BIOL1110 M2 Lecture 3 PDF

BIOL1110 From Molecules to Cells (Lecture 3) Cytoskeleton & intercellular connection Dr. Gary Ying Wai Chan Office: Rm4N11 KBSB Email: [email protected] Cytoskeleton - Both microtubule and microfilament are formed from bead- like, globular pro...

BIOL1110 From Molecules to Cells (Lecture 3) Cytoskeleton & intercellular connection Dr. Gary Ying Wai Chan Office: Rm4N11 KBSB Email: [email protected] Cytoskeleton - Both microtubule and microfilament are formed from bead- like, globular protein subunits - Intermediate filament are made from fibrous protein subunits Microfilament Intermediate filament Microtubule Microtubule: centrosome made of tubulin Microtubules are long, hollow cylinders made of the protein tubulin (alpha tubulin and beta tubulin). With an outer diameter of 25 nm, they are much more rigid than actin filaments. Microtubules are long and straight and typically have one end attached to a single microtubule- organizing center (MTOC) called a centrosome. Function: cell division, organization of intracellular structure, and intracellular transport, as well as ciliary and flagellar motility. Microtubule and centrosome Centriole Centriole is made up of tubulin (picture ii), two centrioles from a centrosome, which is the major microtubule- organizing center (MTOC) in human cells. Centriole consist of 9 sets of 3 attached microtubules arranged to form a hollow cylinder Centrosomes organize microtubules in cytoplasm (picture iii), microtubules from a framework such as mitotic spindle that appears during cell division (picture iv).  Microtubule: form cilia and flagella v There are two types of cilia: 1. primary cilia serve as sensory organelles; 2. motile cilia present on a cell's surface in large numbers and beat in coordinated waves Flagella are a long, whip-like structure that help cells move. Microtubules can also from the whip-like organelles known as cilia and flagella (picture v). Microtubule: Microtubules are ever-changing, with reactions constantly adding and subtracting tubulin dimers dynamic at both ends of the filament.The rates of change at either end are not balanced — one end grows instability more rapidly and is called the plus end, whereas the other end is known as the minus end. Dynamic instability of microtubules centrosome Microfilament Actin filaments (also known as microfilaments) are two-stranded helical polymers of the protein actin. They appear as flexible structures, with a diameter of 5-9 nm and less rigid than MT, and they are organized into a variety of linear bundles, two- dimensional networks, and three-dimensional gels. Microfilament: functions Although actin filaments are dispersed throughout the cell, they are most highly concentrated in the cortex, just beneath the plasma membrane. They are important for many important cellular processes, including muscle contraction, cell motility, cell division and cytokinesis, vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape. Actin filament and myosin work together for muscle movement Corpses of victims of the 1991 Bangladesh cyclone in Sandwip displaying signs of rigor mortis (Author: Rahat) Rigor mortis (or postmortem rigidity), why?  Intermediate filament Intermediate filaments are ropelike fibers with a diameter of around 10 nm; they are made of intermediate filament proteins, which constitute a large and heterogeneous family (type I to type VI and unclassified). Intermediate filaments do not participate in cell motility One type of intermediate filament forms a meshwork called the nuclear lamina just beneath the inner nuclear membrane. Other types extend across the cytoplasm, giving cells mechanical strength. In an epithelial tissue, they span the cytoplasm from one cell-cell junction to another, thereby strengthening the entire epithelium. How individual cells assemble themselves to form tissues and organs? e.g., How do individual epithelial cells form an epithelium (epithelial tissue)? Specialized junctions offer a solution Structural and functional links between cells Apical surface: facing the lumen Tight junctions produce impermeable or semipermeable barriers between cells, especially in epithelial membranes. Gap junctions allow electrolytes and other molecules to pass between cells. Also known as communicating junctions. They are found in epithelial tissues, in cardiac and smooth muscle, in nervous and certain connective tissues. Adherens junctions provide strong mechanical attachment between adjacent cells. They hold cardiac muscle cells tightly together as the heart expands and contracts. They hold epithelial cells together. Desmosomes are like spot welds which hold cells together and help prevent lateral tearing of tissues Basal surface: other sides Adherens junction Lock cells together - Major transmembrane protein: cadherin - Major peripheral proteins : catenin - Cytoskeletal anchor: actin Actin catenin Cadherin catenin cell cell Cadherins The human skin cell shown in this LM were grown in culture and stained with fluorescent antibodies. Cadherins, a group of membrane proteins, are seen as green belts around each cell in this sheet of cells. The nuclei appear as blue spheres; myosin in the cells appears red. Cadherin molecule as an example of cell i nteractions : http://www.youtube.com/watch?v=pBUIb5jchVo ArkitekStudios Summary of the video: -Cadherin molecules are transmembrane proteins, the extracellular segments with 5 domains that form dimers in the same cell in a Ca2+-dependent manner, which interacts with cadherin dimers from another cell. -if you take away the Ca2+, then this cell- cell interaction is disrupted, and the cells will dissociate from each other. -the intracellular domain of cadherin interacts with catenin which adhere the cadherin to intracellular cytoskeleton. Gap junctions allow the transfer of small molecules and ions between adjacent cells. Gap junction - Allow ions and small molecules flow between the cytoplasm of neighboring cells; e.g., electrical coupling in cardiac and nerve cells. - Major protein component: connexin - Connexon is made up of 6 connexins Tight junctions prevent the passage of materials through spaces between cells.  Tight junction - Rows of proteins form tight seal by fusing each cell to its neighbors - Prevent substances from leaking across the free surface - Major transmembrane proteins: occludin, claudin, - Major peripheral protein for interaction with cytoskeleton: ZO-1 occludin, claudin ZO-1 Actin Actin cell cell Tight sealing What happen if tight junctions no longer function?  Question time! True or False Q: Actin filament is more rigid than microtubules Q: Cilia are made of microtubules Q: Calcium ion controls the interaction between cells via adherens junctions. Q: Major transmembrane protein in gap junction is claudin. Q: Gap junction permits the free passage of small proteins of ~10kDa between the cells