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EVOLUTIONARY HOMOLOGS (part 2/3)

Abl tyrosine kinase

Fertilization causes tyrosine phosphorylation of several sea urchin egg proteins within minutes of sperm-egg binding, although the identity of the kinase(s) involved and the mechanism of regulation is not known. Antibodies against a egg homolog of the ABL family of protein tyrosine kinases identify a 220-kDa protein kinase, highly enriched in the egg cortex, where it is tightly associated with detergent-insoluble cytoskeletal elements. The enzyme is capable of phosphorylating synthetic peptide substrates that were used to characterize the kinase activity in an immune-complex assay. Measurement of the protein tyrosine kinase activity immunoprecipitates at different times after fertilization reveals that the level of kinase activity is transiently elevated during the first few minutes postinsemination. The amount of the 220-kDa protein does not increase significantly during this period, so the increased kinase activity probably results from activation of the enzyme. These in vitro studies indicate that the 220-kDa abl-related kinase is one of the protein kinases activated during fertilization and suggest that it may play a role in the egg activation process (Moore, 1994).

The myristoylated form of c-Abl protein, as well as the P210bcr/abl protein, have been shown to associate with F-actin stress fibers in fibroblasts. The domain responsible for this interaction maps to the extreme COOH-terminus of Abl. The actin-binding domain is localized to a 58 amino acid region, including a charged motif at the extreme COOH-terminus that is important for efficient binding. F-actin binding by Abl is calcium independent; Abl competes with gelsolin for binding to F-actin. In addition to the F-actin binding domain, the COOH-terminus of Abl contains a proline-rich region that mediates binding and sequestration of G-actin, and the Abl F- and G-actin binding domains cooperate to bundle F-actin filaments in vitro. The COOH terminus of Abl thus confers several novel localizing functions upon the protein, including actin binding, nuclear localization, and DNA binding. Abl may modify and receive signals from the F-actin cytoskeleton in vivo, and is an ideal candidate to mediate signal transduction from the cell surface and cytoskeleton to the nucleus (Van Etten, 1994).

Chronic myelogenous leukemia (CML) is a myeloproliferative disorder caused by the t(9;22) translocation. This translocation creates a unique tyrosine kinase oncogene, bcr/abl, whose product, p210BCR/ABL, is localized to the actin cytoskeleton. One of the major tyrosine phosphoproteins in cells transformed by p210BCR/ABL is the protooncoprotein p120c-Cbl. p210BCR/ABL induces formation of a multimeric complex of proteins that include p120c-Cbl, phosphotidylinositol-3' kinase, and p210BCR/ABL itself. Certain focal adhesion proteins are also part of this complex, including paxillin and talin. The sites in paxillin required to bind to p120c-Cbl in this complex have been partially mapped. The interaction of pl20c-Cbl with paxillin is specific, since other focal adhesion proteins, such as p125FAK, vinculin, and alpha-actinin, are not in this complex. The binding of p120c-Cbl to the focal adhesion protein paxillin could contribute to the known adhesive defects of CML cells (Salgia, 1996).

Transformation of B-lineage precursors by the Abelson murine leukemia virus appears to arrest development at the pre-B stage. Abelson-transformed pre-B cell lines generally retain transcriptionally inactive, unrearranged immunoglobulin kappa alleles. Nontransformed pre-B cells expanded from mouse bone marrow efficiently transcribe unrearranged kappa alleles. In addition, they contain activated complexes of the NF-kappa B/Rel transcription factor family, in contrast with their Abelson-transformed counterparts. Using conditionally transformed pre-B cell lines, it has been shown that the v-abl viral transforming protein, a tyrosine kinase, blocks germ-line kappa gene transcription and negatively regulates NF-kappa B/Rel (Drosophila homologs Dorsal and Dif) activity. An active v-abl kinase specifically inhibits the NF-kappa B/Rel-dependent kappa intron enhancer, which is implicated in promoting both transcription and rearrangement of the kappa locus. v-abl inhibits the activated state of NF-kappa B/Rel complexes in a pre-B cell via a post-translational mechanism that results in increased stability of the inhibitory subunit I kappa B alpha (Drosophila homolog: Cactus). This analysis suggests a molecular pathway by which v-abl inhibits kappa locus transcription and rearrangement (Klug, 1994).

