Figure 6. G protein hypothesis in bone cells. In osteoblast-like cells, fluoride forms a complex with aluminum (AlF_x), which interacts with GDP to form a GTP-like molecule. Activation of the Gi protein stimulates the tyrosine phosphorylation of signaling proteins by an unknown protein tyrosine kinase (PTK), such as the recently identified Pyk2. Activation of the MAPK pathway through the Ras pathway leads to enhanced cell proliferation (adapted from Caverzasio et al., 1998). Reprinted with permission from Elsevier Science.