Week 9 Flashcards
3 types of cytoskeletal polymers
actin filaments, intermediate filaments, and microtubules
cytoskeleton is made of
protein polymers
Cytoskeleton can form….
polarized, highly dynamic, self-organizing structures
Microtubule function
function in concert with molecular motors that generate force and move vesicles and other complexes along the microtubule surface.
Cells use microtubules to provide…
structural support because microtubules are the strongest of the cytoskeletal polymers
Do microtubules have polarity?
Yes. Positive end is crowned by β-tubulin and assembles faster.
The negative end is crowned by α-tubulin and assembles slower
Tubulin Polymerization
Microtubule polymerization begins with the formation of a small number of nuclei (small polymers).
Microtubules polymerize by addition of tubulin subunits to both ends of the polymer.
critical concentration
A critical concentration of tubulin subunits always remains in solution. The concentration of tubulin must be above the critical concentration (Cc) for assembly to occur.
is there a net change in polymerization at critical concentration?
No. There is no net change
Dynamic Instability
the process where microtubules are constantly switching between phases of growth and shortening
Driven by GTP hydrolysis
microtubule rescue
The transition from shortening to growing
microtubule catastrophe
The transition from growing to shortening states
Straight proto-filaments indicate…
the growing end and have lateral and longitudinal bonds to stabilize them
curled proto-filaments indicate…
the shortening end. They bend back & away form the microtubule lattice. Eliminates the lateral bonding
how GTP Hydrolysis Drives Dynamic Instability
Growing microtubules have a cap of GTP-tubulins at their tip because the GTP associated with β-tubulin is hydrolyzed to GDP shortly after a subunit adds to a microtubule
Why Do Cells Have Dynamic Microtubules?
Dynamism of microtubules is vital for cell function
slide 15
Interactions Between Microtubules and Actin Filaments
Microtubules and actin filaments function together during cell locomotion and cell division.
In general, microtubules direct where and when actin assembles or generates contractile forces.
Microtubules influence the actin cytoskeleton through direct binding or indirect signaling.
Actin
Exists as both a monomer called G-actin and as a filamentous polymer called F-actin
Actin structure
The actin filament is structurally polarized and the two ends are not identical.
Actin Polymerization
De novo actin polymerization is a multistep process that includes nucleation and elongation steps.
The rates of monomer incorporation at the two ends of an actin filament are not equal
When do Actin Subunits Hydrolyze ATP
after polymerization.
Actin monomers can be bound to ATP, ADP+Pi, or ADP alone.
The critical concentration for actin assembly depends on whether actin has bound ATP or ADP
Actin-Binding Proteins
Regulate actin polymerization and organization
Associate with monomers or filaments and influence the organization of actin filaments in cells
The cell uses these proteins to regulate motility
G-protein Regulation of Actin Polymerization
Members of the Rho family of small G proteins regulate actin polymerization and dynamics.
Activation of Rac, Rho, and Cdc42 proteins induces formation of lamellipodia, contractile filaments, and filopodia, respectively
Myosins cellular role
Myosin proteins are energy transducing machines that use ATP to power motility and generate force along actin filaments.
Some myosins power muscle and cellular contractions, whereas others power membrane and vesicle transport, regulate cell shape and polarity and participate in signal transduction and sensory perception pathways.
myosin
actin-based molecular motors with essential roles in many cellular processes.
myosin head/motor domain
Contains the ATP- and actin-binding sites and is responsible for converting the energy from ATP hydrolysis into mechanical work.
Myosin regulatory domain
Acts as a force transducing lever arm.
Myosin tail domain
Interacts with cargo proteins or lipid and determines its biologic function
Mechanochemical Pathway of Myosin
Myosin’s affinity for actin depends on whether it’s bound to ATP, ADP-Pi, or ADP.
Myosins with bound ATP or ADP-Pi are in weak binding states, and will rapidly associate and dissociate from actin
Myosins with either bound ADP or no nucleotide are in strong binding states.
Myosin regulation
regulated by phosphorylation and by interactions with actin- and myosin-binding proteins
Intermediate Filaments
are major components of the cytoplasmic and nuclear cytoskeletons.
Essential in maintaining correct tissue structure and function.
Extracellular Matrix (ECM)
Network of material secreted from the cells forming a complex meshwork outside of cells
Major component of certain parts of plants and animals
(Bone and cartilage of animals
Woody parts of plants)
Adhesive ECM proteins
Fibronectin and laminin
Adhere ECM components together and to the cell surface
Structural ECM proteins
Collagen provides tensile strength
Main protein found in bone, cartilage, tendon, skin
Elastin provides elasticity
Expands and returns to original shape
Collagens
common structure, what differentiates a collagen
Many different genes encode procollagen
All collagens have a common triple helix structure
Similar yet different amino acid sequences affect structure and function of each specific type of collagen
Laminins
location, function
Found in virtually all tissues of vertebrate and invertebrate animals.
The principal functions of laminins are to provide an adhesive substrate for cells and to resist tensile forces in tissues.
