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“Wunder” F-BAR Domains: Going from Pits to Vesicles

2007; Cell Press; Volume: 129; Issue: 4 Linguagem: Inglês

10.1016/j.cell.2007.05.006

1097-4172

Klaus Fütterer, Laura M. Machesky,

Cell Adhesion Molecules Research

Clathrin-mediated endocytosis is a key mechanism by which cells take up extracellular cargo. In this issue, Shimada et al., 2007Shimada A. Niwa H. Tsujita K. Suetsugu S. Nitta K. Hanawa-Suetsugu K. Akasaka R. Nishino Y. Toyama M. Chen L. et al.Cell. 2007; (this issue)Google Scholar reveal the mode of action of the F-BAR domain, which deepens the initial membrane pit that forms during clathrin-mediated endocytosis. Clathrin-mediated endocytosis is a key mechanism by which cells take up extracellular cargo. In this issue, Shimada et al., 2007Shimada A. Niwa H. Tsujita K. Suetsugu S. Nitta K. Hanawa-Suetsugu K. Akasaka R. Nishino Y. Toyama M. Chen L. et al.Cell. 2007; (this issue)Google Scholar reveal the mode of action of the F-BAR domain, which deepens the initial membrane pit that forms during clathrin-mediated endocytosis. Lipid membranes are highly flexible and can be deformed into a wide range of shapes or broken apart to form smaller entities. In some way, this is what happens in clathrin-mediated endocytosis, a fundamental mechanism by which cells take up liquids and particles from the environment. In clathrin-mediated endocytosis, cells first form hemispherical plasma membrane invaginations around the cargo, such as a ligand-bound receptor (Figure 1A). The invaginations, known as clathrin-coated pits, gradually morph into vesicles that are cut off from the plasma membrane, a process aided by the GTPase dynamin (Itoh et al., 2005Itoh T. Erdmann K.S. Roux A. Habermann B. Werner H. De Camilli P. Dev. Cell. 2005; 9: 791-804Abstract Full Text Full Text PDF PubMed Scopus (480) Google Scholar) (Figure 1A). Membrane bending during endocytosis requires energy and is driven by membrane-associated proteins that insert parts of their structure into one leaflet of the bilayer and/or in binding impress their own shape onto the membrane (Gallop et al., 2006Gallop J.L. Jao C.C. Kent H.M. Butler P.J. Evans P.R. Langen R. McMahon H.T. EMBO J. 2006; 25: 2898-2910Crossref PubMed Scopus (398) Google Scholar). A recent example is the Bin/amphiphysin/Rvs (BAR) domain. The BAR domain self-associates into a crescent-shaped dimer (Figure 1B) that when bound to the membrane induces membrane bending (Peter et al., 2004Peter B.J. Kent H.M. Mills I.G. Vallis Y. Butler P.J. Evans P.R. McMahon H.T. Science. 2004; 303: 495-499Crossref PubMed Scopus (1263) Google Scholar). Incubation of BAR domains with liposomes (artificial spherical bilayered membranes) reshapes them into tubules whose diameter matches the curvature of the BAR-domain crescent (∼200 Å in diameter). A variant of the BAR domain, the N-BAR domain, inserts flanking amphipathic sequences into the membrane to enhance tubulation. Endophilins and amphiphysins contain N-BAR domains which together with dynamin are responsible for forming the narrowly curved neck region of the nascent vesicle (Figure 1A) (Gallop et al., 2006Gallop J.L. Jao C.C. Kent H.M. Butler P.J. Evans P.R. Langen R. McMahon H.T. EMBO J. 2006; 25: 2898-2910Crossref PubMed Scopus (398) Google Scholar, Masuda et al., 2006Masuda M. Takeda S. Sone M. Ohki T. Mori H. Kamioka Y. Mochizuki N. EMBO J. 2006; 25: 2889-2897Crossref PubMed Scopus (211) Google Scholar). Yet, what is happening prior to formation of the neck is less clear. Shimada et al., 2007Shimada A. Niwa H. Tsujita K. Suetsugu S. Nitta K. Hanawa-Suetsugu K. Akasaka R. Nishino Y. Toyama M. Chen L. et al.Cell. 2007; (this issue)Google Scholar now elucidate the structure and mode of action of an evolutionary "cousin" of the BAR domain, termed the EFC (extended FCH) or F-BAR ("FCH and BAR") domain. These names