BIOLOGICAL OVERVIEW

Enabled is cytoskeletal regulator that facilitates continued actin polymerization at the barbed ends of actin filaments, induces cellular projections when overexpressed, and functions together with several different receptors (including Robo and UNC-40/DCC) implicated in axon guidance. This overview of Enabled starts with ABL, one of a number of mammalian cancer causing proteins (oncoproteins). ABL is a highly conserved nonreceptor tyrosine kinase containing SH3 and SH2 protein interaction domains. In less technical terms, ABL functions to modify other proteins by phosphorylation, and it attaches to other proteins by means of SH3 and SH2, two different protein interaction domains. One target of Drosophila ABL is Enabled, a protein that binds to ABL and other oncoproteins; Ena possess the Src homology 3 (SH3) protein interaction domain. SH2 and SH3 domains are found in many "adaptor" proteins that bind to other proteins in the progress of carrying out their function.

Recently a breakthrough in understanding Enabled and Abl has come from the cloning of a Mouse homolog of Enabled (Mena). Both Enabled and Mena are relatives of VASP (Vasodilator-Stimulated Phosphoprotein). A conserved domain of Mena targets it to proteins containing a specific proline-rich motif. The association of Mena with the surface of the intracellular pathogen Listeria monocytogenes and the G-actin binding protein Profilin suggests that Mena may participate in bacterial movement by facilitating actin polymerization. That is, Enabled and Mena, and indirectly ABL, serve to modify the actin based cytoplasm, an important agent in maintaining cell shape, regulating cell migration, and many other cell functions. In Drosophila, ABL and ENA act in the developmental processes of axon pathfinding and eye morphogenesis.

First the relationship of ABL to cancer will be examined because of the insight this provides to an understanding of ABL and Enabled function. Derivatives of ABL protein tyrosine kinase are involved in Philadelphia chromosome positive chronic myelogenous leukemia and acute lymphocytic leukemia in humans, and the pre-B-cell leukemia caused by Abelson murine leukemia virus in mice (from which ABL derives its name). The Philadelphia chromosome is generate by fusion between two normal human chromosomes. These translocations are not found at random, but in the middle of two genes, Abl and Bcr (named after the breakpoint cluster region). The product of this gene fusion is BCR-ABL protein, a hybrid protein able to confer a leukemia phenotype on cells in which it is expressed (Gertler, 1995 and references).

BCR is a multifunctional protein with a pseudo Kinase domain (apparently non-functional), a CDC24 domain and a GAP domain. CDC24 is a yeast gene that when mutated results in defective bud formation. It is believed that CDC24 plays a critical role in the establishment of cell polarity, development of normal cell shape, localization of secretion, and cell-surface deposition. The GAP (GTPase-activating protein) function targets p21 RAC, an important signaling protein in determining and modifying cell shape. Not only does BCR form a hybrid protein with ABL in the Philadelphia translocation, but BCR itself serves to target ABL, resulting in ABL activation (Arlinghaus, 1992 and references). When occuring on a composite protein, the interaction of BCR with ABL causes cell transformation. The chimeric protein products of these oncogenes exhibit elevated tyrosine kinase activity, oligomerization and association with the actin cytoskeleton, tyrosine phosphorylation of BCR sequences, association with signal transducing proteins GRB2, SHC, CBL, SYP and members of the 14-3-3 protein family (Gertler, 1995 and references).

