Participation in Membrane Transport

In protozoa and some algae, cation sequestration is one of the functions of acidocalci-some (Docampo and Moreno, 2001 Ruiz et al, 2001a,b). This is an electron-dense acidic organelle, which contains pyrophosphate and PolyP bound with Ca2+ and other cations. Its membrane possesses a number of pumps and exchangers for the uptake and release of these elements. 

It should be noted that the PolyPs of the cell envelope could also be the first barrier on the route of penetration of heavy metal cations into a cell, both in prokaryotes and eukaryotes. 

PolyP is a participant of transmembrane ion transport processes, both in procaryptes and eukaryotes. It is widely accepted that ion channels are exclusively proteins, but recently the formation of ion-selective, voltage-activated channels by complexes of PolyP and poly-(A )-3-hydroxybutyratcs (PHBs) has been demonstrated (Reusch and Sadoff, 1988 Reusch, 1992, 1999a, 2000). Each of these have unique molecular characteristics that facilitate ion selection, solvation and transport. 

Complexes of the two polymers, isolated from bacterial plasma membranes or prepared from synthetic polymers, form voltage-dependent, Ca2+-selective channels in planar lipid bilayers that are selective for divalent over monovalent cations, permeant for Ca2+, Sr2+ 

To still further determine the composition of the channels, the PHB-Ca2+-PolyP complexes were reconstituted. PHB was recovered from E. coli and carefully purified, and Ca2+-PolyP was prepared from commercial sodium PolyP and calcium chloride. Single-channel currents similar to those described above were obtained by three different experimental procedures, as described by Reusch et al. (1995). The chain length of chemically synthesized PolyP was determined by acrylamide gel electrophoresis to be in the same range (55-65 residues) as in the E. coli complexes (Castuma et al., 1995). 

---