Type 4 P-type ATPases (flippases) are implicated in the generation of phospholipid asymmetry in membranes by the inward translocation of phospholipids. In budding yeast, the DRS2/DNF family members Lem3p-Dnf1p/Dnf2p and Cdc50p-Drs2p are putative flippases that are localized, respectively, to the plasma membrane and endosomal/trans-Golgi network (TGN) compartments. Herein, we identified a protein kinase gene, FPK1, as a mutation that exhibited synthetic lethality with the cdc50⌬ mutation. The kinase domain of Fpk1p exhibits high homology to plant phototropins and the fungus Neurospora crassa NRC-2, both of which have membrane-associated functions. Simultaneous disruption of FPK1 and its homolog FPK2 phenocopied the lem3⌬/dnf1⌬ dnf2⌬ mutants, exhibiting the impaired NBD-labeled phospholipid uptake, defects in the early endosome-to-TGN pathway in the absence of CDC50, and hyperpolarized bud growth after exposure of phosphatidylethanolamine at the bud tip. The fpk1⌬ fpk2⌬ mutation did not affect the subcellular localization of Lem3p-Dnf1p or Lem3p-Dnf2p. Further, the purified glutathione S-transferase (GST)-fused kinase domain of Fpk1p phosphorylated immunoprecipitated Dnf1p and Dnf2p to a greater extent than Drs2p. We propose that Fpk1p/Fpk2p are upstream activating protein kinases for Lem3p-Dnf1p/Dnf2p.

Accumulating evidence, including experiments using cytochrome P450 1a2 (Cyp1a2) gene knock-out mice (Cyp1a2(-/-)), indicates that the development of chemically induced porphyria requires the expression of CYP1A2. It has also been demonstrated that iron enhances and expedites the development of experimental uroporphyria, but that iron alone without CYP1A2 expression, as in Cyp1a2(-/-) mice, does not cause uroporphyria. The role of iron in the development of porphyria has not been elucidated. We examined the in vivo effect of iron deficiency on hepatic URO accumulation in experimental porphyria. Mice were fed diets containing low (iron-deficient diet (IDD), 8.5 mg iron/kg) or normal (normal diet (ND), 213.7 mg iron/kg) levels of iron. They were treated with 3-methylcholanthrene (MC), an archetypal inducer of CYP1A, and 5-aminolevulinate (ALA), precursors of porphyrin and heme. We found that uroporphyrin (URO) levels and uroporphyrinogen oxidation (UROX) activity were markedly increased in ND mice treated with MC and ALA, while the levels were not raised in IDD mice with the same treatments. CYP1A2 levels and methoxyresorufin O-demethylase (MROD) activities, the CYP1A2-mediated reaction, were markedly induced in the livers of both ND and IDD mice treated with MC and ALA. UROX activity, supposedly a CYP1A2-dependent activity, was not enhanced in iron-deficient mice in spite of the fact of induction of CYP1A2. We showed that a sufficient level of iron is essential for the development of porphyria and UROX activity.