I'm a G-protein coupled receptor (GPCR) fan, and all of you should be too. Most drugs, from pot to tylenol, act on G-protein Coupled Receptors. The pharmaceutical industry would be helpless without them.

For a quick primer on GPCRs, you can try to decipher this picture:

Or just check out Wikipedia.

Anyway, GPCRs, in response to hormones or drugs, activate a three-component G-protein, which in turn switches on a signaling pathway. The key is that all three-part G-proteins are basically the same - they need each of the three components, and those components hang out at the plasma membrane of the cell.

Or do they? This interesting paper shows that a completely different protein, a phosphatidylinositol-3-kinase, can act as the beta subunit of a heterotrimeric G-protein. This allows GPCRs to signal from inside the cell, away from plasma membrane - inside of endosomes. The paper shows that the phosphatidylinositol-3-kinase (named Vps15), is structurally very similar to the beta subunit - an impressive piece of convergent evolution. A protein, Vps15 with one function (a PI-3 kinase), is tweaked to perform a new one.