Protein interactions with lipid bilayers: the channels of kidney plasma membrane proteolipids

Proteolipids extracted from bovine kidney plasma membrane induce irreversible changes in the electrical properties of lipid bilayers formed from diphytanoyl phosphatidylcholine. The interaction with the proteolipid produces channels which are cation selective. At low protein concentrations (i.e., less than 0.6 microgram/ml), the single-channel conductance is approximately 10 pS in 100 mM KCl and 3 pS in 100 mM NaCl. In the presence of protein concentrations above 1 microgram/ml, another population of channels appears. These channels have a conductance of about 100 pS in 100 mM KCl and 30 pS in 100 mM NaCl. Further, these channels are voltage dependent in KCl, closing when the voltage is clamped at values greater than or equal to 30 mV. The steady-state membrane conductance, measured at low voltages, was found to increase proportional to a high power (2-3) of the proteolipid concentration present in one of the aqueous phases. In 100 mM NaCl, the conductance increases at protein concentrations above 5 microgram/ml, whereas in 100 mM KCl it increases at protein concentrations above 0.6 microgram/ml. These measurements indicate that the higher steady-state conductance observed in KCl at a given proteolipid concentration in a multi-channel membrane presumably results because more channel incorporate in the presence of KCl than in the presence of NaCl. The two major fractions which comprise the proteolipid complex were also tested on bilayers. It was found that both fractions are required to produce the effects described.