Question

• Describe how/why the polarity of the microfilament is associated with it having a “pointed” end and a “barbed” end. What is the functional significance of such a shape?

Answer 1

Microfilaments, also called actin filaments, are polymers of the protein actin that are part of a cell’s cytoskeleton. The cytoskeleton is the network of protein filaments that extends throughout the cell, giving the cell structure and keeping organelles in place. Microfilaments are the smallest filaments of the cytoskeleton. They have roles in cell movement, muscle contraction, and cell division

Microfilament Structure

Microfilaments are composed of two strands of subunits of the protein actin (hence the name actin filaments) wound in a spiral. Specifically, the actin subunits that come together to form a microfilament are called globular actin (G-actin), and once they are joined together they are called filamentous actin (F-actin). Like microtubules, microfilaments are polar. Their positively charged, or plus end, is barbed and their negatively charged minus end is pointed. Polarization occurs due to the molecular binding pattern of the molecules that make up the microfilament. Also like microtubules, the plus end grows faster than the minus end.

Microfilaments are the thinnest filaments of the cytoskeleton, with a diameter of about 6 to 7 nanometers. A microfilament begins to form when three G-actin proteins come together by themselves to form a trimer. Then, more actin binds to the barbed end. The process of self-assembly is aided by autoclampin proteins, which act as motors to help assemble the long strands that make up microfilaments. Two long strands of actin arrange in a spiral in order to form a microfilament.