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Strombidium rassoulzadegani is a planktonic ciliate that retains chloroplasts from its food and uses them to obtain a nutritional supplement from photosynthesis. Unlike most members of the Oligotrichida, it is not difficult to grow in culture and thus it can serve as an experimental model for this kind of mixotrophy. We report here on its distribution, seasonal pattern of occurrence in the western North Atlantic, and on experiments to elucidate patterns of encystment and excystment, preferred food algae, and heterotrophic growth. Among ten different microalgae, including members of the Dinophyceae, Chlorophyceae, Haptophyceae, Cryptophyceae, and Bacillariophyceae, only four could support growth for more than one week, and only the chlorophyte Tetraselmis chui (PLY 429) could consistently support sustained growth in the dark. Of the four algae that supported growth, three also resulted in longer survival when the ciliate was subsequently starved in the light, compared to the dark, suggesting that all of them provided some photosynthetic benefit to the ciliate. The dinoflagellate Prorocentrum minimum (JA) supported similar survival in the light and dark and likely does not undergo chloroplast retention in the ciliate.

MinK-related peptides (MiRPs) are single-span membrane proteins that assemble with specific voltage-gated K+(Kv) channel α-subunits to establish gating kinetics, unitary conductance, expression level, and pharmacology of the mixed complex. MiRP3 (encoded by the KCNE4 gene) has been shown to alter the behavior of some Kv α-subunits in vitro but its natural partners and physiologic functions are unknown. Seeking in vivo partners for MiRP3, immunohistochemistry was used to localize its expression to a unique subcellular site, the apical membrane of renal intercalated cells, where one potassium channel type has been recorded, the calcium- and voltage-gated channel BK. Overlapping staining of these two proteins was found in rabbit intercalated cells, and MiRP3 and BK subunits expressed in tissue culture cells were found to form detergent-stable complexes. Electrophysiologic and biochemical evaluation showed MiRP3 to act on BK to reduce current density in two fashions: shifting the current-voltage relationship to more depolarized voltages in a calcium-dependent fashion (∼10 mV at normal intracellular calcium levels) and accelerating degradation of MiRP3-BK complexes. The findings suggest a role for MiRP3 modulation of BK-dependent urinary potassium excretion.

Summary Mutations in the inositol 5-phosphatase OCRL are responsible for Lowe syndrome, whose manifestations include mental retardation and renal Fanconi syndrome. OCRL has been implicated in membrane trafficking, but disease mechanisms remain unclear. We show that OCRL visits late-stage, endocytic clathrin-coated pits and binds the Rab5 effector APPL1 on peripheral early endosomes. The interaction with APPL1, which is mediated by the ASH-RhoGAP-like domains of OCRL and is abolished by disease mutations, provides a link to protein networks implicated in the reabsorptive function of the kidney and in the trafficking and signaling of growth factor receptors in the brain. Crystallographic studies reveal a role of the ASH-RhoGAP-like domains in positioning the phosphatase domain at the membrane interface and a clathrin box protruding from the RhoGAP-like domain. Our results support a role of OCRL in the early endocytic pathway, consistent with the predominant localization of its preferred substrates, PI(4,5)P 2 and PI(3,4,5)P 3 , at the cell surface.