure 17.3). Only Abl-B, which is 19 amino acids longer than Abl-A, is myristoylated at the N-terminus,59 a feature that has recently been linked to autoinhibition of kinase activity.6061 Further toward the C-ter-minus, there is a Src homology (SH) 3 domain that interacts with proline-rich domains of other proteins, an SH2 domain capable of binding to phosphorylated tyrosine residues of interacting proteins, and an SH1 domain, which carries tyrosine kinase activity.36 While the N-terminus exhibits a high degree of homology to other nonreceptor tyrosine kinases, such as Src, the C-terminus is unique to Abl. Three DNA-binding domains, a nuclear localization signal, nuclear export signal, a proline-rich region, and an actin-binding domain can be defined within the C-terminus. Under physiological circumstances, Abl is nuclear, and its tyrosine kinase activity is tightly regulated.

The function of Abl has been studied extensively, and a very complex picture has emerged (for reviews, see Refs. 62 and 63). There is evidence for an inhibitory role of Abl in cell cycle regulation, which led to the notion that it may be considered a tumor suppressor.64 A number of studies have implicated Abl tyrosine kinase in the regulation of the cellular response to DNA damage, by interaction with several proteins involved in DNA repair or response to genotoxic stress.65-70 Yet other data suggest a role in the signal transduction from and to integrin receptors on the cell surface,7172 and there is evidence for activation of Abl kinase upon ligand binding to the platelet-derived growth factor receptor.73 Mice with homologous disruption of the ABL locus suffer from high perinatal mortality and have multiple defects, including defective immune function and skeletal abnormalities, and suffer from a poorly characterized wasting syndrome.7475 However, there is no indication that the rate of spontaneous tumors is increased in these mice, arguing against a tumor suppressor function of Abl. Importantly, simultaneous disruption of the ABL-related gene, ARG, also referred to as ABL-2, is embry-onically lethal, due to a failure of neuronal development, which argues that the ARG may partially compensate for the loss of Abl function in the ABL knockout mice.76 The mechanism underlying the tight regulation of Abl kinase activity in physiological conditions has recently been clarified, at least for the Abl-B isoform, by a combination of mutational structure function analysis and crystallography.6061 These studies revealed a critical role for the N-terminal cap region of the protein, which inhibits the kinase by an intramolecular interaction. This does not exclude that other mechanisms, such as transacting molecules, may also be involved in regulating the kinase.77

Figure 17.3 Structure of the Abl protein. The type Ia isoform is slightly shorter than type Ib, which contains a myristoylation (myr) site for attachment to the plasma membrane. Note the three SrC-homology (SH) domains situated toward the NH2-terminus. Y393 is the major site of autophosphorylation within the kinase domain, while phenylalanine 401 (F401) is highly conserved in PTKs containing SH3 domains. The middle of the protein is dominated by proline-rich regions (PxxP) capable of binding to SH3 domains and also harbors one of three nuclear localization signals (NLS). The carboxy-terminus contains DNA as well as G- and F-actin-binding domains. Phosphorylation sites by Atm, cdc2, and PKC are shown. The arrowhead indicates the position of the breakpoint in the Bcr-Abl fusion protein. (Blood 96:3343, 2000; with permission)

Figure 17.3 Structure of the Abl protein. The type Ia isoform is slightly shorter than type Ib, which contains a myristoylation (myr) site for attachment to the plasma membrane. Note the three SrC-homology (SH) domains situated toward the NH2-terminus. Y393 is the major site of autophosphorylation within the kinase domain, while phenylalanine 401 (F401) is highly conserved in PTKs containing SH3 domains. The middle of the protein is dominated by proline-rich regions (PxxP) capable of binding to SH3 domains and also harbors one of three nuclear localization signals (NLS). The carboxy-terminus contains DNA as well as G- and F-actin-binding domains. Phosphorylation sites by Atm, cdc2, and PKC are shown. The arrowhead indicates the position of the breakpoint in the Bcr-Abl fusion protein. (Blood 96:3343, 2000; with permission)

Figure 17.4 Structure of the Bcr protein. Note the dimerization domain (DD) and the two cAMP kinase homologous domains at the N-terminus. Y177 is the autophosphorylation site crucial for binding to Grb-2. The center of the molecule contains a region homologous to Rho guanidine nucleotide exchange factors (Rho-GEF) as well as dbl-like and pleckstrin homology (PH) domains. Toward the C-terminus a putative site for calcium-dependent lipid binding (CaLB) and a domain with activating function for Rac-GTPase (Rac-GAP) are found. Arrowheads indicate the position of the breakpoints in the Bcr-Abl fusion proteins. (Blood 96:3343, 2000; with permission)

Figure 17.4 Structure of the Bcr protein. Note the dimerization domain (DD) and the two cAMP kinase homologous domains at the N-terminus. Y177 is the autophosphorylation site crucial for binding to Grb-2. The center of the molecule contains a region homologous to Rho guanidine nucleotide exchange factors (Rho-GEF) as well as dbl-like and pleckstrin homology (PH) domains. Toward the C-terminus a putative site for calcium-dependent lipid binding (CaLB) and a domain with activating function for Rac-GTPase (Rac-GAP) are found. Arrowheads indicate the position of the breakpoints in the Bcr-Abl fusion proteins. (Blood 96:3343, 2000; with permission)

The function of BCR (Figure 17.4) is even less well understood than that of Abl. The N-terminus contains a dimerization domain as well as a serine/threonine kinase activity. The center of the molecule is dominated by dbl-like and pleckstrin homology (PH-) domains that have guanidine nucleotide exchange function for small G proteins, including RhoA, Rac, and Cdc42. The C-terminus has GTPase-activating function for Rac (reviewed in Ref. 36). In contrast to ABL knockout mice, BCR knockout mice are practically normal, and an increased oxidative burst of their neu-trophils is the only recognized anomaly.78

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