Decoding P4-ATPase substrate interactions

Abstract

Cellular membranes display a diversity of functions that are conferred by the unique composition and organization of their proteins and lipids. One important aspect of lipid organization is the asymmetric distribution of phospholipids across the plasma membrane. The unequal distribution of key phospholipids between the cytofacial and exofacial leaflets of the bilayer creates physical surface tension that can be used to bend the membrane; and like Ca2+, a chemical gradient that can be used to transduce biochemical signals. Phospholipid flippases in the type IV P-type ATPase (P4-ATPase) family are the principle transporters used to set and repair this phospholipid gradient, and the asymmetric organization of these membranes are encoded by the substrate specificity of these enzymes. Thus, understanding the mechanisms of P4-ATPase substrate specificity will help reveal their role in membrane organization and cell biology. Further, decoding the structural determinants of substrate specificity provides investigators the opportunity to mutationally tune this specificity to explore the role of particular phospholipid substrates in P4-ATPase cellular functions. The present work reviews the role of P4-ATPases in membrane biology, presents our current understanding of P4-ATPase substrate specificity, and discusses how these fundamental aspects of P4-ATPase enzymology may be used to enhance our knowledge of cellular membrane biology.

Structure and function analysis of the C. elegans aminophospholipid translocase TAT–1

“…On the other hand, the P335L mutation alters the first residue of a highly conserved PISL motif in the fourth transmembrane domain (M4). The M4 transmembrane domain, based on a modelled structure of the human P4 ATPase ATP8A2 and related mutational studies, is proposed to play a critical role in determining the specificity of the lipid substrate, with the isoleucine residue in the PISL motif contacting the head group of the phospholipid substrate and mediating its release to the cytoplasmic side and the M4 domain serving as a pump rod to move the phospholipid substrate (Andersen et al, 2016;Vestergaard et al, 2014;Roland and Graham, 2016). Interestingly, a missense mutation (I376M) in the PISL motif in ATP8A2 was found to cause a neurodegenerative disorder in humans (Onat et al, 2013) and found to impair the ATPase and the PS flipping activity of ATP8A2 without affecting its expression or stability in cultured cells (Vestergaard et al, 2014), further highlighting the importance of this motif in P4 ATPases.…”

“…P4-type ATPases are highly conserved transmembrane proteins that are suggested to promote ATP-dependent inward movement of aminophospholipids such as PS (Auland et al, 1994;Tang et al, 1996;Paulusma and Oude Elferink, 2005;Andersen et al, 2016;Roland and Graham, 2016), resulting in the restriction of PS in the cytosolic leaflet and PS asymmetry in the plasma membrane. The mechanisms by which these large lipid substrates are transported specifically across the membrane have remained an enigma (Vestergaard et al, 2014;Andersen et al, 2016).…”

Transport Mechanism of Class-3 P4 ATPase Lipid Flippases

“…Combining the finding of two substrate binding sites and the transport cycle, we propose that the lipid head group transports through the groove between TMH2 and TMH4, while lipid tails move accordingly on the hydrophobic surface of the TM2 and TM4. This is essentially the same as the previously described "credit card model" (Andersen et al, 2016;Roland and Graham, 2016).…”

Substrates of P4‐ATPases: beyond aminophospholipids (phosphatidylserine and phosphatidylethanolamine)

“…TM1-6 have been suggested to form the principal unit responsible for transport of phospholipids across bilayers, whereas TM7-10 seem to play an ancillary role (30-32, 59-61). Three hypotheses have been proposed to describe the mechanism underlying substrate transport by P4-ATPases: 1) a 2-gate model, 2) a hydrophobic gate model, and 3) a central cavity model (32,(59)(60)(61)(62). The 2-gate model suggests that entry and exit gates at the interface of the exoplasmic and cytoplasmic membranes, respectively, sequentially bind to and transfer specific substrates by using TM1-4 (60).…”

“…Three hypotheses have been proposed to describe the mechanism underlying substrate transport by P4-ATPases: 1) a 2-gate model, 2) a hydrophobic gate model, and 3) a central cavity model (32,(59)(60)(61)(62). The 2-gate model suggests that entry and exit gates at the interface of the exoplasmic and cytoplasmic membranes, respectively, sequentially bind to and transfer specific substrates by using TM1-4 (60). The hydrophobic gate model proposes that a cluster of hydrophobic residues centered around the middle of TM4 propels water in a groove between TM1, TM2, TM4, and TM6 together with the phospholipid headgroup to the cytoplasmic side (59).…”