FIGURE 7:

Rescue activity is determined during depolymerization. (A) Under rescue-promoting conditions, depolymerization is inhibited by rescue sites in the microtubule lattice, and the microtubule end returns to a polymerizing state. These sites may include, but are not limited to, regions of missing subunits (i.e., “protofilament gaps”) and subunits that maintain GTP at the E-site (i.e., “GTP islands”). (B) Under rescue-inhibiting conditions, divalent cations promote depolymerization and inhibit the activity of rescue sites in the lattice. Here we depict a proposed role for magnesium ions in promoting the outward curling of protofilaments.