View
4
Download
0
Category
Preview:
Citation preview
© 2012 Pearson Education, Inc.
Lectures by
Kathleen Fitzpatrick Simon Fraser University
Cytoskeletal
Systems
Chapter 15
© 2012 Pearson Education, Inc.
Table 15-1 - Microtubules
© 2012 Pearson Education, Inc.
Table 15-1 - Microfilaments
© 2012 Pearson Education, Inc.
Table 15-1 – Intermediate Filaments
© 2012 Pearson Education, Inc.
Table 15-3
© 2012 Pearson Education, Inc.
Microtubules
• Microtubules are the largest of the
cytoskeletal components of a cell
• There are two types of microtubules
• They are involved in a variety of functions in
the cell
© 2012 Pearson Education, Inc.
Two Types of Microtubules Are
Responsible for Many Functions in
the Cell
• Cytoplasmic microtubules pervade the
cytosol and are responsible for a variety of
functions
- Maintaining axons
- Formation of mitotic and meiotic spindles
- Maintaining or altering cell shape
- Placement and movement of vesicles
© 2012 Pearson Education, Inc.
Two types of microtubules (MTs)
• Axonemal microtubules include the
organized and stable microtubules found in
structures such as
- Cilia
- Flagella
- Basal bodies to which cilia and flagella attach
• The axoneme, the central shaft of a cilium or
flagellum, is a highly ordered bundle of MTs
© 2012 Pearson Education, Inc.
Tubulin Heterodimers Are the Protein
Building Blocks of Microtubules
• MTs are straight, hollow cylinders of varied length
that consist of (usually 13) longitudinal arrays of
polymers called protofilaments
• The basic subunit of a protofilament is a
heterodimer of tubulin, one a-tubulin and one b-
tubulin
• These bind noncovalently to form an ab-
heterodimer, which does not normally dissociate
© 2012 Pearson Education, Inc.
Figure 15-2
© 2012 Pearson Education, Inc.
Figure 15-2A
© 2012 Pearson Education, Inc.
Figure 15-2B
© 2012 Pearson Education, Inc.
Figure 15-2C
© 2012 Pearson Education, Inc.
Subunit structure
• a and b subunits have very similar 3-D structure, but only 40% amino acid identity
• Each has an N-terminal GTP binding domain, a central domain to which colchicine can bind, and a C-terminal domain that interacts with MAPs (microtubule-associated proteins)
• All the dimers in the MT are oriented the same way
© 2012 Pearson Education, Inc.
MT polarity and isoforms
• Because of dimer orientation, protofilaments
have an inherent polarity
• The two ends differ both chemically and
structurally
• Most organisms have several closely related
genes for slight variants of a- and b-tubulin,
referred to as isoforms
© 2012 Pearson Education, Inc.
Microtubules Can Form as Singlets,
Doublets, or Triplets
• Cytoplasmic MTs are simple tubes, or singlet
MTs, with 13 protofilaments
• Some axonemal MTs form doublet or triplet MTs
• Doublets and triplets contain one 13-
protofilament tubule (the A tubule) and one or two
additional incomplete rings (B and C tubules) of
10 or 11 protofilaments
© 2012 Pearson Education, Inc.
Microtubules Form by the Addition
of Tubulin Dimers at Their Ends
• MTs form by the reversible polymerization of
tubulin dimers in the presence of GTP and Mg2+
• Dimers aggregate into oligomers, which serve
as “nuclei” from which new MTs grow
• This process is called nucleation; the addition
of more subunits at either end is called
elongation
© 2012 Pearson Education, Inc.
Microtubule assembly
• MT formation is slow at first, the lag phase,
due to the slow process of nucleation
• The elongation phase is much faster
• When the mass of MTs reaches a point
where the amount of free tubulin is
diminished, the assembly is balanced by
disassembly; the plateau phase
© 2012 Pearson Education, Inc.
Figure 15-3
© 2012 Pearson Education, Inc.
Critical concentration
• Microtubule assembly in vitro depends on concentration of tubulin dimers
• The tubulin concentration at which MT assembly is exactly balanced by disassembly is called the critical concentration
• MTs grow when the tubulin concentration exceeds the critical concentration and vice versa
© 2012 Pearson Education, Inc.
Addition of Tubulin Dimers Occurs
More Quickly at the Plus Ends of
Microtubules
• The two ends of an MT differ chemically, and one
can grow or shrink much faster than the other
• This can be visualized by mixing basal bodies
(structures found at the base of cilia) with tubulin
heterodimers
• The rapidly growing MT end is the plus end and
the other is the minus end
© 2012 Pearson Education, Inc.
Figure 15-4
© 2012 Pearson Education, Inc.
