Shared Flashcard Set


Chapter 12
Undergraduate 4

Additional Biology Flashcards




Describe Actin

1. Basic Structure
2. Abundance
1. polymerizes to form actin filaments
-thin and flexible
-7 nm in diameter and several um in length
-AKA microfilaments organized into bundles and 3D networks

2. 5-10% of total protein in all cells
6 kinds of genes- 4 in muscle and 2 in non muscle
Describe Actin

1. Naming
2. Shape of each protein
1. a. [G] Monomer- globular
b. [F] Filamentous- globular actin binds head to tail with two other actins to form filaments

2. Pointed end- point
Barbed- mouth
*establishes polarity
Describe Actin: Polymerization
1. trimer is formed and monomers are added to either end
*trimerization is most difficult step
-barbed end is growing end (10x faster)

2. polymerization is reversible
-dynamic process
3. rate at which monomers are added is dependent upon concentration [G]
Describe Actin: Treadmilling
1. Barbed end grows faster
2. ATP bound actin dissociates less than ADP bound actin
3. ATP is hydrolyzed and is released at pointed end
4. ADP is converted to ATP and binds at barbed end
-net less at pointed end and net gain at barbed
Describe the following drugs

1. Cytochalasins
2. Phalloidin
1. bind to barbed ends of actin and block elongation
-inhibit movement because actin cannot grow

2. bind to actin and prevents dissociation
-stabilizes to the point where it cannot dissociate
-there is no free actin to polymerize to make new actin

*both block phagocytosis
Describe Actin Binding Proteins
Regulate assembly and disassembly of actin filaments

1. bind along length of actin to stabilize or cross link
2. bind to caps of filaments to stabilize ends and prevent dissociation
3. promote dissociation by severing or stimulating depolymerization
4. regulate exchange of ATP/ADP
Describe the following Actin Binding Proteins

1. Formin
2. Tropomyosins
3. Arp 2/3
4. ADF/Colin
5. Profilin
1. dimers that bind to barbed end of initial monomer and move along filament to add monomers and extend
-unbranched filaments
2. stabilize long unbranched filaments by binding lengthwise along the groove of filaments
3. facilitate branch formation by binding to ATP near barbed end
4. facilitates breakdown by
a. binding to released actin/ADP and not allowing it to be charged to ATP for reincorporation
b. binds at center and cleaves
5. promotes binding by exchanging ADP for ATP
-new actin/ATP can be added to barbed end
Describe Organization of Actin Filaments

1. Actin Bundles
2. Actin Networks
1. filaments are cross-linked into closely packed parallel arrays
-cross linking (actin bundling proteins) are small rigid proteins that bind actin and align filaments closely
ex. intestinal villi

2. filaments are cross-linked in arrays that for 3D meshwork
-these proteins are large and flexible
Describe Actin Bundles

1. Parallel Bundles
2. Contractile Bundles
3. Network
1. closely spaced parallel filaments
-all the same polarity
ex. Fimbrin

2. more widely spaced filaments
-increased spacing allows motor protein in myosin to interact with actin filaments
ex. alpha actinin

3. Filamin forms cross-links
-dimer that flares at bottom to bind

*both rely on Calcium
Describe Actin Proteins that provide structural support for the membrane

1. Spectrin
2. Ankyrin
3. Dystrophin
1. tetramer of two peptide chains (alpha and beta)
-ends associate with short actin filament
2. links spectrin-actin to membrane by binding to Band 3 (transmembrane protein)
-Protein 4.1 binds spectrin to glycophorin
3. links actin to transmembrane proteins in muscle cells
-gives stability during muscle contraction
ex. MD- dystrophin is easier absent or abnormal
Describe attachment of actin and ECM
1. Cells excrete ECM onto dish
2. attach via integrin binding
3. attachment sites (focal adhesions) also attach large actin bundles called stress fibers
-stress fibers are contractile bundles
-cross linked with alpha actinin, tropomyosin

Talin and Vinculin- binding to stress fibers

*only occurs in cell culture
Describe Tight Junctions
1. actin binds to p120 and Catenins
2. Cadherins are transmembrane proteins that bind to catenins
3. keeps tissue in contact
Describe Actin and Microvilli
1. parallel bundles of 20 to 30 actin filaments
2. filaments are cross linked by villin
-also fimbrin
3. attached to membrane by calmodulin and myosin I
Describe the following Protrusions

1. Pseudopodia
2. Lamellipodia
3. Filopodia (Microspikes)
1. extension responsible for phagocytosis and movement
2. broad, sheetlike extensions at edge of fibroblasts
3. thin projections of plasma membrane supported by actin
Define Myosin
protein that converts chemical energy (ATP) into mechanical energy generating movement
Describe the organization of muscle
1. Muscles are bundles of muscle fibers
2. the cytoplasm of these cells consists of myofibrils
-bundles of thick myosin and thin actin filaments
3. each myofibril is organized into sarcomeres
-a chain of contractile units
Describe the sarcomere
-alternating actin and myosin
-from Z disc to Z disc is 1 sarcomere
-barbed end of actin attaches to Z disc
-cross linked by alpha actinin

Contraction occurs when two Z discs come together and actin and myosin remain in place
Describe the following Sarcomere proteins

1. Titin
2. Nebulin
1. extremely large; single molecules extend from M line to Z disc
-act as springs to keep myosin centered
-maintain tension

