The Intrinsically Disordered Nuclear Localization Signal and Phosphorylation Segments Distinguish the Membrane

Membranephosphatidylcholinehomeostasis ismaintained in part by a sensing device in the key regulatory enzyme, CTP: phosphocholine cytidylyltransferase (CCT). CCT responds to decreases in membrane phosphatidylcholine content by reversible membrane binding and activation. Two prominent isoforms, CCT and 2, have nearly identical catalytic domains and very similar membrane binding amphipathic helical (M) domains but have divergent and structurally disordered N-terminal (N) and C-terminal phosphorylation (P) regions. We found that the binding affinity of purified CCT 2 for anionic membranes was weaker than CCT by more than an order of magnitude. Using chimeric CCTs, insertion/deletion mutants, and truncatedCCTs, we show that the stronger affinity of CCT can be attributed in large part to the electrostatic membrane binding function of the polybasic nuclear localization signal (NLS)motif, present in the unstructuredN-terminal segment of CCT but lacking in CCT 2. The membrane partitioning of CCT 2 in cells enriched with the lipid activator, oleic acid, was also weaker than that of CCT and was elevated by incorporation of the NLSmotif. Thus, the polybasic NLS can function as a secondary membrane binding motif not only in vitro but in the context of cell membranes. A comparison of phosphorylated, dephosphorylated, and region P-truncated forms showed that the in vitromembrane affinity of CCT 2 is more sensitive than CCT to phosphorylation status, which antagonizes membrane binding of both isoforms. These data provide a model wherein the primary membrane binding motif, an amphipathic helical domain, works in collaboration with other intrinsically disordered segments thatmodulatemembrane binding strength. The NLS reinforces,whereas thephosphorylated tail antagonizes the attraction of domain M for anionic membranes.