1.C.127. The Pore-forming Trypanolytic Apolipoprotein A1 Factor (APOL1) Family
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 increased ion permeability of the plasma membrane. Thomson and Finkelstein 2015 demonstrated that nanogram quantities of full-length recombinant APOL1 cause the formation of cation-selective macroscopic conductances in planar lipid bilayers. The conductances are 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 (pKa = 7.1). This conductance was 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 (Q8T309) and prevents trypanosome lysis by APOL1. Thus, lysis involves endocytic recycling of APOL1 and the formation of cation-selective channels at neutral pH in the parasite plasma membrane (Thomson and Finkelstein 2015).
APOL1 is the channel-forming component necessary for innate immunity. The common human APOL1 variant G0 provides protection against infection with certain Trypanosoma and Leishmania parasite species, but it cannot protect against the trypanosomes responsible for human African trypanosomiasis. Human APOL1 variants G1 and G2 protect against human-infective trypanosomes, but also confer a higher risk of developing chronic kidney disease. Trypanosome-killing activity is dependent on the ability of APOL1 to insert into membranes at acidic pH and form pH-gated cation channels. Coiled-coil binding of the leucine zipper domains of APOL1 is necessary for the open cation channel conformation (Schaub et al. 2021).
This family includes two distantly related sub-families (1.C.127.1, exclusively from animals, with no homologues in other eukaryotes), and subfamily 1.C.127.2 (exclusively from bacteria). Further, these proteins all appear to be distantly related to the pore-forming constituents of TC family 1.C.36, especiallly sub-family 1.C.36.3. Family 1.C.36 includes proteins that form pores in host animal cell plasma membranes in conjunction with Type III protein secretion systems of bacteria. The proposed relationships between these families are not yet fully established but are under investigation (M Saier, preliminary observations).
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Human Apolipooprotein A1, APOL1, a tripanolytic factor of 398 aas and 4 possible α-helical TMSs in a 1 + 2 + 1 TMS arrangement. Pore-formation has been demonstrated in planar bilayer membranes. APOL1 inserts into such bilayers at acidic pH to form pH-gated non-selective cation channels that open upon pH neutralization. This corresponds to the pH changes encountered during endocytic-recycling, suggesting that APOL1 forms a cytotoxic cation channel in the parasite plasma membrane. Pore-formation is blocked by the serum resistance-associated VSG protein, SRA. See family discussion for a published description of this protein (Thomson and Finkelstein 2015). APOL1 risk variants induce opening of the mitochondrial permeability transition pore (Carney 2019). Cation channel conductance and pH gating of the innate immunity factor APOL1 is governed by pore lining residues in the C-terminal domain (Schaub et al. 2020). Two residues in the C-terminal domain (CTD), tyrosine-351 and glutamate-355 influence pH gating properties, and a single residue, aspartate-348, determines both cation selectivity and pH gating. Thus, the predicted transmembrane region closest to the APOL1 C-terminus is the pore-lining segment of this channel-forming protein (Schaub et al. 2020).
APOL1 of Homo sapiens
Uncharacterized protein of 566 aas and 4 TMSs in a 1 + 2 + 1 TMS arrangement.
UP of Takifugu rubripes (Torafugu)
Apolipoprotein L domain-containing protein 1 isoform X1 of 319 aas and 2 TM
Apolipoprotein of Chrysemys picta bellii
Uncharacterized protein of 1010 aas and 4 TMSs in a 1 (N-terminal) + 2 + 1 TMS arrangement.
UP of Tigriopus californicus (Marine copepod)
Uncharacterized protein of 313 aas and 3 TMSs in a 2 + 1 TMS arrangement.
UP of Pygocentrus nattereri (red-bellied piranha)
Uncharacterized protein of 1075 aas and possibly 6 TMSs in a 1 (N-terminal) + 1 + 2 + 2 TMS arrangement.
UP of Crassostrea virginica (eastern oyster)
Serine/threonine-protein kinase domain (N-terminus) and Lipoprotein domain (C-terminus) protein, Nek5, of 543 aas and 4 TMSs in a 2 + 2 TMS arrangement.
Kinase of Danio rerio
Uncharacterized protein of 289 aas and 4 TMSs in a 2 + 2 TMS arrangement.
UP of Acropora millepora (cnidaria)
Uncharacterized protein of 536 aas and 2 TMSs. This bacterial protein shows sequence similarity to animal members of the family in the region showing the 2 TMSs.
UP of Helicobacter suis
Prolipoprotein of 553 aas and 0 TM
Lipoprotein of Helicobacter phage PtB89G
Uncharacterized protein of 700 aas and 1 - 3 TMSs, two of which may be adjacent to each other. This protein is annotated as a divalent metal ion transporter, but this is probably an error.
UP of Helicobacter canis