Which of the following statements about actin is correct?
a. If the actual amount of available ATP-bound G-actin is below the critical concentration at both ends of a filament, then the actin filament will depolymerize.
b. Actin monomers are called G-actin because they have a nucleotide binding site that accommodates a molecule of GTP.
c. The plus end of an actin monomer has a net positive charge.
d. Actin generally polymerizes more rapidly at the pointed end.
e. Polymerization of an actin filament will only occur at the plus end.