microtubules Flashcards
what is the structure of microtubules?
they are long hollow cylinders made of the protein tubules. They are more rigid than actin
how are microtubules formed? where are they formed?
They are made of protein tubule dimers (always beta plus alpha minus) that bind together in protofilaments which then bind to form a cylinder. They usually originate from some kind of microtubule-organizing center (MTOC) such as the centrosome, spindle pole, and basal body
what is the difference between alpha and beta tubulin?
both can bind GTP but only beta can hydrolyze. beta is plus end and alpha is minus end
what are the two microtubule structures that can exist?
T-form: bound to GTP
D-form: bound to GDP
what is a GTP cap?
when subunits are added at a high rate, the GTP has not had time to by hydrolyzed and the tip of the polymer is in T form. This T form tip is a GTP cap. The cap stabilizes the plus end and keeps it straight.
why is the GTP cap stabilizing?
the protofilaments are straight when they contain GTP and they can form more stable lateral bonds with neighboring protofilaments. hydrolysis to GDP curves the protofilament and destabilizes the microtubule
what is dynamic instability?
individual microtubules alternate between a period of slow growth and a period of rapid disassembly. Each microtubule grows/shrinks independently of others.
catastrophe is the change from growth too shrinkage.
rescue is the change from shrinkage to growth
what is cholchicine?
toxic drug that depolymerizes microtubules. comes from Coleus houseplant
what is Paclitaxel?
a microtubule polymerizing drug that binds to beta subunit and stabilizes microtubule. comes from the Pacific Yew.
can be cancer treatment by interfering with growth of cancer cells by blocking mitosis
how are microtubules nucleated?
by gamma-TURCs found on the centrosome. They have a ring structure that makes a good template for tubular microtubule formation.
what is a centrosome? what role does it play in microtubules?
the centrosome is a major microtubule organizing center in animal cells. It is non membrane bound and made of dense mass of protein, always containing two centrioles. Tubulin polymerizes from gamma-TURCs in centrosome
what is self centering?
microtubules can grow, push into the walls of their container, and eventually center the centrosome. This is important for establishing a coordinate system (positioning organelles/chromosomes)
what are the nucleation sites of cilia or flagella?
Basal bodies: modified centrioles, one (not a pair) is found at the base of cilium/flagellum
what are the types of accessory proteins for microtubules? (MAPs)
polymerizing nucleating capping severing and depolymerizing bundling motor proteins
what are polymerizing proteins? (MAP)
they stabilize microtubules
what are nucleating proteins? (MAP) example
they start assembly (nucleate assembly). gamma-TURC
what are capping proteins? (MAP)
they localize microtubules within cells and may anchor them to bilayers. this can polarize a cell
What are severing and depolymerizing proteins? (MAP) example
they destabilize microtubules. Stathmin sequesters free tubulin dimers and causes microtubule shrinkage
what are bundling proteins? (MAP)
they bind to microtubules and determine the spacing between them. ensures uniform spacing
what are molecular motors?
enzymes that generate force and walk along microtubules toward either plus or minus end.
what are the domains of motor proteins?
head domain: binds microtubules and generates force
tail domain: binds bilayers or other cargo
What is kinesin? how does it function?
a dimer of two motor domains connected by coiled tail. Its rear head detaches, passes over other head, and rebinds. When motor domain is ATP bound it binds to binding site strongly, when hydrolyzed to ADP it is loose. One head is always bound so it doesn’t let go of microtubule track
What are the two motor proteins we discussed and which end do they walk towards?
Kinesin: walks toward plus end
Dynein: walks toward minus end
cargo don’t necessarily bind to just one of these proteins, a tug of war or coordinate motor activity can occur
what is cytoplasmic Dynein and axonemal Dynein? how do they differ from other motor proteins?
cytoplasmic Dynein: have 2 larger head domains. used similarly to other motor proteins
axonemal Dynein: have three large head domains. found in cilia and flagella, specialized for sliding movements of mictrotubules that drive beating of cilia/flagella
how does intracellular cargo become loaded onto the correct motor protein?
the motor protein’s tail domain mediates cargo binding. adaptor proteins associate with motor proteins to regulate motor activity and to link motor proteins to appropriate cargo
how are microtubules arranged in flagellum and cilium?
one basal body is found at the base of the cilium/flagellum and serves as the template for formation of axoneme (microtubules). there is a 9 +2 arrangement of Microtubules in the axoneme
what are the structural elements of the axoneme that allow for bending?
there are outer doublet held in place by nexin to prevent sliding and Dynein is walking along neighboring microtubules. the walking of Dynein leads to bending of the axoneme and flagellum
what are the motions used by flagella and cilia to move?
flagella: continuous sinusoidal wave
cilia: power stroke followed by recovery stroke
what is kartageners syndrome?
the flagella and cilia can’t move. results in inversion of normal asymmetry of organs, male sterility, and susceptibility to lung infections
How are microtubules able to transport cargo in long axons and dendrites?
microtubules can transport cargo long distances within cells. microtubules in axons and dendrites have their plus ends and minus ends oriented in both directions relative to soma. fibroblasts are only oriented with plus end away from soma
