Mutating the PDZ motif of SLC26A9 restores constitutive current when coexpressed with F508del CFTR. Whole cell patch-clamp measurement of membrane current (Im) observed in HEK293 cells transiently transfected with myc-SLC26A9 ± T789A (PDZ motif mutation) and CFTR. A: constitutive current was measured 1 min after break-in and before any treatments. The indicated myc-SLC26A9 construct was coexpressed with either wt (black bars) or F508del (gray bars) CFTR. B: after measurement of constitutive Im, cells were treated with 10 µM forskolin and the change in Im recorded when the response plateaued (~2 min after forskolin addition). F508del coexpressors did not respond to forskolin (not shown). Holding potential = −40 mV; each bar, n ≥ 5, *P < 0.05 compared with coexpression with wt CFTR. C–D: IF indicates that mutating the PDZ motif of myc-SLC26A9 increases its plasma membrane expression. HEK293cells expressing myc-SLC26A9 ± T789A (PDZ motif mutation) with F508del CFTR were fixed, permeabilized, and labeled with anti-myc antibody to assess distribution of myc-SLC26A9. C: coexpression of myc-SLC26A9 + F508del CFTR. D: coexpression of myc-SLC26A9 T789A + F508del CFTR.