Cell Bio Chapter 17

cell crawling

A Cortex Rich in Actin Filaments Underlies the Plasma Membrane of Most Eukaryotic Cells Cell Crawling Depends on Cortical Actin Forces generated in the actin-filament-rich cortex help move a cell forward -lamellipodium (has a high concentration of actin filaments) -actin polymerization causes lamellipodium to protrude and then lamellododium attaches to focal contacts (contain integrins) and then myosin motor proteins slide actin filaments thereby contracting and moving forward Actin filaments allow animal cells to migrate A web of polymerizing actin filaments pushes the leading edge of a lamellipodium forward this is due to the ARP (actin related protein complex). ARP polymerize new actin filaments (always have same degree of deflection) capping proteins will stabilize the end lamellopodium are sheet like whereas filopodium are fingerlike

contractile structures

Actin Associates with Myosin to Form Contractile Structures Myosin-I is the simplest myosin about 70 nm long have head group (has ATPase activity) have tail that is important in selecting cargo vesicles have receptors that are recognized but he tail domain of myosin-1 (plus end directed motor) can have myosin-1 attached to the membrane

skeletal muscle anatomy

Actin Filaments Slide Against Myosin Filaments During Muscle Contraction A skeletal muscle cell is packed with myofibrils they are multinucleate cuz cells fuse. nucleus near surface Sarcomeres (2.2 micro meter) are the contractile units (fundamental subunits) of muscle. they are connected to each other. dark band at the end is the Z disc ( end) plus end of the actin is attached at the z disc Muscles contract by a sliding-filament mechanism The head of a myosin-II molecule walks along an actin filament through an ATP-dependent cycle of conformational changes

summarize ACTIN FILAMENTS

Actin filaments are helical polymers of globular actin (gAct) monomers. They are more flexible than microtubules and are generally found in bundles or networks. Like microtubules, actin filaments are polarized, with a fast-growing plus end and a slow-growing minus end. Their assembly and disassembly are controlled by the hydrolysis of ATP tightly bound to each actin monomer and by various actin-binding proteins. The varied arrangements and functions of actin filaments in cells stem from the diversity of actin-binding proteins, which can control actin polymerization, cross-link actin filaments into loose networks (microvillus) or stiff bundles, attach actin filaments to membranes, or move two adjacent filaments relative to each other. A concentrated network of actin filaments underneath the plasma membrane forms the bulk of the cell cortex, which is responsible for the shape and movement of the cell surface, including the movements involved when a cell crawls along a surface.

Extracellular Signals

Can Alter the Arrangement of Actin Filaments Activation of Rho family GTPases can have a dramatic effect on the organization of actin filaments in fibroblasts - Unstimulated - Rho activated (form stress fibers) - Rac activation (forms a whole lamillopodium around the edge of the cell) - Cdc42 activation (cell is completely covered in fillipdium)

cilia and flagella

Cilia and Flagella Contain Stable Microtubules Moved by Dynein Many hairlike cilia project from the surface of the epithelial cells that line the human respiratory tract A cilium beats by performing a repetitive cycle of movements, consisting of a power stroke followed by a recovery stroke Flagella propel a cell through fluid using repetitive wavelike motion. during movement, the microtubules are sliding past each other Microtubules in a cilium or flagellum are arranged in a "9 + 2" array. ring of 9 doubles and 2 singlets in the middle. dynein reach from one microtubule to the next The movement of dynein causes the flagellum to bend...sliding motion gets turn into a bend (prevent microtubules from sliding right off

other types of muscle

Different Types of Muscle Cells Perform Different Functions Smooth muscle - Found in the walls of the stomach, intestine, uterus, arteries, and many other structures which undergo slow and sustained involuntary contractions - Contraction triggered by phosphorylation of myosin-II - Contraction triggered by adrenaline, serotonin, prostaglandins, and several other signaling molecules Nitrous oxide relaxes smooth muscle Cardiac muscle - Drives the circulation of blood - The heart contracts involuntarily for the life of the organism, up to 3 billion times during the average life span of a human - Subtle abnormalities in actin or cardiac myosin-II can lead to serious disease (sudden death disease in athletes) - Mutations in cardiac myosin-II genes in the sarcomere cause familial hypertrophic cardiomyopathy, a hereditary disorder responsible for sudden death in athletes

