title: Arp2/3 complex activity in filopodia of spreading cells.
abstract: BACKGROUND
  Cells use filopodia to explore their environment and to form new adhesion contacts for motility and spreading. The Arp2/3 complex has been implicated in lamellipodial actin assembly as a major nucleator of new actin filaments in branched networks. The interplay between filopodial and lamellipodial protrusions is an area of much interest as it is thought to be a key determinant of how cells make motility choices.
  
  
  RESULTS
  We find that Arp2/3 complex localises to dynamic puncta in filopodia as well as lamellipodia of spreading cells. Arp2/3 complex spots do not appear to depend on local adhesion or on microtubules for their localisation but their inclusion in filopodia or lamellipodia depends on the activity of the small GTPase Rac1. Arp2/3 complex spots in filopodia are capable of incorporating monomeric actin, suggesting the presence of available filament barbed ends for polymerisation. Arp2/3 complex in filopodia co-localises with lamellipodial proteins such as capping protein and cortactin. The dynamics of Arp2/3 complex puncta suggests that they are moving bi-directionally along the length of filopodia and that they may be regions of lamellipodial activity within the filopodia.
  
  
  CONCLUSIONS
  We suggest that filopodia of spreading cells have regions of lamellipodial activity and that this activity affects the morphology and movement of filopodia. Our work has implications for how we understand the interplay between lamellipodia and filopodia and for how actin networks are generated spatially in cells.
authors.affiliation: University of Birmingham School of Biosciences, Edgbaston, Birmingham, UK. s.a.johnston@bham.ac.uk
authors.forename: Simon A
authors.initials: SA
authors.lastname: Johnston
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Bramble
authors.forename: Chun L
authors.initials: CL
authors.lastname: Yeung
authors.forename: Paula M
authors.initials: PM
authors.lastname: Mendes
authors.forename: Laura M
authors.initials: LM
authors.lastname: Machesky
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D051378:Actin-Related Protein 3 / Q000378:metabolism*; D000091302:Angiopoietin-Like Protein 2; D000075802:Angiopoietin-like Proteins; D042682:Angiopoietins / Q000378:metabolism*; D000818:Animals; D002448:Cell Adhesion; D002460:Cell Line; D002465:Cell Movement; D003593:Cytoplasm / Q000648:ultrastructure; D005347:Fibroblasts / Q000378:metabolism* / Q000648:ultrastructure; D005353:Fibronectins / Q000378:metabolism; D051379:Mice; D009479:Neuropeptides / Q000378:metabolism; D011485:Protein Binding; D021381:Protein Transport; D011554:Pseudopodia / Q000378:metabolism* / Q000648:ultrastructure; D015398:Signal Transduction; D020744:rac GTP-Binding Proteins / Q000378:metabolism; D020830:rac1 GTP-Binding Protein
chemical_list: D051378:Actin-Related Protein 3; D000091302:Angiopoietin-Like Protein 2; D000075802:Angiopoietin-like Proteins; D042682:Angiopoietins; C485793:Angptl2 protein, mouse; D005353:Fibronectins; D009479:Neuropeptides; C484602:Rac1 protein, mouse; D020744:rac GTP-Binding Proteins; D020830:rac1 GTP-Binding Protein