Cell Biology Lectures No. 9 & 10: The Cytoskeleton
Wednesday February 6 & Monday February 11 , 2013
-Both the cellular trafficking of organelles and cell migration are achieved using the cytoskeleton. The
cytoskeleton is an intricate network of protein filaments that extend throughout the cytoplasm. It is
composed of three primary filaments: microfilaments (7-9 nm) < intermediate filaments (10 nm) <
microtubules (25 nm). The various functions carried out along the cytoplasm often require energy from
ATP or GTP hydrolysis.
Microtubule Structure & The Dimeric Tubulin Subunit:
-Microtubules are a repeating structure (polymer) of α and β tubulin monomers. Together, an α and β
tubulin form an αβ dimer as the basic microtubule subunit. Because they conjugate into a dimer, α and β
tubulin are almost never found separately. Instead they stretch end to end in a dimeric form, creating a
protofilament (striated chain of tubulin dimers) that has polarity (one end α and one end β). It takes 13
protofilaments to form the hollow tube structure of a microtubule.
-The tubulin dimer subunit is quite abundant in the cell and very easy to isolate and crystallize. Due to its
high degree of stability, microtubules depolymerize to these dimers and not to monomers. GTP can bind
to both ends of the dimer (binds permanently to α-tubulin), but because β-tubulin can GTP, it can bind
to GTP or GDP. In fact, as the protofilament polymer grows, the GTP bound to β-tubulin hydrolyzes.
The Arrangement Of Microtubule Protofilaments:
-Three basic arrangements are observed with microtubule protofilaments: singlets (one ring), doublets
(A and B rings) and triplets (A, B and C rings). The singlet configuration of 13 protofilaments is most
common and this arrangement can grow or shrink (polymerize or depolymerize). Doublets are observed
in cilia and flagella structures and consist of a 10 protofilament B ring in addition to the 13 protofilament
A ring. Triplets are observed in centrioles and basal body structures and consist of 10 protofilament B
and C rings in addition to the 13 protofilament A ring. Both doublets and triplets don’t tend to
polymerize or depolymerize given the high stability of their structure.
Microtubule Organization & The Centrioles:
-Two types of microtubules are known for organizing the interior of a cell: axonemal (found in cilia and
flagella; not found in axon) and cytoplasmic (nerve cells only have cytoplasmic microtubules). The
centrosome is the major microtubule-organizing center of (MTOC) of animal cells (not in plant cells);
there can be many different kinds of MTOCs in animal cells. The centrosomes contain mother and
daughter centrioles which are small barrel-shaped structures made of triplet microtubules that lie 90° to
each other. Note that the microtubules don’t actually touch the centrioles, but rather make contact with the pericentriolar matrix, which is a matrix of protein (γ-tubulin, augmin complex, etc.) outside of the
centrioles that allows for the polymerization of microtubules.
-The MTOC functions to nucleate (act as a starting point for) the assembly of microtubules. Examples of
MTOCs are present in spindle poles (mitotic apparatus), cilia and basal bodies. The minus end is
associated with the α-tubulin end (towards the MTOC), while the plus end is always associated with the
β-tubulin end (grows away from the MTOC). Note that nerve cell axons have minus ends pointing
towards their respective MTOCs, but they can also possess dendritic minus ends (which don’t lie in
parallel with plus ends) that actually grow away from the MTOC.
Γ-Tubulin & The Polymerization Of Microtubules:
-In the MTOC called the centrosome, γ-tubulin associates with other proteins to form the γ-tubulin ring
complex and provides the nucleating site for growing microtubules in the centrosome of an animal cell.
To this γ-tubulin ring complex, α and β dimers are added, extending in a positive (plus end) direction.
-The amount of α and β tubulin dimers increases until it hits a critical concentration (C ),cwhereby they
form protofilaments. Microtubules polymerize above the critical concentration (free dimer