Question

2. Actin filaments are polarized. Intermediate filaments are non-polar. Myosin thick filaments are bipolar. a. Explain how the assembly of these filaments from their soluble monomers determines whether they are polar, nonpolar or bipolar. b. Why are the two ends of actin filaments designated as being ‘plus’ or ‘minus’? c. Why do intermediate filaments have such high tensile strength?

Answer 1

A. Intermediate filaments are made up of smaller subunits that are themselves elongated and fibrous, whereas actin filaments and microtubules are made of subunits that are compact and globular. The actin subunit is a single globular polypeptide chain and is thus a monomer rather than a dimer.  Many biological polymers—including DNA, RNA, and proteins—are held together by covalent linkages between their subunits. The structural polarity of actin filaments and microtubules is created by the regular, parallel orientation of all of their subunits. This orientation makes the two ends of each polymer different in ways that have a profound effect on filament growth rates.

B. As all the actin monomers are oriented in the same direction, actin filaments have a distinct polarity and their ends called the plus and minus ends, are distinguishable from one another. This polarity of actin filaments is important both in their assembly and in establishing a unique direction of myosin movement.

C. The tight association between protofilaments provide intermediate filaments with a high tensile strength. This makes them the most stable component of the cytoskeleton. Intermediate filaments are therefore found in particularly durable structures such as hair, scales and fingernails. The primary function of intermediate filaments is to create cell cohesion and prevent the acute fracture of epithelial cell sheets under tension. This is made possible by extensive interactions between the constituent protofilaments of an intermediate filament, which enhance its resistance to compression, twisting, stretching and bending forces.