cytoskeleton proteins Flashcards
How many protofilaments are in one microtubule
13 protofilaments, arranged circularly around a central lumen.
A protofilament is a line of stacked alpha-tubulin and beta-tubulin heterodimers.
What indicates the plus end and minus end of a microtubule
Only beta tubulin subunits are exposed on on the plus end.
Only alpha tubulin is exposed on the minus.
Start with alpha tubulin (alpha, first), then add beta tubulin on top of it.
How are the dimers arranged laterally?
Most of the alpha subunit is next overlaps another alpha subunit and beta with beta, but they are just slightly staggered, overall creating a slow spiral.
All of the protofilaments are in parallel arrangement
What is the third component of a tubulin heterodimer?
2 molecules of bound GTP.
The GTP bound to the alpha subunit is not accessible and is considered and integral part of the tubulin structure.
The GTP bound to the beta subunit can be either GTP or GDP and is exchangeable with other proteins.
What is the basic subunit of actin?
G-actin or globular actin, a Monomer.
What other integral component does an actin protofilament contain besides actin monomers?
ATP/ADP
What is an actin filament composed of?
Two f-actin protofilaments coiled around one another in a parallel helix.
What do the terms plus and minus end of a filament indicate functionally?
The plus end is the side where growth/shrinkage polymerization/depolymerization occurs faster,
and the minius end where it occurs slower.l
What are the historical terms for the plus and minus ends of an actin filament?
Plus end: Barbed end
Minus end: Pointed end
like the filament is an arm giving the peace sign.
Where does ATP bind in the G-actin?
The ATP binding cleft is on the minus end of the monomer.
Describe the role of GTP and ATP in formation of the filaments
Monomeric G-Actin is bound to ATP, on polymerization and F-actin formation, ATP is hydrolyzed to ADP.
The Tubulin heterodimer is bound to two GTP units, one by the alpha and one by the beta subunit. In the polymerized form, the BETA subunit hydrolyzes its GTP to GDP.
This hydrolysis is NOT IMMEDIATE. It occurs after a period of time, and the longer a subunit has been incorporated, the more likely it is to have hydrolyzed its ATP or GTP. And it is more likely to depolymerize once in the ADP or GDP form.
Intermediate filaments
The monomers of intermediate filaments are large, filamentous proteins, containing long alpha-helical regions.
They form coiled-coil dimers, then staggered tetramers of two coiled-coils.
These tetramers then twist into a ropelike filament.
They are cell-specific.
Evolutionarily related to the nuclear laminin proteins which form the filament meshwork on the inside of the nuclear envelope.
What are the intermediate filaments of:
Muscle
Epithelial cells
Muscle: Desmin
Epithelial cells: Keratins
