FIGURE 1.

TβRIII is basolaterally localized. (A) We plated 2.5 × 10⁵ NMuMG or MDCK cells on Transwells and grew them for 5 d to allow for polarization. A FITC-dextran tracer was placed in the upper chamber, and the cells were incubated at room temperature for 30 min. Apical and basolateral media samples were excited at a wavelength of 490 nm, and the absorbance at 520 nm was measured. Percentage leakage = basolateral signal/apical signal × 100. **p < 0.01 (Student's t test). (B) Cells were plated as in A and transfected with WT TβRIII, NAAIRS mutant TβRIII, or P826A TβRIII. Two days posttransfection, the cells were fixed and stained with primary antibody against TβRIII and an Alexa 488 secondary (green). Nuclei (blue) were stained with DAPI. Images were collected at a magnification of 400× and show the localization of TβRIII to cell junctions in the flat sections (XY, top). The cross-sectional images (XZ, bottom) show the basolateral localization of WT TβRIII and the apical–basolateral localization of NAAIRS and P826A TβRIII. Bar, 25 μm. (B) The 42–amino acid sequence of the C-terminus of human TβRIII. Each 6–amino acid–long group altered via NAAIRS mutagenesis is shown in alternating blue and black. Residue P826, which is critical for TβRIII's basolateral localization, is starred and shown in red.