Figure 22-15 The Molecular Structure of G-Actin Monomer X-ray crystallography shows that the G-actin monomer is shaped somewhat like a U A nucleotide (ATP or ADP) binds reversibly in a groove in the protein. When G-actin monomers polymerize into F-actin, the mouth of the groove is covered by another G-actin monomer, trapping the bound nucleotide inside. In addition, binding of one G-actin to another forces the mouth of the groove to close more tightly on the bound nucleotide, promoting hydrolysis of the ATP