Lecture 15 Flashcards
Main Myosin
Myosin II
Structure of myosin
-Dimer
Each dimer has a heavy and light chain and a long alpha-helices tail and hinge region
Role of the heavy chain?
-Binds and interacts with ATP (force generating)
Where does myosin walk?
Toward the plus end of actin
When myosin is not bound to ATP/ADP ?
Myosin head domain(heavy chain) associates with the actin filament
When myosin is bound to ATP?
Reduces the affinity of the myosin head to actin and induces a conformational change
Myosin head when bound to ADP + Pi?
Hydrolysis of ATP causes head group to cock toward plus end of actin
Myosin head releases ADP?
Myosin head rebinds actin and head shifts back to regular position
Are both myosin domains coordinated?
No
Myofibrils?
Many actin and myosin together in muscles
Sarcomere?
Actin surrounding myosin molecules in cylindrical fashion
CapZ in sarcomeres?
The plus end is bound by CapZ in sarcomeres to prevent the addition of monomers
Tropomodulin sarcomeres?
The minus end is bound by tropomodulin which prevents the addition of monomers to the minus end
What happens to tropmyosin when calcium is released?
Troponin which is bound to tropomyosin binds the calcium and then pushes tropomyosin out of the way from the myosin binding site
What regulates non-muscle myosin?
Phosphorylation
Myosin light chain kinase?
Phosphorylates non muscle myosin and causes it to unfold into its active state, now it can interact with actin
Why would a cell want non-muscle myosin?
Cytokinesis
Cytokinesis
Near where the cells will divide there is a bunch of myosin light chain kinases that phosphorylate the myosin light chains so that it is in its active form and can assemble with actin. Contraction of actin helps divide the cell
What are microtubules made up of?
Tubulin subunits (dimers) either beta or alpha
Alpha and Beta subunits and GTP
-Both subunits can combine with GTP
-Alpha GTP cannot hydrolyze
-Beta GTP can hydrolyze
How do microtubules assemble ?
Alpha-beta tubulin subunits can assemble in a head to tail fashion into protofilaments. 13 protofilaments then form a hollow tube.
What is the plus end of the microtubule?
Beta subunits
Why are microtubules stiffer than actin?
Form bonds with multiple subunits on many sides
T/F: nucleation in microtubules is not spontaneous
True
Plus end vs Minus end of microtubules ?
Site of nucleation is the minus end
-Growth of the filament occurs at the plus end normally
Free tubulin subunits are bound by?
GTP and have high affinity for other tubulins
GDP bound subunits?
Have less affinity for tubulin
GTP-cap
When new GTP-bound subunits are added quickly enough a stable GTP cap is bound and the microtubule is less likely to dissociate
Do microtubules treadmill?
No
No GTP bound cap?
Catastrophic event
What happens when GDP subunits reach the end of the microtubule?
-Subunits start curving and peel away from the microtubule
-Microtubule will shrink to nothing
Dynamic Instability?
Growing/shrinking of microtubules
Microtubule Nucleating proteins?
-Gamma tubulin
-Augmin
Gamma tubulin and nucleation?
2 gamma tubulin associate into the gamma tubulin small complex. This complex mimics the plus end of a microtubule and binds alpha-subunits
Gamma tubulin ring structure?
7 gamma tubulin small complexes assemble into a ring structure and all start growing (14 exposed gammma tubulins but one is a binding site to close the ring, so 13 protofilaments created)
Microtubule Organizing Center?
A collection of gamma tubulin ring complexes
Example of an MTOC?
Centrosome
Centrioles?
These are microtubules which bind to the centrosome and grow/shrink
Minus end is in the ring complex and plus end grows outward
