Question

Describe the function of scramblase in the ER and in the plasma membrane. Mention all relevant enzyme characteristics necessary to support this function.

Answer 1

The plasma membrane of cell and the ER membrane has an inner and outer leaflet. In inner leaflet there is phosphatidyl serine (PS), phosphatidylethanolamine (PE) and phosphatidylinositol (PI). The outer leaflet has sphingomyelin (SM) and phosphatidylcholine (PC). Specific proteins such as flippases, floppases and scramblases maintain this asymmetry of lipids in the plasma membrane via transbilayer lipid motion. Flippases and floppases move the lipids to inner and outer leaflets of the plasma membrane. Their functions require ATP. The scramblases, on the other hand, maintain transbilayer lipid movement, especially of negatively charges phospholipids such as PS in absence of ATP and in non-selective fashion. In the ER, the scramblase present is TMEM16. In ER, scramblases are present in the luminal leaflet. These ER scramblases will translocate the PS from the luminal leaflet across the ER membrane. From the ER membrane, the PS is then translocated via leaflets to cytoplasmic leaflet of plasma membrane or mitochondria. TMEM16 are also activated by intracellular calcium.

The action of scramblases will allow phosphatidyl serine present in the inner cytoplasmic leaflet to move to the outer leaflet and vice versa. Scramblases are type II single pass transmembrane proteins with a central alpha helix that is surrounded by barrel beta helices. They have an EF hand calcium binding domain that is required for their activation. There is a cysteine Palmitoylation motif that anchors the enzyme to the membrane. The enzyme has a proline rich domain which is required for activity. The C-terminal helix helps the enzyme to integrate in membrane and also for its activity. Various mechanisms have been proposed for scramblase mediated movement of PS. One mechanism proposes that the PS can diffuse through a scramblase groove formed at interface between lipids and protein. It is also possible that the scramblase groove can thin the plasma or ER membrane. This cause movement of lipids between the two leaflets as the lipid barrier for movement is reduced. No energy is required for movement. The hydrophilic head of PS is shielded by the scramblase while the hydrophobic end attaches to membrane.

Scramblases are activated when calcium level in the cytoplasm increases. The calcium binds to the EF-hand (helix-loop helix structure) calcium binding domain. As a result, the enzyme is activated and moves PS to the outer leaflet as well, creating a symmetric distribution of PS in the plasma membrane. Increased calcium in cytoplasm will inactivate flippases and floppases. Scramblases are activated by apoptosis that cause increase in cytoplasmic calcium. The movement of PS to outer leaflet provides the signal for binding of macrophages for phagocytosis. Scramblase action is also involved in blood coagulation via action on red blood cells membrane.