Presumptive Insulin Pathway in Xenopus oocytes

1. Insulin binds to IGF-1/insulin receptor
2. IGF-1/insulin receptor autophosphorylates (little yellow circles are phosphate). This "turns on" the receptor so that it will phosphorylate other proteins.
3. IRS-1 (the green circle) binds to phosphotyrosine on IGF-1/insulin receptor and IRS-1 is activated by phosphorylation on its tyrosines. IRS-1 or the receptor itself activates phospholipase C (or PLC).
4. PIP2 (a polyphosphoinositide or PI) breakdown is catalyzed by PLC; PIP2 is cleaved to DAG and IP3.
5. IP3 releases intracellular calcium that may play a role in cell division. DAG stimulates protein kinase C (an enzyme that is also activated by turmor promoters).