Microtubule treadmilling
• The plus and minus ends of microtubules have
different critical concentrations
• If the [tubulin subunits] is above the critical
concentration for the plus end but below that of
the minus end, treadmilling will occur
• Treadmilling: addition of subunits at the plus end,
and removal from the minus end
© 2012 Pearson Education, Inc.
Figure 15-5
© 2012 Pearson Education, Inc.
Drugs Can Affect the Assembly of
Microtubules
• Colchicine binds to tubulin monomers,
inhibiting their assembly into MTs and promoting
MT disassembly
• Vinblastin, vincristine are related compounds
• Nocodazole inhibits MT assembly, and its
effects are more easily reversed than those of
colchicine
© 2012 Pearson Education, Inc.
Antimitotic drugs
• These drugs are called antimitotic drugs
because they interfere with spindle assembly
and thus inhibit cell division
• They are useful for cancer treatment
(vinblastine, vincristine) because cancer cells
are rapidly dividing and susceptible to drugs
that inhibit mitosis
© 2012 Pearson Education, Inc.
Taxol
• Taxol binds tightly to microtubules and
stabilizes them, causing a depletion of free
tubulin subunits
• It causes dividing cells to arrest during mitosis
• It is also used in cancer treatment, especially for
breast cancer
© 2012 Pearson Education, Inc.
Microtubules Originate from
Microtubule- Organizing Centers
Within the Cell
• MTs originate from a microtubule-organizing
center (MTOC)
• Many cells have an MTOC called a centrosome
near the nucleus
• In animal cells the centrosome is associated with
two centrioles, surrounded by pericentriolar
material
© 2012 Pearson Education, Inc.
Centriole structure
• Centriole walls are formed by 9 pairs of triplet microtubules
• They are oriented at right angles to each other
• They are involved in basal body formation for cilia and flagella
• Cells without centrioles have poorly organized mitotic spindles
© 2012 Pearson Education, Inc.
Figure 15-8A,B
© 2012 Pearson Education, Inc.
Figure 15-8C
© 2012 Pearson Education, Inc.
g-tubulin
• Centrosomes have large ring-shaped protein complexes in them; these contain g-tubulin (along with gamma tubulin ring proteins: GRiPs)
• g-tubulin ring complexes (g-TuRCs) nucleate the assembly of new MTs away from the centrosome
• Loss of g-TuRCs prevents a cell from nucleating MTs
© 2012 Pearson Education, Inc.
Figure 15-9
© 2012 Pearson Education, Inc.
Figure 15-9A
© 2012 Pearson Education, Inc.
Figure 15-9B
© 2012 Pearson Education, Inc.
MTOCs Organize and Polarize the
Micotubules Within Cells
• MTOCs nucleate and anchor MTs
• MTs grow outward from the MTOC with a fixed
polarity—the minus ends are anchored in the
MTOC
• Because of this, dynamic growth and shrinkage
of MTs occurs at the plus ends, near the cell
periphery
© 2012 Pearson Education, Inc.
Figure 15-10A-C
© 2012 Pearson Education, Inc.
Figure 15-10D
© 2012 Pearson Education, Inc.
Microtubule Stability Is Tightly Regulated
in Cells by a Variety of Microtubule-
Binding Proteins
• Cells regulate MTs with great precision
• Some MT-binding proteins use ATP to drive
vesicle or organelle transport or to generate
sliding forces between MTs
• Others regulate MT structure
© 2012 Pearson Education, Inc.
Microfilaments
• Microfilaments are the smallest of the
cytoskeletal filaments
• They are best known for their role in muscle
contraction
• They play a role in cell migration, amoeboid
movement, and cytoplasmic streaming
© 2012 Pearson Education, Inc.
Additional roles of microfilaments
• Development and maintenance of cell shape
(via microfilaments just beneath the plasma
membrane at the cell cortex)
• Structural core of microvilli
© 2012 Pearson Education, Inc.
Actin Is the Protein Building Block
of Microfilaments
• Actin is a very abundant protein in all eukaryotic
cells
• Once synthesized, it folds into a globular-shaped
molecule that can bind ATP or ADP
(G-actin; globular actin)
• G-actin molecules polymerize to form
microfilaments, F-actin
© 2012 Pearson Education, Inc.
Figure 15-12
© 2012 Pearson Education, Inc.
Figure 15-12A
© 2012 Pearson Education, Inc.
Figure 15-12B
© 2012 Pearson Education, Inc.
Figure 15-12C
© 2012 Pearson Education, Inc.
Different Types of Actin Are Found
in Cells
• Actin is highly conserved, but there are some
variants
• Actins can be broadly divided into muscle-specific
actins (a-actins) and nonmuscle actins (b- and
g-actins)
• b- and g-actin localize to different regions of a cell
© 2012 Pearson Education, Inc.
