Cytoskeletal Systems Notes

Chapter 13 outlines the different cytoskeletal systems in cellular biology, focusing specifically on microfilaments and intermediate filaments.

Microfilaments are the smallest of the cytoskeletal filaments.
Key Functions:
- Muscle contraction.
- Development and maintenance of cell shape through their presence just beneath the plasma membrane at the cell cortex.
- Structural core of microvilli, which are tiny protrusions on cells for absorption and surface area increase.

Actin is a highly abundant protein found in all eukaryotic cells.
Upon synthesis, actin folds into a globular shape capable of binding ATP or ADP, known as G-actin (globular actin).
G-actin molecules polymerize into filaments called F-actin (filamentous actin).

G-actin monomers undergo a reversible polymerization process into F-actin filaments, exhibiting a lag phase followed by an elongation phase, akin to tubulin assembly.
F-actin filaments consist of two linear strands of polymerized G-actin organized in a helical structure.
All actin monomers in these filaments maintain the same orientation.

When myosin subfragment 1 (S1) is incubated with microfilaments, the resultant binding forms a distinctive arrowhead pattern.
This indicates the polarity of the microfilaments:
- Plus end referred to as the "barbed end."
- Minus end referred to as the "pointed end."

Actin monomers in the cytosol bind ATP, and upon binding, ATP is converted to ADP, trapping ADP in the polymer.
Microfilament polarity shows a quicker rate of G-actin addition or loss at the plus end compared to the minus end.
Newly assembled G-actin monomers at the growing filament ends are ATP-bound, while most of the filament is composed of ADP-actin.

Cells can dynamically organize actin into various structures, including:
- Trailing edge and Leading edge of migrating cells.
- Stress fibers which provide mechanical support.
- Actin arrays forming the cell cortex, lamellipodia, and filopodia.

Actin-binding proteins are crucial in regulating the polymerization, length, and organization of actin filaments.
Key Functions of Actin-Binding Proteins:
- Control at various stages, including nucleation, elongation, and severing of microfilaments.
- Association of filaments into functional networks.

Monomer-sequestering proteins (e.g., thymosin β4): Bind to G-actin to prevent assembly.
Actin polymerizing proteins (e.g., formin): Promote the addition of G-actin to the filament.
Filament-severing proteins (e.g., gelsolin): Cut filaments, thereby modifying length and creating new ends.
Filament-capping proteins (e.g., CapZ): Prevent assembly or disassembly at filament ends.
Filament-crosslinking proteins (e.g., filamin): Create networks by connecting filaments where they intersect.
Filament-bundling proteins (e.g., α-actinin, fimbrin): Organize actin into tightly packed arrays.

High levels of ATP-bound G-actin promote filament assembly until G-actin concentration becomes limiting.
In healthy cells, free G-actin is usually bound by proteins like thymosin β4, while profilin competes for G-actin binding.
ADF/cofilin: Binds to ADP-G-actin and F-actin to accelerate turnover at the minus end, enhancing the dynamics of the filament.

The addition of capping proteins determines whether filaments can grow or shrink:
- CapZ: Binds to the plus end.
- Tropomodulins: Bind to the minus end.
Actin networks often consist of loose crosslinked filaments, where proteins like filamin help to organize these networks.

Some actin structures, such as focal contacts or focal adhesions, have highly ordered arrangements facilitated by proteins like α-actinin and fascin.

MFs connect to the plasma membrane through various linking proteins (e.g., band 4.1, ezrin, radixin) creating mechanical tension during cell movement or cytokinesis.

Actin can form dendritic networks via the Arp2/3 complex that helps nucleate new branches on existing filaments, activated by WASP and WAVE/Scar proteins.

Intermediate filaments (IFs) are abundant in many animal cells but absent in plant cell cytosol.
Main characteristics:
- Most stable and least soluble components of the cytoskeleton.
- Provide structural support to the cytoskeleton, prominently featuring keratin in animal tissues.

The basic building blocks of IF proteins are fibrous dimers, characterized by a central rod-like domain of 310 to 318 amino acids and N/C-terminal domains unique to each type.
Assembly model:
- Dimeric proteins twist into a coiled-coil configuration, aligning in parallel to form a tetrameric protofilament, with multiple protofilaments merging to create the final filament structure.

Intermediate filaments offer tensile strength in tissues and are less susceptible to chemical degradation than microtubules or microfilaments.

Cytoskeletal integrity is maintained by linker proteins like spectraplakins connecting all three filaments: microtubules, microfilaments, and intermediate filaments.
Plectin serves as an important linker at various sites where these components converge.