4 major classes of intermediate filaments

Intermediate filaments are divided into four major classes Cytoplasmic - Keratin filaments (in epithelial cells) - Vimentin and vimetin-related filaments (in connective tissue cells, muscle cells, and glial cells) - neurofilaments (in nerve cells...associated with ALS) Nuclear - nuclear lamins (in all animal cells) A mutant form of keratin makes skin more prone to blistering Plectin aids in the bundling of intermediate filaments and links these filaments to other cytoskeletal protein networks...one role of pectin is to cross link intermediate filaments to microtubules to form a mesh

summarize intermediate filaments

Intermediate filaments are stable, ropelike polymers—built from fibrous protein subunits—that give cells mechanical strength. Some intermediate filaments form the nuclear lamina that supports and strengthens the nuclear envelope; others are distributed throughout the cytoplasm.

INTERMEDIATE FILAMENTS

Intermediate filaments form a strong, durable network in the cytoplasm of the cell Intermediate Filaments Are Strong and Ropelike Intermediate filaments are like ropes made of long, twisted strands of protein connect desmosomes provide tensile strength found only in vertebrates each filament is formed of ten strands. at the lowest level two monomers form a twisted coiled coil dimer (48 dimer in length). dimers will associate to form a staggered tetramer. 8 tetramers line up to form the network and join end to end Intermediate Filaments Strengthen Cells Against Mechanical Stress basically.. -they don't bind to nucleotides -they are very strong -assemble into pre-existing filaments -less dynamic -unpolarized -no motors -function in cell tissue and integrity

what role do microtubules and motor proteins play in relation to organelles?

Microtubules and Motor Proteins Position Organelles in the Cytoplasm Microtubules help position organelles in a eukaryotic cell

summarize microtubules

Microtubules are stiff, hollow tubes formed by globular tubulin dimers. They are polarized structures, with a slow-growing minus end and a fast-growing plus end. Microtubules grow out from organizing centers such as the centrosome, in which the minus ends remain embedded. Many microtubules display dynamic instability, alternating rapidly between growth and shrinkage. Shrinkage is promoted by the hydrolysis of the GTP that is tightly bound to tubulin dimers, reducing the affinity of the dimers for their neighbors and thereby promoting microtubule disassembly. Microtubules can be stabilized by localized proteins that capture the plus ends, thereby helping to position the microtubules and harness them for specific functions. Kinesins and dyneins are microtubule-associated motor proteins that use the energy of ATP hydrolysis to move unidirectionally along microtubules. They carry specific organelles, vesicles, and other types of cargo to particular locations in the cell. Eukaryotic cilia and flagella contain a bundle of stable microtubules. Their rhythmic beating is caused by bending of the microtubules, driven by the ciliary dynein motor protein. Table From Lodish et al.

MICROTUBULES

Microtubules usually grow out from an organizing center - non dividing - dividing - ciliated Microtubules Are Hollow Tubes with Structurally Distinct Ends Microtubules are hollow tubes made of globular tubulin subunits. they are built from an alpha beta heterodimer. these subunits associate to form protofilaments and then the protofilaments (have a plus end and a minus end...have a alpha on one end and a beta on the other) associate with each other. microtubules are a helical array. always 13 protofilaments in a single microtubule (25 nm in diameter) basically.. -atpases -alpha-beta tubulin bind GTP -rigid and not easily bent polarized -regulated assmebly from a small number of locations -highly dynamic tracks for kinesisn and dyneins function in organization and long-range transport of organelles

