Figure 1.

Two examples of autophosphorylation in cell biology. (A) In Src family kinase (SFK) signaling pathway, the phosphorylation of a membrane receptor, from R to R^∗, is catalyzed by an activated SFK. The activation of the SFK, however, is assisted by its association with phosphorylated receptor and the formation of R^∗SFK complex. Dephosphorylation of R^∗ is catalyzed by a protein tyrosine phosphatase (PTP). (B) In endocytic pathway, the activation of Rab5 GTPase, from its GDP-bound state G^GDT to GTP-bound state G^GTP, is catalyzed by activated Rabex-5, a guanine nucleotide exchange factor. The activation of the Rabex-5, however, is assisted by its association with the G^GTP via another regulator called Rabaptin-5. The GTP hydrolysis carried out by Rab5 is accelerated by a GTPase accelerating protein (GAP).