HS1 (haematopoietic lineage cell-specific gene protein 1), a prominent substrate of intracellular protein tyrosine kinases in haematopoietic cells, is implicated in the immune response to extracellular stimuli and in cell differentiation induced by cytokines. Although HS1 contains a 37-amino acid tandem repeat motif and a C-terminal Src homology 3 domain and is closely related to the cortical-actin-associated protein cortactin, it lacks the fourth repeat that has been shown to be essential for cortactin binding to filamentous actin (F-actin). In this study, we examined the possible role of HS1 in the regulation of the actin cytoskeleton. Immunofluorescent staining demonstrated that HS1 co-localizes in the cytoplasm of cells with actin-related protein (Arp) 2/3 complex, the primary component of the cellular machinery responsible for de novo actin assembly. Furthermore, recombinant HS1 binds directly to Arp2/3 complex with an equilibrium dissociation constant (Kd) of 880nM. Although HS1 is a modest F-actin-binding protein with a Kd of 400nM, it increases the rate of the actin assembly mediated by Arp2/3 complex, and promotes the formation of branched actin filaments induced by Arp2/3 complex and a constitutively activated peptide of N-WASP (neural Wiskott–Aldrich syndrome protein). Our data suggest that HS1, like cortactin, plays an important role in the modulation of actin assembly.
- actin cytoskeleton haematopoietic cell
- tyrosine kinase
- Wiskott–Aldrich syndrome protein (WASP)
Abbreviations used: Arp, actin-related protein; DTT, dithiothreitol; F-actin, filamentous actin; G-actin, globular actin; GFP, green fluorescent protein; GST, glutathione S-transferase; HS1, haematopoietic lineage cell-specific gene protein 1; N-WASP, neural WASP; SH3, Src homology 3; VCA, verprolin–cofilin–acidic motif; WASP, Wiskott–Aldrich syndrome protein.
- The Biochemical Society, London ©2003