*2.3.* ∆*RIS, but Not* ∆*RES, Abolishes the Plasma Membrane Rescue of F508del-CFTR by Revertants*

In order to test whether the stabilized immature form of ∆RES-F508del-CFTR could be rescued to mature form, we next tested the effects of removing the RE<sup>S</sup> from F508del-CFTR with R1070W and G550E revertants. Indeed, both the R1070W and G550E revertants rescued ∆RES-F508del-CFTR to 33 ± 7% and 37 ± 5%, respectively (Figure 1D, lanes 6,10; Table 1).

However, removal of RI<sup>S</sup> from F508del-CFTR with R1070W and G550E revertants, virtually abolished the rescue of F508del-CFTR by both revertants (Figure 1D, lanes 4,8; Table 1). Moreover, processing of <sup>∆</sup>RIL-F508del-CFTR was further increased by either R1070W or G550E up to 96 <sup>±</sup> 2% and 92 ± 4%, respectively (Figure 1D, lanes 5,9; Table 1). Alone, R1070W and G550E rescued F508del-CFTR from 8 ± 1% to 34 ± 3% and 42 ± 4%, respectively (Figure 1D, lanes 3,7; Table 1), as we previously reported [5].

Interestingly, the impaired processing of <sup>∆</sup>RIS-wt-CFTR (44 <sup>±</sup> 2%) was significantly rescued by G550E (to 82 ± 3%), but curiously it was further reduced by R1070W (to 11%) i.e., close to levels of F508del-CFTR processing (Figure 1C, lanes 8,4; Table 1). Of note that R1070W alone also reduced wt-CFTR processing to 69 ± 7%, but this reduction could be compensated by the removal of RI<sup>L</sup> (92 ± 2%) (Figure 1C, lanes 3,5; Table 1), while G550E alone caused no effect on wt-CFTR processing (Figure 1C, lane 7).

In summary, ∆RIS, but not ∆RES, abolishes the plasma membrane rescue of F508del-CFTR by genetic revertants R1070W and G550E.