refer to the previously identified FER/CIP4 homology (FCH) region, a defining feature of members of the Pombe Cdc15 homology (PCH) family of proteins, which are key players in endocytosis. The crystal structures of the F-BAR domain of two PCH family proteins—FBP17 and CIP4—solved by Shimada et al. firmly establish that the FCH region is the first of three extended α helices that define the monomer of the F-BAR domain (Figure 1B). The architecture of the F-BAR domain mirrors the structures of the BAR and N-BAR domains (Tarricone et al., 2001Tarricone C. Xiao B. Justin N. Walker P.A. Rittinger K. Gamblin S.J. Smerdon S.J. Nature. 2001; 411: 215-219Crossref PubMed Scopus (200) Google Scholar, Peter et al., 2004Peter B.J. Kent H.M. Mills I.G. Vallis Y. Butler P.J. Evans P.R. McMahon H.T. Science. 2004; 303: 495-499Crossref PubMed Scopus (1263) Google Scholar, Gallop et al., 2006Gallop J.L. Jao C.C. Kent H.M. Butler P.J. Evans P.R. Langen R. McMahon H.T. EMBO J. 2006; 25: 2898-2910Crossref PubMed Scopus (398) Google Scholar, Masuda et al., 2006Masuda M. Takeda S. Sone M. Ohki T. Mori H. Kamioka Y. Mochizuki N. EMBO J. 2006; 25: 2889-2897Crossref PubMed Scopus (211) Google Scholar) as well as that of the cigar-shaped IRSp53/MIM homology domain (IMD) (Millard et al., 2005Millard T.H. Bompard G. Heung M.Y. Dafforn T.R. Scott D.J. Machesky L.M. Futterer K. EMBO J. 2005; 24: 240-250Crossref PubMed Scopus (182) Google Scholar) (Figure 1B). This fits an emerging theme among protein domains that induce membrane bending: domain dimerization results in a central six-helix bundle with two helices protruding on either side, generating a crescent-shaped molecule with a family-specific radius of curvature (Figure 1B). The dimer interface is very large, suggesting that dimerization is constitutive. By examining the sedimentation of the FBP17 and CIP4 F-BAR domains using analytical ultracentrifugation, Shimada et al. confirm that the F-BAR domain is indeed a dimer in solution. Compared to the BAR domain, the F-BAR domain features longer helices and a distinctly shallower curvature (∼600 Å versus 200–280 Å in diameter). This correlates with the markedly larger diameter of F-BAR versus BAR domain-induced tubules (Itoh et al., 2005Itoh T. Erdmann K.S. Roux A. Habermann B. Werner H. De Camilli P. Dev. Cell. 2005; 9: 791-804Abstract Full Text Full Text PDF PubMed Scopus (480) Google Scholar). Also, to generate tubules the F-BAR domain requires liposomes above a certain size (∼400 Å) (Shimada et al., 2007Shimada A. Niwa H. Tsujita K. Suetsugu S. Nitta K. Hanawa-Suetsugu K. Akasaka R. Nishino Y. Toyama M. Chen L. et al.Cell. 2007; (this issue)Google Scholar). Thus, it is plausible to postulate that F-BAR and N-BAR domain-containing proteins shape different parts of a nascent vesicle (Figure 1A). An outlier of sorts is the IMD domain dimer, which displays actin filament crosslinking activity (Millard et al., 2005Millard T.H. Bompard G. Heung M.Y. Dafforn T.R. Scott D.J. Machesky L.M. Futterer K. EMBO J. 2005; 24: 240-250Crossref PubMed Scopus (182) Google Scholar). Two recent studies showed that the IMD domain, despite its straight conformation (Figure 1B), also deforms membranes (Mattila et al., 2007Mattila P.K. Pykalainen A. Saarikangas J. Paavilainen V.O. Vihinen H. Jokitalo E. Lappalainen P. J. Cell Biol. 2007; 176: 953-964Crossref PubMed Scopus (279) Google Scholar, Suetsugu et al., 2006Suetsugu S. Murayama K. Sakamoto A. Hanawa-Suetsugu K. Seto A. Oikawa T. Mishima C. Shirouzu M. Takenawa T. Yokoyama S. J. Biol. Chem. 2006; 281: 35347-35358Crossref PubMed Scopus (134) Google Scholar), hence its precise function remains to be clarified. It is consequential that in the crystal lattices the distal ends of symmetry-related dimers form tight interactions because of hydrogen bonding, resulting in F-BAR-domain filaments extending across the crystals. Analogous