Myosin motors and force generation Flashcards
What does myosin I do?
Binds membranes and its involved in endocytosis
What does myosin II do?
Forms dimers that associate to each other
What does myosin V do?
Dimerises but not to form large complexes. Binds to vesicles for organelle transport and to microtubules and RNA
How are myosin II filaments formed.
Formed from bi-symmetrical bouquets of myosin II dimers.
How do motor proteins work?
1-ATP binds to myosin head, head is released from actin
2-ATP to ADP, myosin head rotates into cocked state
3-Myosin head binds actin filament
4-POWER STROKE, Release of Pi = energy to straighten myosin and move actin filament to the left
5-ADP released, ATP binds, head released from actin
Difference between the myosin and kinesis motor domain structure?
Myosin releases from the actin filament when ATP binds
Power stroke via dissociation of Pi
Kinesis is released when ATP is hydrolysed to ADP
Power stroke when ATP binds
Describe the sarcomere
Myofibrils each of which consists of a regular repeating array of sarcomeres.
There are alternating light and dark bands with a line (the Z line) down the middle of a light band. Each sarcomere is defined as the region between the Z lines and has one dark band and one half of a light band on either side.
The dark band are composed of myosin II filaments and the light band of actin filaments.
The darkest region is the region of overlap containing both myosin filaments and actin filaments arranged in a regular pattern. When the muscle contracts the light band disappears as the region of overlap between the myosin and actin filaments increases.
What surrounds myosin thick filaments?
Each myosin thick filament is surrounded by 6 thin microfilaments (actin filaments) for it to work on.
What is CapZ and tropomodulin’s role?
The plus ends of the actin filaments are capped by CapZ which allows them to be buried in the Z disk. The minus ends of the actin filaments are capped by tropomodulin.
What is Nebulin’s role?
Nebulin is a large molecule that has repeated actin binding sites and is thought to determine the length of the actin filaments.
What is titin’s role?
Titin is one of the biggest proteins in the human body and is an elastic molecule that prevents over-stretching of the sarcomere.
When you pull a muscle you rip the titin protein apart (ouch).
How does myosin bind to actin?
. An action potential causes opening of a voltage gated Ca++ channel which release a small burst of Ca++ into the cytosol. This Ca++ binds to a channel in the SR which triggers massive and explosive release of Ca++ into the cytosol. The increase in calcium concentration causes myosin to bind to actin.
How does Ca2+ effect tropomyosin and troponin complex?
Elevated Ca2+ concentration causes a conformational change in two accessory proteins, tropomyosin (TM) and the troponin (TN) complex.
In the absence of Ca2+ tropomyosin, which is a rope like protein, covers up the binding site for myosin on the actin filament. In the presence of Ca2+ the myosin binding site is exposed.
Troponin complex is composed of 3 molecules, TH-T, TN-I and TN-C.
Association of Ca++ to TN-C causes a conformational change which ultimately results in a conformational change in tropomyosin.
How are cells divided
Formation of the cytokinetic actomyosin ring helps to split cells.
During mitosis Cyclin B / CDK1 phosphorylates myosin light chain (MLC) and this prevents Myosin II from functioning. When CDK1 is inactivated at anaphase dephosphorylation of MLC allows myosin light chain kinase (MLCK) to phosphorylate MLC. This allows myosin bundle formation of the actomyosin ring.
