The diagram represents the secreted isoform of the A P O L 1 protein after the removal of its N-terminal signal peptide. It includes various structural features such as helices, hydrophobic clusters (H C 1, H C 2), leucine zippers (L Z 1, L Z 2), potential cardiolipin-binding sites (C L), a membrane-addressing domain (M A D), and cholesterol recognition amino acid consensuses (C R A C-1, C R A C-2). The diagram also shows a potential transmembrane double-stranded hairpin helix (T M). The helices are numbered, with helix 5 adopting two positions, either bound or open, affecting the accessibility of helix 4 (L Z 1). When accessible, helix 4 interacts strongly with L Z 2, promoting interaction between the H C 1-L Z 1 and H C 2-L Z 2 regions. This interaction is influenced by acidic conditions or mutations in L Z 1 or L Z 2. Mutations termed G 1 and G 2 target L Z 2 and are significant in the context of A P O L 1's interaction with pathogens and nephropathy.
Structural features of APOL1. This scheme represents the secreted APOL1 isoform after removal of its specific N-terminal signal peptide. It also summarizes the structure of the other human APOLs or mAPOLs, none of which appears to be secreted. The colored cylinders represent different α-helices, some of which are numbered (4). HC1, HC2 = hydrophobic clusters; LZ1, LZ2 = leucine zippers; CL = potential cardiolipin-binding sites; MAD = membrane-addressing domain; CRAC-1, CRAC-2 = cholesterol recognition amino acid consensuses (represented by red stars); TM = potential transmembrane double-stranded hairpin helix. Whereas transmembrane APOL1 insertion strictly requires acidic conditions, such insertion can occur at neutral pH for APOL3 (5). In silico structural folding of the MAD and C-terminal helices was determined by I-TASSER modeling (https://zhanggroup.org/I-TASSER/). The boxes illustrate the folding of the APOL1 or APOL2 N-terminal domain, as determined by X-ray scattering and nuclear magnetic resonance (4). In the isolated N-terminal domain, helix 5 can adopt two positions, preventing (bound) or not (open) helix 4 (or LZ1) accessibility. When accessible, like probably occurs in the full protein, helix 4 (or LZ1) can strongly interact with LZ2. Such LZ1-LZ2 pairing can promote cis-interaction between the HC1-LZ1 and HC2-LZ2 regions (6). This interaction is affected either by acidic conditions, as in trypanosome endosomes, or by LZ1 or LZ2 mutations (3). C-terminal mutations termed G1 and G2, which target LZ2, allow APOL1 to kill the APOL1-resistant human pathogen T. brucei rhodesiense, to the detriment of APOL1-mediated nephropathy (7).