Notes 1: Intro, Components of the cytoskeleton
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Introduction to Skeletons
There are two types of skeleton in nature
Organisms that have an exoskeleton
invertebrates usually have an exoskeleton
vertebrates have an endoskeleton
there are transition organisms that are the bridge between a fully developed exoskeleton
like for example a wasp or a snail; the snail has its own house, the house is basically the exoskeleton that serves the typical function of support and protection, same thing with the with the insect
it's a rigid structure that supports the organism, the internal organs, and protects them from predators, from the elements of nature and a bunch of different things even from infections
mollusks are the linking whatever in the chain that goes from exoskeleton to fully developed to endoskeleton
what happened is that the soft organism, at some point during evolution, wrapped around the exoskeleton and the exoskeleton became internalized by the organism and became an endoskeleton
can see in the in the mollusks especially
this is a cuttlefish right, this is the skeleton of the cuttlefish
find this sometimes on the beach
feel like they're made of chalk but that's the endoskeleton of the cuttlefish
it's not fully developed like ours because it's the residual of when the mollusks that has an exoskeleton
like the snail during evolution has wrapped around and engulfed the exoskeleton into itself and then it evolved further to become this beautiful structure that is our endoskeleton of the vertebrates
Components of the Eukaryotic Cytoskeleton
Cells have their own skeleton too
for animal cells it's an endoskeleton because it's inside the cell and it's called the cytoskeleton
Bacteria an external structure, the cell wall, that also serves function like a skeleton
that would be the equivalent of an exoskeleton on the cellular level
today we're gonna talk about cytoskeletons of animal cells; not gonna mention too much about nothing at all about plant cells
there are three main components of the cytoskeleton
intermediate filaments
microtubules
actin filaments
Intermediate filaments
Intermediate filaments
very strong in the sense
has great tensile strength
allow cells to sustain mechanical stress
they are found almost in all animal cells
mostly form a mesh throughout the cytoskeleton and even within the nucleus
Intermediate Filaments are Structured Like Ropes
Intermediate filaments are so strong because of structure
The basic structure of an intermediate filament is the same of a rope
It's basically a filament that keeps coiling with another filament
The two filaments align together and form a tetramer
The tetramers align together to form a group of eight tetramers
Tetramer composed by two filaments
Tetramers align head to tail
the parts that are that are loose on the C terminus and the N terminus let's say if, if you take one, the first one is a reference, they get together like this you know like the parts that are loose like if you're crossing your fingers of the two hands in this way and this goes along with the entire length of the filament
so in the end it's pretty strong because there are all these filaments wrapped around in this coiled-coil helices and they're wrapped around each other, they also interact like this so it becomes really strong
Types of Intermediate Filaments and Their Functions
Two different types of intermediate filaments
ones that are cytoplasmic
ones that are nuclear
in the cytoplasm the most Commons are keratin filaments that are found in every epithelial cells
Epithelial cells need to be strong because they are the layer that's outside every organ also inside actually
our skin is made of epithelial cells
so they have to be strong because they also serve an important support function
there's a lot of keratin in your hair I
vimentin is another intermediate filament
it's present in all connective tissues in muscle cells and in glial cells
then the neurofilaments are a type of intermediate filaments that is very specific for neural cells
the nuclear filaments are mostly the so-called Lamins, Lamin AC, Lamin B
Nuclear Intermediate Filaments
nuclear intermediate filaments
they are the only ones that don't form the rope like structure
they make this coiled-coil structure but not the rope structure
they form this mesh
this is a beautiful electron microscopy picture, well there are a little ruptures because you know when you prepare a sample sometimes you get artifacts, but you can see the mesh it really looks like a piece of fabric it's very beautiful
the nuclear lamina, just underneath the intranuclear side of the nuclear envelope interacts with this anchoring dimers that are formed
there's that KASH-domain protein which has one end in the intraperinuclear space and the other end towards the cytosol
then there's the SUN-domain proteins which are the other way around with one end in the perinuclear space and the other end inside the nucleus
so the nuclear lamina interacts with the intranuclear end of the SUN-proteins
basically as this complex anchors the nuclear lamina so effectively the skeleton of the nucleus to the actin cytoskeleton of the cytoplasm of the cell
but then importantly the SUN-domain proteins also directly interact with chromatin so with the chromosomes
so basically everything that is in the nucleus it's not just floating freely, they're actually anchored to the nuclear envelope