motor proteins in microtubules

Motor Proteins Drive Intracellular Transport 2 class, -dyneins (ATPases, moves to the minus end) can have tails of various lengths. have two globular head groups kinesins- have 2 globular head groups. they are plus end directed. tails can bind to cargo Both kinesins and dyneins move along microtubules using their globular heads Different motor proteins transport different types of cargo along microtubules. the lagging head group will detach and the linking filament will allow it to swing forward and become the leading head group. this is called processive motion. its driven by ATP hydrolysis tails are what recognizes the specific types of cargo most kinesis are plus end directed but all the dyneins are minus ends directed Kinesin causes microtubule gliding in vitro A single molecule of kinesin moves along a microtubule. flexible filaments are different lengths so stepwise can be different. binding of kinesis to the microtubule causes it to release ADP and bind ATP

muscle contraction process

Muscle Contraction Is Triggered by a Sudden Rise in Cytosolic Ca2+ myosin is attached to a single actin molecule. ATP will bind causing a change in the myosin head group which reduces the affinity of the head for actin and allows it to move along the filament. then the cleft closes like a clam shell around the ATP molecule, triggering a large shape change that causes the head to be displaced along the actin filament . Hydrolysis of ATP occurs (enters the cocked state), but the ADP and inorganic phosphate (Pi) produced remain tightly bound to the myosin head.A weak binding of the myosin head to a new site on the actin filament causes release of the inorganic phosphate produced by ATP hydrolysis. This release triggers the power stroke—the force-generating change in shape during which the head regains its original conformation. In the course of the power stroke, the head loses its bound ADP, thereby returning to the start of a new cycle. At the end of the cycle, the myosin head is again bound tightly to the actin filament in a rigor configuration. Note that the head has moved to a new position on the actin filament, which has slid to the left along the myosin filament. T tubules (key of getting Ca2+ in...invaginations of plasma membrane) and the sarcoplasmic reticulum surround each myofibril Skeletal muscle contraction is triggered by the release of Ca2+ from the sarcoplasmic reticulum into the cytosol -voltage gated calcium channel and clcim release channel Skeletal muscle contraction is controlled by tropomyosin and troponin complexes sarcoplasmic reticulum calcium atp ase brings calcium back into the lumen of the SR. for every ATP it hydrolyzes it bring in 2 calcium ions tropomyosin (binds abut 7 actin subunits) blocks myosin from touching actin. tropomyosin is regulated by the troponin complex (3 subunits) binds calcium and cause change in organization of tropomyosin so it slides off of actin binding sites

summarize muscle contration

Myosins are motor proteins that use the energy of ATP hydrolysis to move along actin filaments. In nonmuscle cells, myosin-I can carry organelles or vesicles along actin-filament tracks, and myosin-II can cause adjacent actin filaments to slide past each other in contractile bundles. In skeletal muscle cells, repeating arrays of overlapping filaments of actin and myosin-II form highly ordered myofibrils, which contract as these filaments slide past each other. Muscle contraction is initiated by a sudden rise in cytosolic Ca2+, which delivers a signal to the myofibrils via Ca2+-binding proteins associated with the actin filaments.

Centrosome

The Centrosome Is the Major Microtubule-organizing Center in Animal Cells centrosomes are made of a pair of centrioles (microtubules) that sit at a 90° angle. centrioles are short stiff rings Tubulin polymerizes from nucleation sites (gamma tubulin ring complex) on a centrosome ring complex greatly accelerate the formation of microtubules microtubules grow at their plus ends from gamma tubulin ring complexes of the centrosome. minus ends are always attached to the centrosome Tubulin polymerizes from nucleation sites on a centrosome

cytoskeleton of prokaryotes vs eukaryotes

The Cytoskeleton The cytoplasm of a eukaryotic cell is supported and organized by a cytoskeleton of intermediate filaments, microtubules, and actin filaments. Prokaryotes contain proteins that are analogous to tubulin, actin, and intermediate filaments and play roles in cell shape, division, protection, and polarity.