filaments were seen in electron micrographs of the FBP17 F-BAR domain in the absence of liposomes (Itoh et al., 2005Itoh T. Erdmann K.S. Roux A. Habermann B. Werner H. De Camilli P. Dev. Cell. 2005; 9: 791-804Abstract Full Text Full Text PDF PubMed Scopus (480) Google Scholar). Based on this evidence the authors propose that deepening of the initial clathrin-coated pit might be driven by F-BAR domains both through membrane bending induced by F-BAR-domain binding and by end-to-end oligomerization of F-BAR dimers (Figure 1A). A pattern of stripes running perpendicular to the tubule axis is indeed seen in phase-contrast cryo-transmission electron micrographs of F-BAR-induced tubules, and the pitch of the pattern (40–50 Å) correlates with the diameter of the central region of the F-BAR dimer (35 Å). Consistent with their hypothesis that oligomerization is essential to F-BAR-domain-induced invagination, Shimada et al., 2007Shimada A. Niwa H. Tsujita K. Suetsugu S. Nitta K. Hanawa-Suetsugu K. Akasaka R. Nishino Y. Toyama M. Chen L. et al.Cell. 2007; (this issue)Google Scholar find that point mutations to conserved residues at the distal end loop completely abrogate membrane invagination in vivo, although to a certain extent they retain liposome binding and tubulation activity. Endocytosis involves the interplay between membrane deformation and the reorganization of the actin cytoskeleton. This is evident from experiments demonstrating colocalization of actin, actin-regulatory proteins, and endocytic proteins—including FBP17 and CIP4—at sites of vesicle formation (see Itoh et al., 2005Itoh T. Erdmann K.S. Roux A. Habermann B. Werner H. De Camilli P. Dev. Cell. 2005; 9: 791-804Abstract Full Text Full Text PDF PubMed Scopus (480) Google Scholar and references therein). Yet, the mechanistic role of actin polymerization in vesicle formation is far from clear. PCH family proteins, in addition to the N-terminal F-BAR domain, contain a central HR1 domain mediating interaction with Rho-family GTPases and a C-terminal Src homology 3 (SH3) domain, which recruits proteins with polyproline repeats. Interaction partners of the SH3 domain of FBP17 and CIP4 include an important activator of actin polymerization, neuronal Wiskott-Aldrich syndrome protein (N-WASP). Shimada et al. demonstrate that FBP17 recruitment to clathrin-coated pits is synchronous with that of N-WASP, suggesting that recruitment of PCH family proteins to sites of endocytosis promotes actin polymerization. Interestingly, the latter may put a brake on vesicle formation: treating cells with inhibitors of actin polymerization results in extensive membrane tubulation even when PCH family proteins are only weakly expressed (Itoh et al., 2005Itoh T. Erdmann K.S. Roux A. Habermann B. Werner H. De Camilli P. Dev. Cell. 2005; 9: 791-804Abstract Full Text Full Text PDF PubMed Scopus (480) Google Scholar). Others have suggested that actin assembly generates a pushing force driving vesicle movement (Kaksonen et al., 2005Kaksonen M. Toret C.P. Drubin D.G. Cell. 2005; 123: 305-320Abstract Full Text Full Text PDF PubMed Scopus (540) Google Scholar). Clearly, this story promises many more interesting chapters. Curved EFC/F-BAR-Domain Dimers Are Joined End to End into a Filament for Membrane Invagination in EndocytosisShimada et al.CellMay 18, 2007In BriefPombe Cdc15 homology (PCH) proteins play an important role in a variety of actin-based processes, including clathrin-mediated endocytosis (CME). The defining feature of the PCH proteins is an evolutionarily conserved EFC/F-BAR domain for membrane association and tubulation. In the present study, we solved the crystal structures of the EFC domains of human FBP17 and CIP4. The structures revealed a gently curved helical-bundle dimer of ∼220 Å in length, which forms filaments through end-to-end interactions in the crystals. Full-Text PDF Open Archive