Types of animal cell junctions
tight, gap, and anchoring junctions
Tight Junctions (function, components/structure, location)
Forms tight seal between adjacent cells
Prevents ECM from leaking between cells
Made by occludin and claudin
Bind to each to form tight seal
Not mechanically strong, not strongly associated to cytoskeleton
Gap Junctions (function, components/structure, location)
Small gap between plasma membranes of cells at junction
Six connexin proteins in one cell align with six connexin proteins in an adjacent cell to form a connexon
Connexon allows passage of ions and small molecules
Allow adjacent cell to share metabolites and directly signal each other
Anchoring Junctions (function, components/structure, location)
Attach cells to each other and to the ECM
Rely on cell adhesion molecules (CAM)
Cadherin and integrin
4 categories of Anchoring junctions
Adherins junctions
Desmosomes
Hemidesmosomes
Focal adhesions
Adherens Junctions (function, components/structure, location)
junctions are a family of related cell surface domains that link neighboring cells together.
Adherens junctions contain transmembrane cadherin receptors and link the cells actin cytoskeletons together
Desmosomes
function, components/structure, location
intermediate filament-based cell adhesion complexes.
The principal function of desmosomes is to provide structural integrity to sheets of epithelial cells by linking the intermediate filament networks of cells.
desmosomes can function as both…
adhesive structures and as signal transducing complexes
Hemidesmosomes
function, components/structure, location
Hemidesmosomes, like desmosomes, provide structural stability to epithelial sheets.
Hemidesmosomes are found on the basal surface of epithelial cells, where they link the ECM to the intermediate filament network via transmembrane receptors.
Anchoring Junction Proteins: Cadherins
function, components/structure, location
Cell Adhesion Molecules (CAMs) that create cell-to-cell junctions
Ca2+ dependent adhering molecule
Extracellular domain of two cadherins, each in adjacent cells, bind to each other to promote cell-to-cell adhesion
Anchoring Junction Proteins: Integrins
function, components/structure, location
Integrins are a type of CAM that connect cells to the ECM
Extracellular portion binds ECM
Intracellular portion binds cytoskeleton and signaling proteins
Integrin structure
Integrins are composed of two distinct subunits, known as α and β chains.
intern function
Integrins are signaling receptors that control both cell binding to ECM proteins and intracellular responses following adhesion.
Integrins have no enzymatic activity of their own. Instead, they interact with adaptor proteins that link them to signaling proteins.
affinity modulation
varying the binding strength of individual receptors
avidity modulation
varying the clustering of receptors
Integrins and Inside-Out Signaling
Changes in receptor conformation result from intracellular signals that originate elsewhere in the cell (e.g., at another receptor).
Components of biological membranes
phospholipid bilayer, proteins, and carbohydrates
Fluid-Mosaic Model
Membrane exhibits properties that resemble a fluid because lipids and proteins can move relative to each other within the membrane
Fluidity
individual molecules remain in close association yet have the ability to readily move within the membrane
types of membrane proteins
integral/intrinsic and peripheral/extrinsic
Semifluid
most lipids can rotate freely around their long axes and move laterally within the membrane leaflet
Flipflop” of lipids
lipids from one leaflet to the opposite leaflet does not occur spontaneously
Flippase requires what molecule to transport lipids from one leaflet to the other
ATP
Factors Affecting Fluidity
Length of fatty acyl tails.
Presence of double bonds in the acyl tails.
Presence of cholesterol.
Cholesterol tends to stabilize membranes.
Effects depend on temperature.
Fatty acid saturation
refers to amount of Hydrogens bonded to Carbon
Saturated means no C=C double bonds
Experiments on Lateral Transport
Larry Frye and Michael Edidin conducted an experiment that verified the lateral movement of membrane proteins
What cells were fused in the Frye and Edidin experiments
Mouse and human cells
How is the movement of some integral membrane proteins restricted?
may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally.
Also, membrane proteins may be attached to molecules that are outside the cell, such as the interconnected network of proteins that forms the extracellular matrix
what type of microscopy can be used to study membranes?
Transmission electron microscopy (TEM)
and Freeze fracture electron microscopy (FFEM), specialized form of TEM, can be used to analyze the interiors of phospholipid bilayers
How is the transfer of Lipids to Other Membranes possible?
Lipids in ER membrane can diffuse laterally to nuclear envelope
Transported via vesicles
Lipid exchange proteins
Cholesterol stabilization of membrane at high temperatures
Lipid becomes more fluid
Cholesterol becomes more rigid
Cholesterol stabilization of membrane at low temperatures
Lipid becomes more rigid
Cholesterol becomes more fluid
where can you find cholesterol in the membrane?
between phospholipids. Polar OH group by the heads and aromatic groups by the tails
K+ channels function
function as water-filled pores that catalyze the selective and rapid transport of K+ ions.
What do K+ channels catalyze?
catalyze selective and rapid ion permeation
K+ channels-Selectivity Filter
catalyzes dehydration of ions, which confers specificity and speeds up ion permeation.
What regulates K+ channels?
Membrane potential
