Human trypanolytic factor APOL1 forms pH-gated cation-selective channels in planar lipid bilayers: relevance to trypanosome lysis

Proc Natl Acad Sci U S A. 2015 Mar 3;112(9):2894-9. doi: 10.1073/pnas.1421953112. Epub 2015 Feb 17.


Apolipoprotein L-1 (APOL1), the trypanolytic factor of human serum, can lyse several African trypanosome species including Trypanosoma brucei brucei, but not the human-infective pathogens T. brucei rhodesiense and T. brucei gambiense, which are resistant to lysis by human serum. Lysis follows the uptake of APOL1 into acidic endosomes and is apparently caused by colloid-osmotic swelling due to an increased ion permeability of the plasma membrane. Here we demonstrate that nanogram quantities of full-length recombinant APOL1 induce ideally cation-selective macroscopic conductances in planar lipid bilayers. The conductances were highly sensitive to pH: their induction required acidic pH (pH 5.3), but their magnitude could be increased 3,000-fold upon alkalinization of the milieu (pK(a) = 7.1). We show that this phenomenon can be attributed to the association of APOL1 with the bilayer at acidic pH, followed by the opening of APOL1-induced cation-selective channels upon pH neutralization. Furthermore, the conductance increase at neutral pH (but not membrane association at acidic pH) was prevented by the interaction of APOL1 with the serum resistance-associated protein, which is produced by T. brucei rhodesiense and prevents trypanosome lysis by APOL1. These data are consistent with a model of lysis that involves endocytic recycling of APOL1 and the formation of cation-selective channels, at neutral pH, in the parasite plasma membrane.

Keywords: APOL1; African trypanosome; SRA; apolipoprotein L-I; pH-gated channel.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Apolipoprotein L1
  • Apolipoproteins / chemistry
  • Apolipoproteins / genetics
  • Apolipoproteins / pharmacology*
  • Cell Membrane / metabolism*
  • Humans
  • Hydrogen-Ion Concentration
  • Lipid Bilayers / metabolism*
  • Lipoproteins, HDL / chemistry
  • Lipoproteins, HDL / genetics
  • Lipoproteins, HDL / pharmacology*
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / pharmacology
  • Trypanocidal Agents / chemistry
  • Trypanocidal Agents / pharmacology*
  • Trypanosoma brucei gambiense / metabolism*


  • APOL1 protein, human
  • Apolipoprotein L1
  • Apolipoproteins
  • Lipid Bilayers
  • Lipoproteins, HDL
  • Recombinant Proteins
  • Trypanocidal Agents