Insights into the oligomerization process of the C-terminal domain of human plasma membrane Ca2+-ATPase

Plasma membrane calcium pumps (PMCAs) sustain a primary transport system for the specific removal of cytosolic calcium ions from eukaryotic cells. PMCAs are characterized by the presence of a C-terminal domain referred to as a regulatory domain. This domain is target of several regulatory mechanisms: activation by Ca2+-calmodulin complex and acidic phospholipids, phosphorylation by kinase A and C, proteolysis by calpain and oligomerization. As far as oligomerization is concerned, the C-terminal domain seems to be crucial for this process. We have cloned the C-terminal domain of the human PMCA isoform 1b, and characterized its properties in solution. The expressed protein maintains its tendency to oligomerize in aqueous solutions, but it is dissociated by amphipathic molecules such as diacylglycerol and sodium dodecyl sulphate. The presence of sodium dodecyl sulphate stabilizes the domain as a compact structure in monomeric form retaining the secondary structure elements, as shown by small angle neutron scattering and circular dichroism measurements. The importance of oligomerization for the regulation of PMCA activity and intracellular calcium concentration is discussed.

Research highlights
► Recombinant C-terminal domain of human PMCA1b oligomerizes in aqueous solutions.
► Amphipathic molecules such as diacylglycerol and SDS stabilize the domain in monomeric form while phosphatidylserine and phosphatidylcholine do not.
► SDS has a stronger effect than DAG in stabilizing the domain as monomer.
► CD spectroscopy shows a minor loss of β-structure of monomeric domain in the presence of SDS.
► SANS shows that the monomeric domain is folded as an elongated globular structure.