Mechanistic Insights into Autoinhibition of the Human Flippase ATP8B1.

P4-ATPases are lipid flippases that maintain membrane phospholipid asymmetry by transporting specific phospholipids from the exoplasmic to the cytosolic leaflet, an essential process for membrane integrity, trafficking and signaling. Several P4-ATPases are tightly regulated by autoinhibitory N- and C-terminal extensions, yet the molecular basis of this regulation remains incompletely understood. Here, we investigated the autoinhibition mechanism of the human flippase ATP8B1 using trans-inhibition assays with synthetic peptides derived from its C-terminal tail. Using purified C-terminally truncated ATP8B1-CDC50A, we systematically assessed the inhibitory properties of peptides corresponding to distinct segments of the C-terminus. We show that the distal disordered region of the C-terminal tail significantly contributes to autoinhibition, likely through transient interactions with the cytosolic domains. We further identify a critical interaction between R1228 in the C-terminal tail and E219 in the A-domain, whose disruption markedly reduces inhibitory potency. In addition, we demonstrate that a minimal peptide spanning residues 1216-1228, which bridges the A- and N-domains in the autoinhibited conformation, is sufficient to inhibit ATPase activity. Together, these results refine the molecular description of ATP8B1 autoinhibition, open the way for structure-based activation strategies and provide insight into conserved regulatory mechanisms among P4-ATPases.