PDZ domains: More than just a glue.

P domains are part of a molecular scaffold that holds multiprotein signaling complexes together. It is generally believed that the role of PDZ domains is to position target ion channels, receptors, or other signaling molecules in correct spatial arrangement in relation to each other and to specialized regions of the cell (1–4). Two recent reports, one published in Cell (5) and another in this issue of PNAS (6) challenge this paradigm. These reports are focused on biochemical and functional analysis of cystic fibrosis transmembrane conductance regulator (CFTR) channel interactions with multi-PDZ domain adaptor proteins CAP70 (CFTR-associated protein 70 kDa) (5) and NHERF (Na1yH1 exchanger regulatory factor) (6). CFTR contains a classical type I PDZ-domain binding carboxyl-terminal consensus 2T-KyR-L, conserved from Xenopus to humans (5). Biochemical association of CFTR with the NHERF PDZ1 domain has previously been reported by several groups (7–9), and the importance of this interaction for apical sorting of CFTR in polarized epithelia has been demonstrated (10, 11). The previous findings fit with the general paradigm regarding the role of the PDZ domain containing proteins in membrane protein targeting in polarized cells. What makes the reports by Raghuram et al. (6) and Wang et al. (5) particularly interesting is the notion that the association with PDZ domains does more than simply position the CFTR in the appropriate location. Both groups concluded that PDZ domains in NHERF and CAP70 also play a modulatory function by directly affecting CFTR channel gating. Raghuram et al. found that an addition of the NHERF PDZ1–2 tandem protein to the cytosolic side of inside-out patches taken from Calu-3 (lung submucosal gland) cells resulted in an increase of endogenous CFTR channel activity. Remarkably, only the tandem construct of 1–2 NHERF PDZ domains activated CFTR, but not when first or second PDZ domains added independently, mixed together or when one of the PDZ domains in the tandem construct was mutated. Moreover, first and second PDZ domains of NHERF inhibit potentiation of CFTR activity by PDZ1–2 NHERF construct, and the tandem construct itself exerts a biphasic effect on CFTR activity. Wang et al. (5) performed similar experiments with CFTR channels transiently expressed in HEK-293 cells. Using inside-out patches or marcopatches, Wang et al. found that the activity of a CFTR channel potentiated in a biphasic manner by the tandem construct of 3–4 PDZ domains of CA P70, whereas CAP70 PDZ 1–2 and 2–3 tandem constructs had no effect. The PDZ 3 domain alone did not activate CFTR channels, but the PDZ 3–3 tandem construct was effective. Moreover, Wang et al. found that the monoclonal antibody directed against CFTR carboxyl termini had a potentiating effect on CFTR channel activity similar to that of the CAP70 PDZ 3–4 tandem construct. Three general models can explain the findings of Raghuram et al. (6) and Wang et al. (5). In the first model PDZ domain tandem links CFTR with an additional membrane or membrane-associated protein (Fig. 1A). In the second model PDZ domain tandem cross-links two CFTR subunits within the dimeric channel (Fig. 1B). In the third model PDZ domain