G-Actin Monomers Polymerize into
F-Actin Microfilaments
• G-actin monomers can polymerize reversibly into filaments with a lag phase, and elongation phase, similar to tubulin assembly
• F-actin filaments are composed of two linear strands of polymerized G-actin, wound into a helix
• All the actin monomers in the filament have the same orientation
© 2012 Pearson Education, Inc.
Demonstration of microfilament polarity
• Myosin subfragment 1 (S1) can be incubated with microfilaments (MFs)
• S1 fragments bind and decorate the actin MFs in a distinctive arrowhead pattern
• The plus end of an MF is called the barbed end and the minus end is called the pointed end, because of this pattern
© 2012 Pearson Education, Inc.
Polarity of microfilaments
• The polarity of MFs is reflected in more rapid addition or loss of G-actin at the plus end than the minus end
• After the G-actin monomers assemble onto a microfilament, the ATP bound to them is slowly hydrolysed
• So, the growing MF ends have ATP-actin, whereas most of the MF is composed of ADP-actin
© 2012 Pearson Education, Inc.
Specific Drugs Affect
Polymerization of Microfilaments
• Cytochalasins are fungal metabolites that prevent the addition of new monomers to existing MFs
• Latrunculin A is a toxin that sequesters actin monomers and prevents their addition to MFs
• Phalloidin stabilizes MFs and prevents their depolymerization
© 2012 Pearson Education, Inc.
Figure 15-14A
© 2012 Pearson Education, Inc.
Figure 15-14B
© 2012 Pearson Education, Inc.
Figure 15-15
© 2012 Pearson Education, Inc.
Actin-Binding Proteins Regulate the
Polymerization, Length, and
Organization of Actin
• Cells can precisely control where actin assembles and the structure of the resulting network
• They use a variety of actin-binding proteins to do so
• Control occurs at the nucleation, elongation, and severing of MFs, and the association of MFs into networks
© 2012 Pearson Education, Inc.
Figure 15-19A
© 2012 Pearson Education, Inc.
Intermediate Filaments
• Intermediate filaments are the most stable and least soluble cytoskeletal components and are not polarized
• An abundant intermediate filament (IF) is keratin, an important component of structures that grow from skin in animals
• IFs may support the entire cytoskeleton
© 2012 Pearson Education, Inc.
Figure 15-22
© 2012 Pearson Education, Inc.
Intermediate Filament Proteins Are
Tissue Specific
• IFs differ greatly in amino acid composition from
tissue to tissue
• They are grouped into six classes
© 2012 Pearson Education, Inc.
Classes of intermediate filament proteins
• Class I: acidic keratins
• Class II: basic or neutral keratins
• Proteins of classes I and II make up the
tonofilaments found in epithelial surfaces
covering the body and lining its cavities
© 2012 Pearson Education, Inc.
Classes of intermediate filament proteins
(continued)
• Class III: includes vimentin (connective tissue),
desmin (muscle cells), and glial fibrillary acidic
(GFA) protein (glial cells)
• Class IV: These are the neurofilament (NF)
proteins found in neurofilaments of nerve cells
© 2012 Pearson Education, Inc.
Classes of intermediate filament proteins
(continued)
• Class V: includes the nuclear lamins A, B, and C that form a network along the inner surface of the nuclear membrane
• Class VI: Neurofilaments in the nerve cells of embryos are made of nestin
• Animal cells can be distinguished based on the types of IF proteins they contain—intermediate filament typing
© 2012 Pearson Education, Inc.
Table 15-4
© 2012 Pearson Education, Inc.
Intermediate Filaments Assemble
from Fibrous Subunits
• IF proteins are fibrous rather than globular
• All have a homologous central rodlike domain conserved in size, secondary structure, and to some extent, in sequence
• Flanking the central helical domain are N- and C-terminal domains that differ greatly among IF proteins
© 2012 Pearson Education, Inc.
Figure 15-23
© 2012 Pearson Education, Inc.
Intermediate Filaments Confer
Mechanical Strength on Tissues
• Cellular architecture depends on the unique properties of the cytoskeletal elements working together
• MTs resist bending when a cell is compressed whereas MFs serve as contractile elements that generate tension
• IFs are elastic and can withstand tensile forces
© 2012 Pearson Education, Inc.
The Cytoskeleton Is a Mechanically
Integrated Structure
• IFs are important structural determinants in many cells and tissues; they are thought to have a tension-bearing role
• IFs are not static structures; they are dynamically transported and remodeled
• The nuclear lamina, on the inner surface of the nuclear envelope, disassemble at the onset of mitosis and reassemble afterward
© 2012 Pearson Education, Inc.
Integration of cytoskeletal elements
• Plakins are linker proteins that connect intermediate filaments, microfilaments, and microtubules
• One plakin, called plectin, is found at sites where intermediate filaments connect to MFs and MTs
© 2012 Pearson Education, Inc.
Figure 15-24
Recommended