Cytoskeleton, Cell Junctions, and Transport Across Membranes
I. Eukaryotic Cytoskeletal Proteins
Introduction:
For a long time, prokaryotes (bacteria) were not believed to have cytoskeletons.
It has been about years since cytoskeletal components were discovered in bacteria; however, these are generally not covered in introductory textbooks.
All eukaryotic cytoskeletal components are protein complexes.
Three Types of Cytoskeletal Proteins in Eukaryotes:
Actin Filaments (also called Microfilaments):
Size: Thinnest of the three types.
Monomer: Actin (only one kind of actin protein).
Structure:
Monomers are often depicted as small, spherical units, not very long but with a consistent diameter.
Individual actin proteins associate repeatedly in a long chain.
Two of these long chains come together to form a double helix (similar to DNA's double helix structure).
Bonds: Individual actin proteins are formed with covalent bonds. However, actin proteins interact with each other (to form the filament) via intermolecular forces (e.g., hydrogen bonding, ionic interactions), not covalent bonds. This makes the filaments flexible, allowing subunits to easily detach or reattach.
Functions:
Maintaining and rapidly changing cell shape.
Muscle contraction.
Cytoplasmic streaming (movement of cytoplasm within the cell, observable as chloroplast movement in living plant cells).
Cell motility (e.g., forming pseudopodia for phagocytosis).
Cell division (facilitating the pinching-in of the cleavage furrow).
Microtubules:
Size: Thickest of the three types.
Monomers: Tubulin (two different types: alpha tubulin and beta tubulin).
Structure:
Alpha and beta tubulin monomers associate to form tubulin dimers.
These dimers stack to form protofilaments.
protofilaments arrange themselves in a cylindrical fashion to create a hollow tube. This structure makes them the most rigid of the three cytoskeletal components.
Functions:
Cell shape.
Nuclear division (e.g., moving chromosomes during mitosis and meiosis).
Forming centrioles and basal bodies.
Moving organelles (act as tracks for motor proteins like kinesin and dynein).
Forming cilia and flagella (for cell movement or movement of extracellular fluid).
Intermediate Filaments:
Size: Intermediate in thickness compared to actin filaments and microtubules ( nm diameter).
Monomer: Diverse group of highly variable proteins, depending on the cell type and tissue (e.g., keratins in epithelial cells, vimentin in connective tissue, desmin in muscle cells, neurofilaments in neurons, lamins in the nuclear envelope).
Structure:
Composed of stable, rope-like fibers formed by intertwining subunits, typically assembling into a . These tetramers then pack together in an overlapping manner to form the final filament.
They are incredibly stable and durable, providing a robust cellular framework. Unlike actin and microtubules, intermediate filaments do not exhibit dynamic instability or treadmilling.
Functions:
Provide mechanical strength and structural support to cells, resisting stretching and compression.
Anchor organelles, such as the nucleus, within the cytoplasm.
Form the nuclear lamina, which provides structural support to the nuclear envelope and plays a role in chromatin organization.
Maintain tissue integrity, particularly in epithelial tissues where they link cells together at desmosomes, resisting external forces and preventing cell tearing.
Are less dynamic than actin filaments and microtubules, meaning they are relatively stable once formed and