Study of phospholamban incorporated in nanoBLMs

Background: Phospholamban (wt-PLN) is an integral membrane protein that regulates cardiac sarco/endoplasmatic CaATPase (SERCA). PLN exists in equilibrium between monomeric and pentameric forms. Monomeric unphosphorylated PLN inhibits SERCA, but when PLN is phosphorylated releases the inhibition and allows calcium translocation. In literature there are four principal proposed structural models of pentameric wt-PLN (1) and it is not clear if the wt-PLN pentamer can form membrane pores. Methods: We are studying PLN using a new experimental model of biological membrane (nanoBLMs) (2). The properties of the nanoBLMs have been characterized by conductivity and electrochemical measurements. Results: The conductivity recorded on nanoBLMs containing wt-PLN in the presence of NaClO4 is ~90% of the value in the presence of NaCl. This result is in agreement with the values of equivalent conductance at infinite dilution for Cl- and ClO4 -. Moreover it is evident that the conductance in the presence of ChoCl is half of that of NaCl at the same concentration. These results suggest that contrary to Na+, Cland ClO4 -, Cho+ cannot pass through the membrane embedding wt-PLN. Conclusion: From our experimental data it seems that wt-PLN can form pores which are not selective for small ions. These data are in agreement with the hypothesis of Oxenoid and Chou (3), according to which physiologically relevant ions such as Na+, K+ and Cl- are small enough to pass through the narrowest part of the wt-PLN pentamer (diameter ~ 0.36 nm). This pore dimension is supported by our experimental data indicating that a big organic cation like Cho+ (radius 0.33 nm) cannot pass through the membrane containing wt-PLN. References: 1. Favero, et al., J Am Chem Soc. 2005; 127(22):8103. 2. Traaseth, et al., Proc Natl Acad Sci USA. 2007; 104(37):14676. 3. Proc Natl Acad Sci USA. 2005; 102(31):10870.