BIO 211 Lecture 9: Microtubules, Intermediate Filaments, and Septins - Drexel University PDF

BIO 211: Cell, Molecular and Developmental Biology II Drexel University, Winter 2025 Lecture #9 – Microtubules, Intermediate Filaments, and Septins  OVERVIEW MICROTUBULES Chapter 16, pages 987 - 1001 INTERMEDIATE FILAMENTS AND SEPTINS Chapter 16,...

BIO 211: Cell, Molecular and Developmental Biology II Drexel University, Winter 2025 Lecture #9 – Microtubules, Intermediate Filaments, and Septins  OVERVIEW MICROTUBULES Chapter 16, pages 987 - 1001 INTERMEDIATE FILAMENTS AND SEPTINS Chapter 16, pages 1007 - 1012 The EB1 protein binds to the plus ends of growing microtubules 2 min time-lapse GFP-EB1 Microtubules are highly dynamic Microtubules plus ends point towards the cell periphery Microtubules help move cargo and signaling proteins around the cell Intermediate filaments and septins form physical barriers F-actin Intermediate filaments (Ifs) and septins form physical barriers Keratin (a type of IF) Intermediate filaments are relatively static Intermediate filaments are non-polarized Intermediate filaments provide mechanical strength to cells and tissues Intermediate filaments (Ifs) and septins form physical barriers Keratin F-actin The Functions of Actin Filaments Are Inhibited by Both Polymer-stabilizing and Polymer- destabilizing Chemicals α (alpha) and β (beta) tubulin form the tubulin heterodimer The tubulin heterodimer bound to a guanine nucleotide is the basic subunit of MTs The GTP cap helps to control microtubule growth Hydrolysis of GTP to GDP on tubulin leads to rapid depolymerization of MTs Microtubules Undergo Dynamic Instability Microtubules Undergo Dynamic Instability Hydrolysis of GTP to GDP on tubulin leads to rapid depolymerization of MTs due to a conformational change that weakens the MT polymer bond. Microtubules Undergo Dynamic Instability The rapid switch between growing and shrinking is a result of dynamic instability within the MT polymer. Dynamic instability means that MTs are able to rapidly change their network architecture in response to extracellular and intracellular signals. A Protein Complex Containing g-Tubulin Nucleates Microtubules γ (gamma) tubulin is localized around centrosomes to form the minus end of the microtubule following nucleation γ-TuSC = gamma tubulin small complex Microtubules Emanate from the Centrosome in Animal Cells A centrosome in a human fibroblast DNA F-actin Pericentrin Samir Jambhekar Undergraduate Scientist Petrie Lab Microtubule-Binding Proteins Modulate Filament Dynamics and Organization J Nucleates microtubules J J Governs microtubule dynamics J J J Connects MTs to other structures J J J Microtubule Plus-End-Binding Proteins Modulate Microtubule Dynamics and Attachments Catastrophe and stabilization factors both target the GTP-bound tubulin dimers at the plus end of the polymer Tubulin-Sequestering and Microtubule-Severing Proteins Destabilize Microtubules Stathmin is similar to thymosin in that it limits the availability of subunits to reduce polymer growth. Also different in that it binds to two tubulin heterodimers instead of a single monomer. Two Types of Motor Proteins Move Along Microtubules Two Types of Motor Proteins Move Along Microtubules Two Types of Motor Proteins Move Along Microtubules Cytoplasmic dynein Microtubules and Motors Move Organelles and Vesicles Intermediate filaments (IFs) and septins Keratin F-actin Introduction Intermediate Filament Structure Depends on the Lateral Bundling and Twisting of Coiled- Coils Step (C) is why intermediate filaments are non-polar when assembled in cells A B C Intermediate Filament Structure Depends on the Lateral Bundling and Twisting of Coiled- Coils This rope-like structure of overlapping subunits is what gives intermediate filaments their mechanical strength. Many bonds within the polymer means they can stretch/bend without breaking. Intermediate Filaments Impart Mechanical Stability to Animal Cells Mutations within the keratin gene can lead to abnormally weak intermediate filaments and blistering diseases like epidermolysis bullosa. Linker Proteins Connect Cytoskeletal Filaments and Bridge the Nuclear Envelope Plectin (green) acts to cross-link different cytoskeleton polymers together Linker Proteins Connect Cytoskeletal Filaments and Bridge the Nuclear Envelope Many of these connections are required simply to apply force to the nucleus in order to move or position it within the cell. Fibroblast lobopodial migration in a three-dimensional matrix 3 hour time-lapse  The Nuclear Piston Mechanism of 3D Cell Migration: Myosin II pulls the nucleus forward to pressurize the cytoplasm and generate lobopodia Primary fibroblast in 3D CDM Nucleus GFP-vimentin Petrie et al. 2014 Science. 345: 1062-1065. Petrie et al. 2017 J. Cell Biol. 216: 93-100. Septins: the final frontier in the eukaryotic cytoskeleton Septin 7 Septins Form Filaments That Regulate Cell Polarity Cdc = septin 1-9 Septins can form filaments, sheets, and/or rings. Septins Form Filaments That Regulate Cell Polarity Septins can form diffusion barriers in dividing yeast cells and at the base of cilia. Septin localization depends on matrix dimensionality DNA Back Front F-actin Septin 7 Nick Sookhoo Undergraduate Scientist Petrie Lab 2D Matrix 3D Matrix SUMMARY OF MTs, IFs, and SEPTINS MICROTUBULES Large, polarized filaments capable of rapid growth and collapse Nucleated by gamma tubulin within centrosomes MT-binding proteins control MT stability and cargo trafficking INTERMEDIATE FILAMENTS Large family non-polar filaments which give mechanical strength to the cell Plectins connect IFs to other cytoskeletal structures SEPTINS Stay tuned!