Molecular Motors
Pages 1010-1034
General Characteristics
of Molecular Motors
Motor proteins – bind to a polarized cytoskeletal filament
and use the energy derived from repeated cycles of ATP
hydrolysis to more steadily along it
-They differ in the type of filament they bind to, the
direction in which they move along the filament, and the
“cargo” they carry.
Myosin II Structure
Myosin was the first motor protein
identified and is responsible for muscle
contraction. It is formed from 2 heavy
chains and 2 light chains.
Myosin II Thick Filament
Motor Activity is Located
in the Myosin Head
Myosin Superfamily Tree
-Myosin tails have diversified during evolution to permit the
proteins to dimerize with other subunits and to interact with
different proteins or cargo. Humans have about 40 myosin genes
Kinesin
Kinesin is a motor protein that moves along microtubules
It was first identified is the giant axon of the squid, where it
functions to carry organelles away from the neuronal cell
body toward the axon terminal by walking toward the + end
of microtubules
Many kinesins or kinesin-related proteins function in
organelle movement or have specific roles in mitotic and
meiotic spindle formation and chromosome separation.
Kinestin and Kinestin-Related
Proteins
Goes towards + end
Goes towards – end
Increases dynamic instability
Monomer
Dynein Structure
Dyneins are a family of – end directed microtubules motors
- are the largest and fastest molecular motors
Cytoplasmic dynein important for vessicle
trafficking and
localization of the
Golgi apparatus
Ciliary dynein important in the beating
of cilia and flagella
Myosin and Kinesin Structure
-They have nearly identical cores
Myosin Walking
Cycle
Cocked
Rigor state
Attached
Force-Generating
Released
Attached
Comparison of Kinesin and
Myosin
Motor Proteins are Adapted
to Cell Functions
Kinesin –moves in a highly processive fashion, traveling
hundred of ATPase cycles on a microtubule before
dissociating
Myosin II – makes just one or a few steps along an actin
filament before dissociating
Two Reasons:
1. the cycles of the two motor heads in a kinesin dimer are
coordinated with each other, so that one kinesin head does
not let go until the other is ready to bind
2. kinesin spends a larger fraction of its ATPase cycle
tightly bound to the microtubule
Walking Direction of Kinesin
Family Proteins
The coiledcoil domain
seems to
determine the
directionality
of movement
Attachment
Model of Dynein
to an Organelle
Motor proteins also have a
significant role in organelle
transport along actin
filaments
Effect of Microtubule
Depolymerization on
the Golgi Apparatus
Green – Golgi apparatus
Red – microtubules
Golgi are being positioned
near the center of the cell
by dyneins moving towards
the – end of microtubules
Myosin V on Melanosomes
Black – melanosomes
Green – Myosin V
Melanosome Movements in Fish
Pigmented Cells
-Both dynein and
kinesin are
associated with
pigment granules
Regulation of Myosin
II
Non-muscle myosin
Skeletal Muscle
Cells
Skeletal Muscle Myofibrils
Longitudinal section
The
Sarcomere
Sliding-Filament Model
for Muscle Contration
Accessory Proteins in a
Sarcomere
Alpha-actinin
Calcium Release in the
Sarcoplasmic Reticulum
Regulation of Skeletal Muscle
Contraction
Troponin
T=tropomyosin binding
I=Inhibitory
C=Ca++ binding
Resting state
I,T pulls tropomyosin out of its
normal binding groove and blocks
myosin
Active state
C binds Ca++ and releases
tropomysoin allow the interaction
of actin and myosin
Effect of Subtle Mutations
in Cardiac Myosin
6-day old mouse
Flagella and Cilia
Movements
Microtubules in a Flagellum
or Cilium
Cilliary Dynein
Bending of an Axoneme
Structure of Basal Bodies
Base of cilia and flagella, and centrioles
Kartagener’s syndrome – defect in ciliary dynein