Deletion of phenylalanine 508 (delta F508) in the cystic fibrosis transmembrane-conductance regulator (CFTR) prevents the otherwise functional protein from reaching the plasma membrane and is the leading cause of cystic fibrosis. Indirect evidence suggests that the mutant protein, delta F508 CFTR, is misfolded. We address this issue directly, using comparative limited proteolysis of CFTR at steady steady state and during biosynthesis in the native microsomal environment. Distinct protease susceptibilities suggest that cytosolic domain conformations of wild type and delta F508 CFTR differ, not only near F508, but globally. Moreover, delta F508 CFTR proteolytic cleavage patterns were indistinguishable from those of the early folding intermediate of wild type CFTR. The results suggest that the delta F508 mutation causes the accumulation of a form of the protein that resembles an intermediate in the biogenesis of the wild type CFTR, rather than induces the production of non-native variant.

Although the cystic fibrosis transmembrane conductance regulator (CFTR) is primarily implicated in the regulation of plasmamembrane chloride permeability, immunolocalization and functional studies indicate the presence of CFTR in the endosomal compartment. The mechanism of CFTR delivery from the cell surface to endosomes is not understood. To delineate the internalization pathway, both the rate and extent of CFTR accumulation in endosomes were monitored in stably transfected Chinese hamster ovary (CHO) cells. The role of clathrindependent endocytosis was assessed in cells exposed to hypertonic medium, potassium depletion or intracellular acid-load. These treatments inhibited clathrin-dependent endocytosis by 90 %, as verified by measurements of "#&I-transferrin uptake. Functional association of CFTR with newly formed endosomes was determined by an endosomal pH dissipation protocol [Lukacs, Chang, Kartner, Rotstein, Riordan and Grinstein (1992) J. Biol. Chem. 267, 14568-14572]. As a second approach, endocytosis of CFTR was determined after cell-surface biotinylation with the

Common conceptual models for unsaturated flow assume that the matric potential is attributed to the capillary force only. These models are successful at high and medium water contents but often give poor results at low water contents. The lower bound of existing water retention functions and conductivity models was extended from residual water content to the oven‐dry condition (i.e., zero water content) by defining a state‐dependent residual water content for a soil drier than a critical value. The advantages of the extended water retention functions include not refitting the retention parameters from the unextended model, its reduction to the unextended form when the soil is wetter than the critical value, and its compatibility with existing relative permeability models. In addition, a hydraulic conductivity model for film flow in a medium of smooth uniform spheres was modified by introducing a correction factor to describe the film flow‐induced hydraulic conductivity for natural porous media. The total unsaturated hydraulic conductivity is the sum of those due to capillary and film flow; it smoothly transits between capillary‐dominated flow and film‐dominated flow over the full range of water content. The film flow is insignificant when the soil is wetter than the critical water content, and, vice versa, the capillary flow is insignificant when the soil is drier than the critical water content. The extended retention and conductivity models were tested with measurements. Results show that, when the soil is at high and intermediate water content, there is no difference between the unextended and the extended models as defined by the theory. When the soil is at low water content, the unextended models overestimate the water content but underestimate the conductivity. The extended models match the retention and conductivity measurements well.

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