The on-off switch of the pump thus has a previously unknown third position, in which the pump changes into the turbo gear. The group of Henning Tidow from Aarhus University and Lisbeth Poulsen from the University of Copenhagen published its studies in the British journal “Nature” (advance online publication). “The discovery not only improves our understanding of a fundamental mechanism in the biology of all higher organisms, but could one day allow for better treatment of certain diseases in which the calcium balance is disturbed,” says Tidow. The researchers used the measuring station of the European Molecular Biology Laboratory EMBL at DORIS.
The element calcium plays a central role in many processes of life, such as cell division, the day-night cycle and the communication of cells. The decisive factor is a gradient in the calcium concentration, which is normally high outside the cell and low inside it. This gradient is maintained among others through a calcium pump, which occurs in all higher organisms (eukaryotes) – from the nettle to the blue whale. For example, under stress the calcium concentration in the cell increases and triggers a corresponding reaction. Afterwards, the concentration must be lowered again.
“The calcium transport from the cell requires a lot of energy. It is therefore important that the pump is activated only when needed,” explains Poulsen. The pump – known as PMCA (plasma-membrane calcium-ATPase) – thus has a switch, which is actuated by the protein calmodulin. When calcium binds to calmodulin, the latter changes its shape so that it can dock onto a binding site of the cell’s calcium pump, thereby activating the pump. When the calcium concentration in the cell increases, more and more pumps are thus switched on.
The researchers led by Tidow viewed the entire switching complex with X-rays to reveal its molecular structure. They chose the switching complex from cells of the plant thale cress (Arabidopsis thaliana), studying it first in crystal form and then in solution, which is closer to the natural environment of the molecule. “Based on this analysis, we were able to create a detailed three-dimensional model of the region of the calcium pump that interacts with calmodulin,” says Tidow. “To our great surprise, we found that the calcium pump has two binding sites for calmodulin and not just one as previously thought.”
The switching complex thus consists of a dumbbell-like structure with two calmodulin binding sites. To determine whether the second site has a biological significance, the researchers tested pumps in which they had disabled one switch. Indeed, these pumps could not run at full power. “Our results show that the calcium pump is controlled in three steps,” explains Poulsen. “It is switched off when no calmodulin is bound to the switching complex. The pump is running at medium speed as soon as one binding site is occupied, and at full speed when calmodulin is bound to both sites.”
The pump is thus activated step by step, depending on how much calcium is present in the cell. When the calcium concentration increases, the pump first operates in an energy-efficient way at moderate speed. If the calcium threatens to reach an amount that is dangerous for the cell, the pump changes into the turbo gear, which enables it to very quickly reduce the concentration.
Bioinformatics analyses revealed that this double switch occurs not only in all plant species, but in general in all cells with a nucleus (eukaryotes). “This study underscores the strength of integrating structural biology in interdisciplinary research,” underlines Poul Nissen of Aarhus University. As the next step, the researchers aim to decipher the structure of the entire calcium pump.
A bimodular mechanism of calcium control in eukaryotes; Henning Tidow, Lisbeth R. Poulsen et al.; Nature 2012 (advance online publication); DOI: 10.1038/nature11539