Figure 2.

Apt1p belongs to the P4A-ATPase clade. Apt1p harbors functional motifs needed for lipid flipping. (A) Membrane topology of P4-ATPases and their β-subunit. P-type ATPases consist of an actuator (A), a phosphorylation domain (P), a nucleotide-binding domain (N), and 10 transmembrane (TM) spanning helices. The Cdc50 subunit consists of two membrane-spanning domains with a large extracellular loop containing four possible N-linked glycosylation sites and two disulfide bridges. (B) Alignment of TM1, 2, 4, and 6 of Apt1p and its nearest orthologs in S. cerevisiae. The sequences were aligned with Mega X using MUSCLE. Numbers above the alignment represent residues in Apt1p. Gray bars below it represent the transmembrane domains. Residues identified by structural information to be positioned along the lipid translocation pathway are highlighted in blue. Residues shown to be involved in a direct interaction with the lipid are shaded in red for lipid-binding site 1 (loading) and in gold for lipid-binding site 2 (unloading). Residues shaded in purple have been suggested to be required for membrane deformation facilitating lipid loading. For accession numbers, see Materials and Methods. For full sequence comparison, see Supplementary Figure S2.