Erythrocytes Ca2* -ATPase

PolyPs and PHB have been found to be associated with ion-conducting proteins such as the human erythrocyte Ca2+-ATPase pump (Reush et al, 1997) and the Streptomyces lividans potassium channel (Reusch, 1999b). Some enzymes of PolyP metabolism, such as polyphosphate glucokinase (Phillips et al, 1999) and yeast high-molecular-weight exopolyphosphatase (Andreeva et al, 2001, 2004), can contain tightly bound PolyP. 

To test the above hypothesis, Reusch et al.30 analyzed the human erythrocyte Ca2+-ATPase pump for polyP and PHB. This protein is the first plasma membrane enzyme demonstrated to work as a Ca2+ pump, and it is the sole transporter of Ca2+ in the cell.121,122 Moreover, the Ca2+-ATPase is ubiquitous in plasma membranes of a variety of cell types in eukaryotic organisms.123... 

The manner in which the PHB and polyP interact with the protein is not known and it remains uncertain whether they play an active role in the export of Ca2+ however, the presence of these highly conserved polymers and enzymatic activities in the erythrocyte Ca2+-ATPase is consistent with the molecular mechanism for Ca2+ export suggested above, i.e. the protein may be viewed as a device that transports Ca2+ out of the cytoplasm by catalyzing the transfer of phosphoryl groups from ATP to polyP. 

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