nuclear envelope

The Nuclear Envelope Is Supported by a Meshwork of Intermediate Filaments Intermediate filaments support and strengthen the nuclear envelope nuclear lamina provides supports for nucleus and anchoring for chromosomes Defects in a nuclear lamin can cause a rare class of premature aging disorders called progeria found only in animals lamina protein not found in plants

protein filaments

The three types of protein filaments that form the cytoskeleton differ in their composition, mechanical properties, and roles inside the cell - Intermediate filaments - Microtubules filaments - Actin filaments

Dynamic Instability of microtubules

abrupt shortening of microtubules Each microtubule grows and shrinks independently of its neighbors The selective stabilization of microtubules can polarize a cell microtubule capping protein can capture the tips of microtubules on one end of the cell and anchor them to a particular shot on the cell Dynamic Instability is Driven by GTP Hydrolysis when heterodimers are trying to bind, they need to b in the GTP bound state. usually add to the plus end cuz have higher GTP concentration there (have a GTP cap). if the addition proceeds faster than hydrolysis than have growing microtubule GDP binds less firmly to heterodimers which leads to them peeling away and get frayed ends and GDP tubulin is released in the cytosol GTP hydrolysis controls the dynamic instability of microtubules

cytoskeletal filaments diameters

actin diameter-7 nm microtubule diameter -25 nm intermediate filaments -11 nm

ACTIN FILAMENTS

aka microfilaments Actin filaments allow animal cells to adopt a variety of shapes and perform a variety of functions..function in microvilli and contractile bundles, actin rings that form at division site Actin Filaments Are Thin (7 nm in diameter) and Flexible -cleft were ATP binding site is. clefts face downward (plus end) -actin filaments are polar -full list of the helix is 37 nm -helical monomers not dimers Actin filaments are thin, flexible protein threads Actin and Tubulin Polymerize by Similar Mechanisms. actin needs ATP to bind. once it binds it will hydrolyze the ATP to ADP and the phosphate will dissociate. actin dissociate wth bound ADP ATP hydrolysis decreases the stability of the actin polymer Treadmilling (addition of actin monomers to the plus end) of actin filaments and dynamic instability of microtubules regulate polymer length in different ways

cytoskeleton

gives a cell its shape and allows the cell to organize its internal components and to move

How do Microtubules Organize the Cell Interior?

microtubules run the length of the axon (plus ends at the axon terminal) movement of molecules is driven by molecular motors Microtubules guide the transport of organelles, vesicles, and macromolecules in both directions along a nerve cell axon Organelles can move rapidly and unidirectionally in a nerve cell axon.

give examples of how Proteins Bind to Actin and Modify Its Properties

monomer-sequestering protiens nucleating proteins severing proteins cross-linking proteisn capping (plus end blocking) protein side-bind proteins myosin motor proteins bundling proteins (in filopodium) Actin-binding proteins control the behavior of actin filaments in vertebrate cells

myosin ii

myosin II -double head, where ATP binding site is (cleft is where ATP binds) 150 nm long tail is coiled coiled domain filaments can associate from side to side to form thick filaments (bipolar) Muscle Contraction Depends on Interacting Filaments of Actin and Myosin Myosin-II molecules can associate with one another to form myosin filaments - myosin-II molecule - myosin-II filament A small, bipolar myosin-II filament can slide two actin filaments of opposite orientation past each other myosin walks toward the plus end

drugs that affect actin filaments

phallodin -binds and stabilizes actin filaments -fungal toxin, comes from death cap mushroom cytochalasin -caps filaments plus end prevent polymerization there lantrunculin -binds actin monomers and prevents their plymerization

drugs that affect microtubules

taxol -binds and stabilizes microtubules colchicine, colcemid -binds tubulin dimers and prevents their polymerization vinblastine, vincristine -binds tubulin dimers and prevents their polymerization Microtubule Dynamics Can be Modified by Drugs