Lecture 22 Flashcards
Microfilaments (MF)
The thinnest cytoskeleton element & a polymer of actin protein
MFs have several important functions:
* Maintenance of cell shape
* Cell movement
* Vesicle transport
* Muscle contraction
* Cytokinesis (contractile ring)
Microfilaments (MF) and Actin Molecules
Actin is the central component of MFs. It exists in cells either as a
monomer (G-actin or globular actin) or as a polymer (F-actin or
fibrous actin).
G-actin
G-actin has four subdomains
and is divided by a central cleft
creating two approximately
equal-sized lobes.
F-actin
An actin filament (F-actin)
appears as two strands of G-
actin monomers. One F-actin
unit has exactly 28 subunits of
G-actin (14 in each strand),
covering exactly a distance of
72 nm.
MFs are Dynamic like MTs:
Microfilaments have properties
similar to microtubules.
F-actin filaments are polar—
they have a ‘plus’ and
‘minus’ end.
Like microtubules, the ‘plus’
end assembles/disassembles
quickly, while the ‘minus’ end
assembles/disassembles
slowly.
Myosins:
A superfamily of motor proteins associated with microfilaments.
Most myosin molecules move toward the ‘plus’ end of microfilaments.
2 broad groups of myosins
1) Conventional myosins
* Type II
* Primary motors for muscle contraction
2) Unconventional myosins
* Type I and types III-XVIII
* Organelle / vesicle movement
Actin-Associated Motor Protein
Unconventional myosins generate
force and contribute to motility in non-muscle cells.
Actin-based protrusion of
leading edge (lamellipodium)
powered by actin growth
Myosin-based contraction pulls
trailing edge forward
Intermediate Filaments (IF)
Intermediate size (~10 nm diameter).
* Exclusive to multicellular animal cells.
* Provide structural support and mechanical strength.
* Stable in comparison to MTs or microfilaments.
* Arrangement of fibrous α-helical proteins.
* Not polar (i.e. no ‘plus’ and ‘minus’ ends). IFs are not used for
transport.
The Nucleus: Function
- Storage, replication, and repair
of genetic material. - Expression of genetic material:
* Transcription: mRNA, tRNA, rRNA
* RNA splicing - Ribosome biosynthesis
The Nuclear Envelope
2 parallel phospholipid bilayers separated by 10-50nm space.
- Outer nuclear membrane (ONM) that binds ribosomes and is continuous with rough ER
* Inner nuclear membrane (INM) that has integral proteins and connects to nuclear lamina.
Importance of the Nuclear Envelope
- Separates nuclear content
from cytoplasm - Separates transcription
and translation processes - Selective barrier that
allows limited movement
of molecules between
nucleus and cytoplasm
The Nuclear Lamina
Supports the nuclear envelop.
* Thin meshwork of filamentous proteins:
– Plants have nuclear lamina, but not made of lamin protein (we
do not know what they are).
The equivalent protein(s) is not known and remains an open
question in plant biology.
* Nuclear lamina is bound to inner membrane of the nuclear
envelop (NE) by integral membrane proteins.
* Provides structural support for nuclear envelop.
* Attachment sites for chromatin (heterochromatin)
Nuclear Pore
Gateways between cytoplasm and nucleus
* 3000-4000 pores/nucleus
* Pores are found where inner and outer membranes fuses.
* Contains a complex protein structure that
involve the arrangement of different types of proteins
Nuclear Pore Complex (NPC)
The nuclear pore complex is:
* Composed of nucleoporins (NUPs)—a
large family of different proteins
* Octagonal symmetry
* Projects into cytoplasm and nucleoplasm
- Passive diffusion of molecules
that are 40 kDA or less: - Rapid
- 100 molecules/minute/pore
- Regulated movement of larger
molecules: - Slow
- 6 molecules/minute/pore
