Topic 4 - Microfilaments
microtubules where the largest cytoskeletal elements with a diameter of 25nm
microfilaments are the smallest with a diameter of only 7nm
microfilaments=string or thread
microtubules made of tubulin dimers
microfilaments are made up of a actin (monomeric protein)
when you have a singular molecule of actin its called G-actin, and in the center of the globular like shape, a
nucleotide can bind (ATP) whereas for tubulin it was GDP. Stacking multiple amount of these you get
filamentous actin, and with this you end up with a filamentous actin. The filament has polarity. two filaments
wound around eachother are a micro filament.
Polarity b/c G-actin always stack the same way, +/- end (+ end grows faster) and same like tubulin actin can
act like ATPase and hydrolyse atp
Acting with ATP present acting tends to be aded to a microfilament, whereas when GDP is present, it tends
to disassemble, similar things to microtubule. Minus end tends to break down, plus end grows.
Acting binding proteins regulate all aspects of microfilament formation:
how quickly they grow, how stable they are (capping proteins to hold at certain length), they determine
whether they are organized into networks or into bundles.
Usufull drugs derived from fungi
1.cytochalasin: prevents actin from being added to a microfilament, and as a result breaks them down.
Microfilaments provide stability to the cell:
1.there is a network of them beneath the cell membrane called a cortex, and they provide support to the cell
2.some microfilaments allow the cell to take on differenr shapes, microvilli small finger like projections of cell
membrane, and they function to increase surface area available for those transport proteins. Seen alot in
cells that do alot of exchange. they are in a nicd paralell bundle. The minus end are at the base of it, and the
plus end are at the tip. they are linked to the plasma membrane to give support to the whole thing.
3. Adherens junctions
Other places you see microfilaments playing structural role are at Adherens junctions (between cells) and
focal adhesions (cell and a substrate)
Purple is cadherin (protein holding cells together) and the fibers coming in to support it are microfilaments
-when you look inside cell membrane you find microfilaments which are made of actin and made of ATP
-whereas when you look inside a cilium you will find microtubules made of tubulin and GTP
2.cilia have motor protein in them whereas microvili do not (i.e. one moves and the other doesn't)
3.Minus ends of cytoskeletal elements are at the base, plus ends at the tip 4.Basal body is in cilium to help organize but there is no such equivalent in microvili
5. 9+2 in cilium vs. a bundle of microtubules in a microvili
6. microvili are quite short
7.they are both finger like projections of the cell membrane
Microfilaments & Movement
the motor protein that works with microfilaments is called myosin, close to kynesin. Moves towards plus end
like Kynesin. Muscle contractions. hydrolyses ATP, and has a long tail. considering the smallest unit of the
sarcomere that is able of contracting, we find the sarcomere extends from the Z-line to the adjacent z-line.
the z line includes a protein called cap z which is a capping protein that stabilizes microfilaments (holds
them at a constant length) minus ends are in the middle of the sarcomere. myosin is organized into thick
filaments that are between the thin filaments located in the center of the sarcomere. A thick filament is a
collection of myosing molecules.
A muscle is a whole bunch of sarcomeres in a row that are then packed together into a muscle cell, they are
able to contract because they are made up of repeated sarcomeres.
The myosin heads are often called cross bridges, becuase they bridge th