Drosophila photoreceptor structure. (A) Longitudinal section through a single photoreceptor showing the relationship of the cell body and rhabdomeres composed of microvilli (m). Organelles located in the cell body are shown. The submicrovillar cisternae (smc) are shown at the junction of the cell body and the rhabdomere. The rhabdomeral plasma membrane is shown in green. The red dotted line shows the level of the cross section in B. n, nucleus; g, Golgi. (B) Cross section through a photoreceptor at the level of the nucleus showing the organelles. The arrow marks the direction of polarized transport to the apical domain. A representative TEM from a wild-type ommatidium is shown with the microvilli and nucleus marked. Bar, 2 µm. (C) Metabolic fates of PA. In addition to de novo biosynthesis from fatty acids and glycerol, PA can be generated by the sequential activity of PLC and DGK or by hydrolysis of PC by PLD. PA can undergo four metabolic fates: (1) allosteric activation of PA-binding proteins, (2) dephosphorylation to DAG by LPP, (3) generation of lyso-PA by phospholipase A₂ (PLA₂), and (4) condensation with CTP by CDP-DAG synthase (CDS) to generate CDP-DAG, a precursor for the biosynthesis of PI, PG, and cardiolipin. PI4P, PI 4-phosphate; SKTL, PI 4-phosphate 5 kinase.