When the sodium pump is scarce, Cav1 (green) loiters near the nucleus instead of infiltrating the cell membrane.

The sodium pump is branching out. As Cai et al. show, the pump not only moves ions, it controls trafficking of a membrane protein that is crucial for intercellular communication and other functions.

The Na/K-ATPase, or sodium pump, ejects sodium ions from the cell and brings in potassium. Besides swapping ions, the pump helps structure the cell membrane. Pump molecules are prevalent in caveolae, pockets in the plasma membrane involved in cell–cell signaling and endocytosis. Instead of assembling these structures at the membrane, cells build pre-fab caveolae in the Golgi apparatus and then ship them for installation. Because pump molecules can bind to caveolin-1 (Cav1), the main structural protein of caveolae, Cai et al. wondered whether the pump helped govern dispersal and positioning of Cav1.

To find out, the researchers used RNA interference to trim the amount of non-pumping Na/K-ATPase. The treatment reduced the amount of Cav1 in the membrane and the number of caveolae. Adding a non-pumping version of Na/K-ATPase restored the normal distribution of Cav1, but a version that couldn't attach to Cav1 had no effect.

The researchers found that in the treated cells, Cav1 emerged from the Golgi apparatus, suggesting that the pump's disappearance doesn't impair caveolin construction. However, Cav1 in the plasma membrane was prone to return to the cytoplasm. A protein called Src promotes this endocytosis of Cav1, but the pump inactivates Src. Overall, the study suggests that by blocking Src and by latching onto Cav1, the sodium pump helps direct Cav1 to the membrane and keep it there.

Cai, T., et al.
2008
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J. Cell Biol.
doi:.