The discovery that the bacterium Listeria recruits the cellular protein VASP, generated great intrigue in the scientific community. Using VASP, Listeria co-opts the cell's cytoskeleton to transport itself within the host cell it infects. VASP is a ligand for profilin, an actin-monomer binding protein that can stimulate the formation of filamentous actin, the non-muscle form of actin that serves as the basis for the actin based cytoskeleton of the cell. Listeria motility results from the rapid polymerization of F-actin at one pole of the bacterium, a process enhanced by host profilin. The properties of VASP make it a candidate host factor that can mediate the recruitment of profilin to the surface of Listeria, and thus promote F-actin assembly. Mena and Evl are two murine proteins highly related to Enabled as well as to VASP. A conserved domain in the N terminal portion of Mena and ENA targets these proteins to proline rich regions of other proteins. VASP interacts with zyxin, a LIM-domain protein localized in focal adhesions that shares a proline-rich motif with vinculin and ActA, two other cytoskeletal proteins. The proline rich region in VASP, which is shared with ENA and Mena, is most likely the profilin-binding site shared by these proteins. Mena itself has been shown to bind to the actin-binding protein Profilin. Localization of Mena to focal adhesions is mediated by its conserved N-terminus, and Mena, like VASP is recruited to the surface of Listera. Mena produces multiple isotypes. One is widely expressed, and a second is enriched in, or specific to, neural cells types. Thus Mena lends some understanding to the possible role of Drosophila ENA in axon pathfinding. (Gertler, 1996).

The well studied migration of neuronal growth cones serves as a paradigm for the actin-driven formation of membrane protrusions (Forscher, 1992). Establishment of proper connections in the central nervous system depends on the ability of neuronal growth cones to guide neurites to their final targets. The ABL-ENA-Profilin pathway is implicated in the process of axonal outgrowth and fasciculation. The mechanism by which the growth cone advances is based on dynamic rearrangement of the actin based cytoskeleton, and it is in this process that Enabled and ABL affect axonogenesis.

Genetic screens for dominant second-site mutations that suppress the lethality of Abl mutations in Drosophila identify alleles of only one gene, enabled. The ENA protein contains proline-rich motifs and binds to ABL and Src SH3 domains. ENA is also a substrate for the Abl kinase. Tyrosine phosphorylation of ENA is increased when it is coexpressed in cells with human or Drosophila Abl, and endogenous ENA tyrosine phosphorylation is reduced in Abl mutant animals. Like Abl, ena is expressed at highest levels in the axons of the embryonic nervous system and ena mutant embryos have defects in axonal architecture. Therefore, it has been concluded that a critical function of Drosophila ABL is to phosphorylate and negatively regulate ENA protein during neural development (Gertler, 1995).

GENE STRUCTURE

PROTEIN STRUCTURE

Amino Acids - 684

Structural Domains

The most notable feature of the predicted ENA protein is a proline-rich core, with 58 proline residues located between amino acids 340-480 and seven matches to the proline-rich consensus site for binding the ABl SH3 domain (Gertler, 1995).

The ActA protein of the intracellular pathogen Listeria monocytogenes induces a dramatic reorganization of the actin-based cytoskeleton. Two profilin binding proteins, VASP and Mena, are the only cellular proteins known so far to bind directly to ActA. This interaction is mediated by a conserved module, the EVH1 domain. E/DFPPPPXD/E, a motif repeated four times within the primary sequence of ActA, is identified as the core of the consensus ligand for EVH1 domains. This motif is also present and functional in at least two cellular proteins, zyxin and vinculin, which are in this respect major eukaryotic analogs of ActA. The functional importance of the novel protein-protein interaction was examined in the Listeria system. Removal of EVH1 binding sites on ActA reduces bacterial motility and strongly attenuates Listeria virulence. ActA-EVH1 binding is a paradigm for a novel class of eukaryotic protein-protein interactions involving a proline-rich ligand that is clearly different from those described for SH3 and WW/WWP domains. This class of interactions appears to be of general importance for processes dependent on rapid actin remodeling (Niebuhr, 1997).

The Enabled/VASP homology 1 (EVH1; also called WH1) domain is an interaction module found in several proteins implicated in actin-based cell motility. EVH1 domains bind the consensus proline-rich motif FPPPP and are required for targeting the actin assembly machinery to sites of cytoskeletal remodeling. The crystal structure of the mammalian Enabled (Mena) EVH1 domain complexed with a peptide ligand reveals a mechanism of recognition distinct from that used by other proline-binding modules. The EVH1 domain fold is unexpectedly similar to that of the pleckstrin homology domain, a membrane localization module. This finding demonstrates the functional plasticity of the pleckstrin homology fold as a binding scaffold and suggests that membrane association may play an auxiliary role in EVH1 targeting (Prehoda, 1999).

date revised: 8 dec 96 

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