Interaction between Tiam1 and the Arp2/3 complex links activation of Rac to actin polymerization

The Rac-specific GEF (guanine-nucleotide exchange factor) Tiam1 (T-lymphoma invasion and metastasis 1) regulates migration, cell-matrix and cell-cell adhesion by modulating the actin cytoskeleton through the GTPase, Rac1. Using yeast two-hybrid screening and biochemical assays, we found that Tiam1 interacts with the p21-Arc [Arp (actin-related protein) complex] subunit of the Arp2/3 complex. Association occurred through the N-terminal pleckstrin homology domain and the adjacent coiled-coil region of Tiam1. As a result, Tiam1 co-localizes with the Arp2/3 complex at sites of actin polymerization, such as epithelial cell-cell contacts and membrane ruffles. Deletion of the p21-Arc-binding domain in Tiam1 impairs its subcellular localization and capacity to activate Rac1, suggesting that binding to the Arp2/3 complex is important for the function of Tiam1. Indeed, blocking Arp2/3 activation with a WASP (Wiskott-Aldrich syndrome protein) inhibitor leads to subcellular relocalization of Tiam1 and decreased Rac activation. Conversely, functionally active Tiam1, but not a GEF-deficient mutant, promotes activation of the Arp2/3 complex and its association with cytoskeletal components, indicating that Tiam1 and Arp2/3 are mutually dependent for their correct localization and signalling. Our data suggests a model in which the Arp2/3 complex acts as a scaffold to localize Tiam1, and thereby Rac activity, which are both required for activation of the Arp2/3 complex and further Arp2/3 recruitment. This 'self-amplifying' signalling module involving Tiam1, Rac and the Arp2/3 complex could thus drive actin polymerization at specific sites in cells that are required for dynamic morphological changes.

The role of the 5' untranslated region of eukaryotic messenger RNAs in translation and its investigation using antisense technologies

This chapter discusses the recent advances in the field of translational control and the possibility of applying the powerful antisense technology to investigate some of the unanswered questions, especially those pertaining to the role of the 5’untranslated region ( UTR) on translation initiation. Translational regulation is predominantly exerted during the initiation phase that is considered to be the rate-limiting step. Two types of translational regulation can be distinguished: global, in which the initiation rate of (nearly) all cellular messenger RNA (mRNA) is controlled and selective, in which the translation rate of specific mRNAs varies in response to the biological stimuli. In most cases of global regulation, control is exerted via the phosphorylation state of certain initiation factors, whereas only a few examples of selective regulation have been characterized well enough to define the underlying molecular events. Interestingly, cis-acting regulatory sequences, affecting translation initiation, have been found not only in the 5’UTRs of selectively regulated mRNAs, but also in the 3’UTRs. Thus, in addition to the protein encoding open reading frames, both the 5’ and 3’UTRs of mRNAs must be considered for their effect on translation.