Key Concepts of Section 17.1

• Microfilaments can be assembled into diverse structures, many of which are associated with the plasma membrane (see Figure 17-4a).

• Actin, the basic building block of microfilaments, is a major protein of eukaryotic cells and is highly conserved.

• Actin can reversibly assemble into filaments that consist of two helices of actin subunits.

• The actin subunits in a filament are all oriented in the same direction, with the nucleotide-binding site exposed on the (−) end (see Figure 17-5).