title: Branching of keratin intermediate filaments.
abstract: Keratin intermediate filaments (IFs) are crucial to maintain mechanical stability in epithelial cells. Since little is known about the network architecture that provides this stiffness and especially about branching properties of filaments, we addressed this question with different electron microscopic (EM) methods. Using EM tomography of high pressure frozen keratinocytes, we investigated the course of several filaments in a branching of a filament bundle. Moreover we found several putative bifurcations in individual filaments. To verify our observation we also visualized the keratin network in detergent extracted keratinocytes with scanning EM. Here bifurcations of individual filaments could unambiguously be identified additionally to bundle branchings. Interestingly, identical filament bifurcations were also found in purified keratin 8/18 filaments expressed in Escherichia coli which were reassembled in vitro. This excludes that an accessory protein contributes to the branch formation. Measurements of the filament cross sectional areas showed various ratios between the three bifurcation arms. This demonstrates that intermediate filament furcation is very different from actin furcation where an entire new filament is attached to an existing filament. Instead, the architecture of intermediate filament bifurcations is less predetermined and hence consistent with the general concept of IF formation.
authors.affiliation: Central Facility for Electron Microscopy, Ulm University, 89081 Ulm, Germany.
authors.forename: Soufi
authors.initials: S
authors.lastname: Nafeey
authors.affiliation: Institute of Experimental Physics, Ulm University, 89081 Ulm, Germany.
authors.forename: Ines
authors.initials: I
authors.lastname: Martin
authors.affiliation: Institute of General Physiology, Ulm University, 89081 Ulm, Germany.
authors.forename: Tatiana
authors.initials: T
authors.lastname: Felder
authors.affiliation: Central Facility for Electron Microscopy, Ulm University, 89081 Ulm, Germany. Electronic address: paul.walther@uni-ulm.de.
authors.forename: Paul
authors.initials: P
authors.lastname: Walther
authors.affiliation: Institute of General Physiology, Ulm University, 89081 Ulm, Germany.
authors.forename: Edward
authors.initials: E
authors.lastname: Felder
mesh_terms: D002460:Cell Line; D055032:Electron Microscope Tomography; D004926:Escherichia coli / Q000378:metabolism; D006801:Humans; D007382:Intermediate Filaments / Q000737:chemistry / Q000648:ultrastructure*; D015603:Keratinocytes / Q000737:chemistry*; D007633:Keratins / Q000378:metabolism*; D008855:Microscopy, Electron, Scanning
chemical_list: D007633:Keratins
keywords: Branching; Electron microscopy (EM); Electron tomography; Intermediate filaments; Keratin; Keratinocyte