Question

Explain how actin capping and actin severing could increase cell migration. Rubric (4): cell migration is linked to actin capping (2) and severing of microfilaments (2).

Answer 1

Addition of salts to a solution of G-actin induces polymerization, creating F-actin filaments. The polymerization process can be monitored by viscometry, sedimentation, and fluorescence spectroscopy. When actin filaments become long enough to become entangled, the viscosity of the solution increases, which is measured as a decrease in its flow rate in a viscometer. The basis of the sedimentation assay is the ability of ultracentrifugation to pellet F-actin but not G-actin. The third assay makes use of G-actin covalently labeled with a fluorescent dye; the fluorescence spectrum of the modified G-actin monomer changes when it is polymerized into F-actin. These assays are useful in kinetic studies of actin polymerization and during purification of actin-binding proteins, which cross-link or depolymerize actin filaments.

The polymerization of actin filaments proceeds in three sequential phases (Figure 18-11a). The first phase is marked by a lag period in which G-actin aggregates into short, unstable oligomers. Once the oligomer reaches a certain length (three or four subunits) it can act as a stable seed, or nucleus, which in the second phase rapidly elongates into a filament by the addition of actin monomers to both of its ends. As F-actin filaments grow, the concentration of G-actin monomers decreases until it is in equilibrium with the filament. This third phase is called steady state because G-actin monomers exchange with subunits at the filament ends but there is no net change in the total mass of filaments.