Transcript Motor Proteins and The Cytoskeleton

Cell Motility
Lecture 17
Cell Motility
• Includes:
– Changes in Cell Location
– Limited Movements of Parts of Cells
• Occurs at the Subcellular, Cellular, and Tissue
Level
• Is Dependent Upon:
– The Cytoskeletal Scaffold
– Motor Proteins
– ATP
Motor Proteins and The Cytoskeleton
Microtubule-based Movements:
Kinesins and Dyneins (MAPS);
Fast Axonal Transport
Movement of Cell Appendages
Movement of Internal Membranes
Movement of Chromosomes during Mitosis
Actin-Based Movements:
Myosins
Muscle Contraction
Cell Contraction
Whole Cell Movement
All Motor Proteins use ATP as an energy source.
Microtubule Motor Proteins
Two Main Families
Kinesins- Primarily Plus End Directed
-Away from Centrosome
Dyneins- Minus End Directed
-Toward Centrosome
-Largest Motor Proteins
2 Globular Heads
-MT Binding
-ATP Binding/Hydrolyzing
Middle Flexible Domain
-Mediates Dimerization
NeckTail For Binding Cargo/Adaptors
Microtubules Function to Maintain Internal Cell
Organization and Cell Shape
MT can be
Capped to
Stabilize
Cell Polarization is a Reflection of the Polarized
System of Microtubules in the Cell Interior
Microtubule Motor Proteins Help Arrange
Membrane Enclosed Organelles in Eukaryotes
Microtubules
Golgi
Depolymerize MT- breakdown of Golgi
The Tail of the MT Motor Proteins Determines
What Cargo Is Transported
Cargo Binding:
Often Involves Adaptor Proteins
Kinesin- Processive
-Fast Axonal Transport
-Arrange ER
Cytoplasmic Dynein- Processive
-Localize Golgi
Axonemal Dynein
Motion of Cilia and Flagella
Kinesin Walking to the Plus End of MTs
(2)
(1)
(1)
(2)
(1)
(2)
(2)
Leading Head (1)
Attaches to
a b Tubulin
(No ATP)
Head 1
Binds ATP
Head 2
Propelled
Forward Past
Leading Head
ADP Release
From Head 2
and Hydrolysis
Of ATP by
Head 1
Cycle Can
Begin Again
Moved
One Step
Processive: At Least One Head is Always Attached
Kinesin in vitro Motility Assays
Axonemal Dynein:
The Motor that Moves Cilia and Flagella
Cilia of
Rabbit
Trachea
Cilia from
Rabbit
Trachea
Flagellum
Of Sperm
++++
Stationary CellsAllows Movement over Tissue
Shorter than Flagella
100+ per cell
Generate force
by bending at base
Flagella Drive Movement
of Individual Cell
Longer than Cilia
1-2 per cell
S shaped wave
MT are StabilizedNo Longer Demonstrate
Dynamic Instability
Organization of the Axoneme
“9 + 2” Pattern
Nexin Connects Adjacent Doublets
Dynein -Head and Tail Bound to
Different MT
Model for Dynein-Mediated Sliding of MTs
(No Nexin)
Actin Based Movements:
The Myosin Superfamily
18 Classes of Myosins
Move toward Plus(Barbed)
End of Actin (except one Class)
4 Broad Groups Based on Function:
1)Power Muscle and Cellular Contraction
Striated Muscle
Contractile Ring-Will cover later lecture
2) Power Membrane and Vesicle Transport
Short Range
Endocytic and Phagocytic Vesicles
3)Cell Shape and Polarity
Head- binds/hydrolyzes ATP and Actin
Regulatory Sites
Tail-
4) Signal Transduction and Sensory
Photoreceptors and Hearing
In vitro Motility Assays with Myosin
+ ATP
Filaments
Glide Along
Surface
Purified S1 Heads Attached to Glass
Membrane and Vesicle Transport:
The Role of Myosin I in Cells
All Cell Types
Transport a
Vesicle or
Organelle
Bind Cortical Actin
Filaments- Lead to
Change in Cell Shape
Muscle Contraction:
The Role of Myosin II
Two Globular Heads
Coiled Coil Tail
Assemble into
Bipolar
Myosin
Filament
Myosin II Movement to the
Plus End of Actin Filaments
Tight
binding
No ATP
ATP
ATP
Hydrolysis
Binding
Occurs
Releases Head is
Head
Cocked
Head Rebinds Filament
Pi Release
Trigger Power Stroke
ADP released
Bind ATP Again
and Release
Cycle Starts Over
Not Processive Like Kinesin
Organization of Muscle Fiber
Organization of the
Skeletal Muscle Fiber
Skeletal Muscle Cell (Muscle Fiber)
Myofilaments make up Myofibrils
Thick- Myosin
Thin- Actin
Hexagonal Arrangement of Actin
Filaments Around the Myosin
Skeletal Muscle Banding Pattern
Light Band= I Band
Thin Actin Fibers
Dark Band=A Band
Thin Actin and Thick
Myosin Fibers Overlap
Structural Proteins of the Sarcomere
Actin Filament Stability
Requires:
Cap Z- Cap Plus End of Actin Filament
Tropomodulin- Cap Minus Ends of Actin Filament
Nebulin –Stabilize Filament/Role in Length
a Actinin- Component of Z Disc - Actin Bundling
Sarcomeres Shorten During Skeletal
Muscle Contraction
No Shortening of Thin or Thick Filaments
Sliding over one another
Myosin Walks Along Actin Filament
Attached
Released
Cocked
Nucleotide Free Form
ATP Binding
ATP Hydrolysis
Conformational Change
Pi release-Initiates
Power Stroke
Force Generating
Attached
Strong BindingPower Stroke
(Conformational change to
Original Shape ) and ADP
Release
The Role of Ca2+ and Regulatory Proteins
in Skeletal Muscle Contraction
The Cytoskeleton in Moving Fibroblast
Meshwork
Thin,
Sheetlike
Thin,Stiff
Protrusions
Crawling Cell
Actin-Rich Cortex Moves a Cell Forward
Myosin II functions in Cell Contraction
• Extension of
lamellipodium
• Lamellipodium anchors
to substrate
• Retraction of the
opposite end of the cell
Formation of Leading Edge of a Lamellipodia
Nucleation of Actin at the Plasma Membrane
Size of Lamellipodia
Remains Constant
As
Assembly at Leading
Edge is Balanced
By
Disassembly
At Rear
Extracellular Signals
Activate Small G proteins
That Regulate
ARP Complex
Lamellipodia
Filopodia