2. filaments are associated with actin
-regulate length at assembly
Describe the Sliding Filament Model
Sarcomere- shortens and brings Z discs closer
A band- stays the same (contains actin/myosin)
I and H- almost disappear
Actin/Myosin- slide past one another
Describe Myosin II

1. Structure
2. Arrangement
1. large protein with two heavy chains and two light chains
Heavy- globular head and long alpha helical tail
-tails twist around each other
Light- Essential light chain and Reg light chain

2. several hundred line up in parallel staggered array
-globular heads bind to actin forming bridge
-orientation flips at end line
Describe the Myosin Model of Function
1. binding of ATP dissociates myosin from actin
2. ATP hydrolysis causes conformational change that displaces myosin head
3. myosin binds to new position on actin and Phos is released
4. myosin head returns to original conformation which pulls actin backwards and ADP is released
*power stroke
Describe the Role of Calcium in Muscle Cells
1. Ca is stored and released from sarcoplasmic reticulum
2. Ca is increased in the cell and allows tropomyosin and troponin to bind
3. Tropomyosin is bound lengthwise and is bound to troponin
4. Without Ca: tropomyosin blocks actin binding site
With Ca: troponin shifts and actin binding site is free
-contraction can proceed
Describe the Role of Actin/Myosin and Cytokinesis
Contractile ring of actin and myosin II pinches the cell in two
Describe the phosphorylation of myosin
-nonmuscle cells, contraction is regulated by phosphorylation of myosin light chain
-catalyzed by myosin light chain kinase (MLCK)
-regulated by Ca dependent Calmodulin

1. Calmodulin is activated by Ca
2. Calmodulin activates MLCK
3. phosphorylates regulatory light chain
Describe Intermediate Filaments: Overview

1. Size
2. Function
3. Structure
4. Formation
5. Location
1. diameters are inbetween actin and microtubules
2. not directly involved in cell movement
-provide mechanical strength and scaffold
-bendable, strong, balance!
3. 8 protofilaments but do not have specific ends
4. more stable and are not dynamic (no treadmilling)
-however phospho can regulate dis/assembly
5. form network in cell, extending from nucleus
-also associate with cytoskeleton to form scaffolding
Describe the following types of Intermediate Filaments

1 and 2. Keratins
3. Vimentin and Desmin and Glial Fibrillary Acidic Protein
4. Nerofilament Proteins
5. Nuclear Lamins
6. Nestins
1. found in epithelial cells (1=acidic 2=neutral/basic)
3. V: forms network extending out from nucleus
D: expressed in muscle cells where it connects Z discs
G: found in glial cells; forms blood barrier
4. found in mature neurons especially long motor axons
-diameter of axon depends on neurofilaments
5. structural support in nucleus
6. expressed during embryonic development in stem cells
Describe the Structure of InterMedFil Proteins
-all have central alpha helix of about 310 AA
-head and tail domains determine function
Describe InterMedFil Assembly
1. central rod domains of two proteins form a coiled coil
2. these dimers associate in staggered antiparallel fashion to form tetramers (head to tail)
-assemble end-to-end to form protofilaments
3. 8 protofilaments come together to form filament
Describe Microtubules

1. Function
2. Structure
3. Assembly
1. cell movement, cell shape, track for kinesmotor, transport of organelles, separation of chromosomes
2. rigid hollow rods, stiff
-made of tubulin dimers (alpha and beta)
-gamma tubulin found in centrosomes
3. dynamic-constantly breaking down and building up
Describe Microtubule Assembly
1. alpha and beta tubulin dimers polymerize to form micros
-consists of 13 protofilaments assembled around a hollow core
2. Protofilaments are composed of head to tail arrays of a/b dimers arranged in parallel
-two distinct ends
3. Treadmilling

*Rapid turnover is critical for remodeling cell during mitosis
Describe Microtubule Assembly: Treadmilling
similar to that of actin but use GTP instead

1. GTP binds to beta and add to growing end (alpha head)
2. GTP is converted to GDP and tubulin is less stable
3. dimers at minus end dissociate (beta tail)
Describe Microtubule Assembly: Dynamic Instability
1. high concentrations of tubulin-GTP, the dimers are added more rapidly than GTP hydrolysis
-micro grows and is longer
2. if concentration drops, GTP at plus end is hydrolyzed and dimer are released
-shorter micros
-GTP cap may fade
Describe Drugs against Microtubule Assembly

1. Colchine and Colcemid
2. Vincristine and Vinblastine
3. Taxol
1. bind tubulin and inhibit polyemerization
2. destabilize micros and inhibit rapidly dividing cells
3. stabilizes micros so the cell can't pinch
-locks together
-no monomers for new micros

*important to block cell cycle division to stop cancer
Describe the Centrosome
-location where microtubules extend in mitosis
-provides orientation of micros.

Key: initiate micro growth
-major protein is tubulin gamma
-forms g-tubulin ring complex
Describe the Stability of Microtubules
1. Phosphorylation of tubulin
-palmitoylation, acetylation, removal of Tyr
2. Microtubule associated proteins (MAPs)
-some cap the ends to stabilize
-other disassemble by severing or increasing rate of breakdown
-some MAPs track tubulin to correct location
ex. Tau
-stabilizes micros
-binds alongside
-makes micros extra long
Supporting users have an ad free experience!