that's why the nuclear envelope is so important
that's why when there are mutations in these proteins that stabilize the nuclear envelope that this mutations can cause very severe diseases
for example some kind of muscular dystrophy are caused by mutations in lamin proteins
some other disorders serious disorders are caused by mutations in this SUN and KASH domain protein
Components of the Eukaryotic Cytoskeleton
The microtubules and actin filaments are in the cytoplasm only
Microtubules
microtubules are the biggest in terms of size, filaments of the cytoskeleton;
they're long
they're very stiff
they are organs that can grow and shrink but they always grow and shrink from the same end which is what we call the plus end whereas the minus end is anchored to a structure called the centrosome
in non-dividing cells there's only one centrosome and one pole of the cell
in dividing cells there are two centrosomes
the first step of mitosis is the duplication of the centrosome and the migration of the second centrosome to the other pole of the cell and then the formation of the mitotic spindle and blah blah blah
in other types of cells like for example the ciliated cell, the basal body which is the structure where the microtubules originate from, instead of being in the centrosome there are multiple basal bodies in the apical end of the cell
apical means at the top end of the cell which is where the cilia also developed from
Microtubule Structure and Assembly
So how is the structure of the microtubule
it all starts with alpha and beta tubulin
alpha and beta tubulin are the components of the microtubule
gamma tubulin that is in the basal body in the centrosome, that's not in the microtubule OK
the components of the microtubule are dimers of alpha and beta tubulin
this dimers stuck one on top of the other with the alpha towards the minus end and the beta towards the plus end and they make a filament
and then this filament arranges with other filaments in this kind of spiral way
there are let's count them 13 filaments that make one microtubule
a prime number remember 13
then all around this this tube structure which that's why it's called microtubule with the lumen inside
this is an electromyography where you can see really well the structure with the with the tubulin dimers, in here you can count the 13 filaments of the microtubule
Microtubule Dynamics Require GTP Hydrolysis
How does it work
The dimer the alpha beta tubulin dimer is a GTPase
A GTPase is an enzyme that hydrolyzes GTP into GDP plus phosphate and the hydrolysis gives the energy to remain attached within the microtubule
so the first step at the plus end of the microtubule is the addition of GTP bound alpha beta dimers
and the addition of this dimers is faster than the kinetics the enzymic kinetics of the dimer to hydrolyze GTP
what happens is that at the plus end there's always a few layers of dimers that are still bound to GTP that have not yet hydrolyzed GTP into GDP
then after you know a few rounds are added then when they're a little bit further down they complete hydrolysis of the GTP released the phosphate and remain bound to GDP and in this configuration there are stable within the microtubule OK
I told you that the hydrolysis this is important to stabilize
So what happens is that if that in fact the kinetics of addition of dimers is faster than the kinetics of GTP hydrolysis then microtubule is stabilized and it grows
however there are certain conditions the reverse occurs hydrolysis of GTP's faster than the kinetics of addition of alpha beta tubulin dimers so what happens is that the that this GTP cap is lost and without GTP bound with the dimers start to come off the microtubual and then we instead of growing we get shrinking
how is that determined which one of the two kinetics is going to be faster well there's a lot of factors that determine that, we're not gonna go into any of the details just know that under certain circumstances one will be faster than the other and that's what regulates growing versus shrinking
Microtubule Polymerization and Growth
capping proteins
there are proteins that associate with the growing microtubule, that stabilize the microtubule
in the absence of this caping protein the microtubule might shrink back
sometimes it depends on whether the microtubule is reached the end of the line, like it's touched the plasma membrane or whatever is under the plasma membrane, could be a layer of actin for example covering the intracellular side of the plasma membrane
so bottom line is it is interactions with other proteins, other structures that determine whether the microtubule is going to grow or shrink because it's going to determine whether the hydrolysis of GTP is gonna be faster or whether the addition of the dimers is gonna be faster OK
but the important thing to learn is to remember because this is different to what we're gonna see shortly about actin is that microtubules can only grow and shrink at one end which is the plus end
the minus end is anchored in the centrosome and it starts from this gamma tubulin rings remember I mentioned that just 5 minutes ago the gamma tubulin so within the centrosome there are these gamma tubulin rings which are really the beginning of the assembly of a microtubule
and then the microtubule grows towards the plus end in any direction of the cell
can grow and shrink grow and shrink