Motors, Actin, Cell Motility (filaments)

Question 1

Intermediate filaments (facts)

Answer 1

• Intermediate size (10-12 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). For that reason, IFs
  are not used for transport.

Question 2

Intermediate Filaments structure and location

Answer 2

• Composed of keratins in the cytoplasm

-Composed of lamins in the nucleus

• Are especially abundant in the axons of neurons

Question 3

Microfilament functions

Answer 3

• Maintenance of cell shape
• Cell movement
• Vesicle transport (especially in plants)
• Muscle contraction
• Cytokinesis (contractile ring)

Question 4

Microfilament and Actin molecules

Answer 4

• Actin is the central component of MFs

-It exists in cells either as a
monomer (G-actin or globular) or as a polymer (F-actin or fibrous).

• Actin is an enzyme that binds and slowly hydrolyzes ATP.
• A microfilament is a double helix of actin monomers.

Question 5

Structures of monomeric G-Actin

Answer 5

• G-actin has four subdomains and is divided by a central cleft creating
  two approximately equal-sized lobes

Question 6

Structures of monomeric F actin filaments

Answer 6

• An actin filament (F-actin) appears as two strands of subunits

-One F-actin unit has exactly 28 subunits of G-actin (14 in each strand), covering exactly a distance
of 72 nm

Question 7

Properties of MF’s

Answer 7

• 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

Question 8

Assembly of F actin microfilaments

Answer 8

• The early nucleation steps of G-
  actin polymerization are slow.
  F-Actin Microfilament Assembly
• ATP binding favors nucleation and
  stability of filaments.

-G-Actin → dimers → trimers → short filaments

-Monomers can be added at both ends, but its always faster at the ‘plus’ end.

-The process is reversible—ATP
hydrolysis stimulates the
destabilization of the polymer

Question 9

Organization of F-Actin microfilaments

Answer 9

• F-actin microfilaments can be arranged in a loose array network (meshwork) or tight bundles/cables/fibers.
• The organization of these structures is regulated by actin-binding proteins.