Chapter Text 35.3
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__serve as tracks for __, which do what
, and __, which do what?
Kinesins are crucial to the __
- carry cargo from the center of the cell to the periphery
- slide microtubules relative to one other during the beating of cilia and flagella on the surfaces of some eukaryotic cells, as well as carries cargo from the cell periphery to the cell center
transport of organelles and other cargo to nerve endings at the peripheries of neurons
Microtubules are a major component of the cytoskeleton and are built from __ and __, which assemble in a __ to form the __.
It can also be regarded as __
The outer diameter is __; and, it is a __structure with the minus end __and a plus end __.
- alpha and beta tubulin
- helical array of alternating tubulin types
- wall of a hollow cylinder
13 protofilaments that run parallel to its long axis
- anchored near the center of the cell
extending toward the cell surface
Microtubules are key components of __, adopting a common architecture in __
A bundle of microtubules called an __is surrounded by a __.
The __is composed of a __.
__drives the __, causing __
- cilia and flagella x2
- membrane contiguous with the plasma membrane
- peripheral group of nine microtubule pairs surrounding two singlet microtubules,c alled a 9+2 array
- motion of one member of each outer pair relative to the other, causing the overall structure to bend
Microtubules are important in __ and in __; they are __ that grow through __ to the ends of existing structures
Tubulin __; but, for tubulin, what is different from actin?
- determining the shapes of cells
- separating daughter chromosomes in mitosis
- highly dynamic structures
- addition of alpha and beta tubulin
- bind and hydrolize NTPs like actin
- the nucleotide is GTP and not ATP
The critical concentration for the __for the __ is lower than that for the __. Thus, a newly formed __consists primarily of __. Through time, the __. The __ in the interior length of a microtubule remain __, whereas __ exposed at an end have a strong tendency to __
- GTP forms of tubulin
- GDP forms
- GTP is hydrolyzed to GDP
- GDP-tubulin subunits
- stably polymerized
- GDP subunits
Some microtubules in a population lengthen while others simultaneously shorten, called __, which arises from __.
- dynamic instability
- random fluctuations in the number of GTP- or GDP-tubulin subunits at the plus end of the polymer
The dynamic character of microtubules is crucial for processes such as __, which require the __.
The structure of tubulin was determed by electron crystallographic methods
__ and __ have similar 3D structures
__is part of the __ and contains a __ adjacent to the __
- assembly and disassembly of elaborate microtubule-based structrues such as spindle fibers
- Alpha and beta tubulin
- P-loop NTPase family
- nucleotide-binding site
The continual __ is essential to their role in cell division
__was discovered through its ability to __. It does what? __and its derivatives have been developed as __ because they affect __
- lengthening and shortening of microtubules
- interfere with cell proliferation
- binds to microtubules and stabilizes the polymerized form
- anticancer agents
- rapidly dividing cells, like those in tumors
Kinesins are __; and, the method is different htan __
When a kinesin molecule moves along a micrtoubue, the two head groups of the kinesin molecule operate how?
It may do what?
- motor protiens that move along microtubules
- myosin movement along actin
- in tandem; one bonds and then the next one does
- take many steps before both head groups are dissociated at the same time; it is a highly processive motion
A single kinesin molecule will take 100 steps toward the __ of a microtubule in a period of seconds before the molecule detaches from the microtubule
This differs from myosin because ATP binding to myosin promotes its dissociation from actin
Despite the differences between __ and __, kinesin-generated movement proceeds by a mechanism similar to that used by myosin
Beginning with a __ in its __ that is __, the __ is bound and is __when __is bound
- myosin and kinesin
- two-headed kinesin molecule in its ADP form
- dissociated from a microtubule
- neck linker binds the head domain
- when ATP
The initial interaction of one of the head domains with a tubulin dimer on a microtubule stimulates the __
The binding of ATP triggers a conformational change in the head domain that leads to two important events:
· __ and __
release of ADP from this head domain and the subsequent binding of ATP
- The affinity of the head domain for the microtubule increases, essentially locking this head domain in place·
- The neck linker binds to the head domain; and this change, transmitted through the coiled-coil domain that connects the two kinesin monomers, repositions the other head domain
In its new position, the second head domain is __
Meanwhile, the intrinsic ATPase activity of the first head domain does what?
When the second head domain binds to the microtubule, the first head __.
Again, ATP binding favors a __ that does what?
- close to a second tubulin dimer, 80 angstroms along the microtubule in the direction of the plus end
- hydrolyzes the ATP to ADP and Pi
- releases ADP and binds ATP
- conformational change
- pulls the first domain forward.
This process can continue for many cycles until what?
Thus, kinesin “__ along the microtubule for approximately 100 steps before __
both head domains are in the ADP form simultaneous and kinesin dissociates from the microtubule
- walks” hand over hand
- both “hands” simultaneously let go
Kinesin hydrolyzes ATP at a rate of approximately __, moving along a microtubule at a speed of 6400 angstroms per second, lower than the maximum rate for myosin, which moves relative to actin at 80,00 angstroms per second
But, myosin movement depends on the __, whereas the movement of kinesin is driven by the __.
- 80 molecules per second,
- independent action of hundreds of different head domains working along the actin filament
- processive action of kinesin head groups working in pairs
Muscle myosin evolved to maximize the __, whereas kinesin functions to __
- speed of motion
- achieve steady, but slower, transport in one direction along a filament
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