The c-abl proto-oncogene encodes a nuclear tyrosine kinase that can phosphorylate the mammalian RNA polymerase II (RNAP II) on its C-terminal repeated domain (CTD) in vitro. Phosphorylation of the CTD has previously been shown to require the tyrosine kinase and the SH2 domain of Abl. A CTD-interacting domain (CTD-ID) at the C-terminal region of c-Abl is also required. Deletion of the CTD-ID causes the Km of binding to polymerase to increase by 2 orders of magnitude. Direct binding of the CTD-ID to CTD and to RNAP II can be demonstrated in vitro. Phosphorylation of the CTD by c-Abl is observed in cotransfected COS cells. Mutant Abl proteins lacking the CTD-ID, while capable of autophosphorylation, neither phosphorylate nor associate with the CTD in vivo. Transient overexpression of c-Abl also leads to a four- to fivefold increase in the phosphotyrosine content of the RNAP II large subunit. Moreover, the large subunit of RNAP II can be coprecipitated with c-Abl. Tyrosine phosphorylation of the coprecipitated RNAP II is dependent on the presence of the CTD-ID in Abl. The ability of c-Abl to phosphorylate and associate with RNAP II can be correlated with the enhancement of transcription by c-Abl. Taken together, these observations demonstrate that c-Abl can function as a CTD kinase in vitro as well as in vivo (Baskaran, 1996).

The carboxyl-terminal repeated domain (CTD) of the mammalian RNA polymerase II is a substrate for the Abl. This specificity is conferred in part by the SH2 domain. The Abl SH2 domain binds the tyrosine-phosphorylated [Tyr(P)] CTD and is required for the processive and stoichiometric phosphorylation of the 52 tyrosines in the CTD. Mutation of the Abl SH2, or exchanging it with that of Src (which does not bind the Tyr(P)-CTD) abolished processivity and reduced the CTD kinase activity without any effect on autophosphorylation or the phosphorylation of nonspecific substrates. These results demonstrate that the SH2 domain of the Abl tyrosine kinase plays an active role in catalysis and suggests that SH2 domain and the tyrosine kinase domain may act in concert to confer substrate specificity (Duyster, 1995).

A novel cellular protein, Abl-interactor-1 (Abi-1), which specifically interacts with the carboxy-terminal region of Abl oncoproteins, has been identified in a mouse leukemia cell line. The protein exhibits sequence similarity to homeotic genes, contains several polyproline stretches, and includes a src homology 3 (SH3) domain at its very carboxyl terminus that is required for binding to Abl proteins. The abi-1 gene has been mapped to mouse chromosome 2 and is genetically closely linked to the c-abl locus. The gene is widely expressed in the mouse, with highest levels of mRNA found in the bone marrow, spleen, brain, and testes. The Abi-1 protein coimmunoprecipitates with v-Abl and serves as a substrate for kinase activity. When overexpressed in NIH-3T3 cells, abi-1 potently suppresses the transforming activity of Abelson leukemia virus expressing the full-length p160v-abl kinase but does not affect the transforming activity of viruses expressing a truncated p90v-abl or v-src kinases. It is suggested that the Abi-1 protein may serve as a regulator of Abl function in transformation or in signal transduction (Shi, 1995).

What is the effect of the v-abl oncogene of the Abelson murine leukemia virus (A-MuLV) on the Jak-STAT pathway of cytokine signal transduction? In murine pre-B lymphocytes transformed with A-MuLV, the Janus kinases (Jaks) Jak1 and Jak3 (See Drosophila Hopscotch) exhibit constitutive tyrosine kinase activity, and the STAT proteins (signal transducers and activators of transcription) (See Drosophila Marelle) normally activated by interleukin-4 and interleukin-7 are tyrosine-phosphorylated in the absence of these cytokines. Coimmunoprecipitation experiments reveal that in these cells v-Abl is physically associated with Jak1 and Jak3. Inactivation of v-Abl tyrosine kinase in a pre-B cell line transformed with a temperature-sensitive mutant of v-abl results in abrogation of constitutive Jak-STAT signaling. A direct link may exist between transformation by v-abl and cytokine signal transduction (Danial, 1995).

p62dok is a docking protein that is a target of the ABL tyrosine kinase. p62dok is a constitutively tyrosine-phosphorylated in chronic myelogenous leukemia progenitor cells and is associated with ras GTPase-activating protein (GAP)-associated protein. Association of p62dok with GAP correlates with its tyrosine phosphorylation. p62dok is rapidly tyrosine-phosphorylated upon activation of the c-Kit receptor, implicating it as a component of a signal transduction pathway downstream of receptor tyrosine kinases. Given the important signaling role that tyrosine kinases play in hematopoietic control, it is not surprising that the appearance of a novel tyrosine kinase activity within a single, primitive erythroid progenitor cell would perturb the intracellular signaling cascades that ensure orderly hematopoiesis (Carpino, 1997).

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