Microfilaments & Actin Structure

What are the three main types of filaments in the eukaryotic cytoskeleton?

1. Microfilaments (Actin, ~7-9 nm) 2. Microtubules (αβ-tubulin dimers, ~25 nm) 3. Intermediate filaments (Various proteins, ~10 nm)

What are the subunit building blocks and diameter of microfilaments?

Actin subunits; diameter is 7 to 9 nm.

What are the subunit building blocks and diameter of microtubules?

αβ-tubulin dimers; diameter is 25 nm.

What is the diameter of intermediate filaments?

Approximately 10 nm.

Which bacterial protein is structurally related to eukaryotic actin?

MreB (which also forms filaments in bacteria).

How many actin genes are expressed in humans, and how identical are their protein sequences?

Humans express 6 actin genes; their protein products are 93% identical.

What percentage of total cellular protein does actin represent in muscle vs. non-muscle cells?

Muscle cells: Up to 10% of total protein. Non-muscle cells: 1% to 5% of total protein.

What is monomeric actin called, and how is its structure organized?

G-actin (Globular actin). It is divided by a central cleft into two lobes and four subdomains (I, II, III, IV).

Where do ATP or ADP bind on a G-actin monomer?

At the bottom of the central cleft, contacting both lobes.

What is the polymer form of actin called, and what is its overall structure?

F-actin (Filamentous actin). It is a long, helical polymer made of two helically wound protofilaments.

How many actin subunits make up one full repeating structural unit of an F-actin filament?

28 subunits total (14 in each of the two strands), covering 36 nm.

How is structural polarity defined in an F-actin filament?

All actin subunits point in the same direction. (-) end: Exposes the ATP-binding cleft. (+) end: The opposite end.

What actin arrangement is found in microvilli and filopodia?

Tight parallel bundles.

What actin arrangement is found in stress fibers, muscle Z-lines, and the adherens belt?

Loose / antiparallel contractile bundles.

What actin organization is present in the lamellipodium / leading edge of a migrating cell?

3D branched networks (gels).

What actin structure forms the cell cortex beneath the plasma membrane?

A membrane-associated network.

What functional actin structure is responsible for splitting a cell during cytokinesis?

The contractile ring.

What technique uses fluorescently labeled antibodies to visualize cytoskeletal filaments under a microscope?

Immunofluorescence microscopy.

How does phalloidin work in fluorescent staining of the cytoskeleton?

Phalloidin is a drug/toxin that specifically binds to F-actin filaments, allowing visualization of microfilament networks.

What supports the apical microvilli structure in intestinal epithelial cells?

Core bundles of actin microfilaments anchored vertically into the terminal web/cell cortex.

How do cell-surface receptors influence the cytoskeleton?

They receive external signals (soluble factors, ECM, neighboring cells) and activate cytosolic signal transduction pathways that alter cytoskeletal arrangement and activity.

What two main cellular outputs are governed by cytoskeletal signal transduction?

1. Organization and movement of organelles. 2. Cell shape, movement, and contraction.

What allows a single cell to create localized, diverse cytoskeletal structures simultaneously?

The integration of signals from multiple receptor types acting on distinct cytosolic pathways.

In a migrating cell, where are microfilaments primarily concentrated?

At the leading edge (lamellipodium and filopodia) to push the cell membrane forward.

How does the structure of F-actin contribute to its function in cell motility?

Its structural polarity and dynamic assembly/disassembly allow directional growth and force generation against membranes.