One of the best-studied examples is in Kv channels where the cytoplasmic N-terminal tetramerization domain facilitates assembly of subunits within the same subfamily (Covarrubias et al., 1991, Kreusch Etoposide et al., 1998 and Li et al., 1992); replacing this T1 domain with an artificial tetramerization domain supports channel assembly but alters channel kinetics, whereas removal of T1 domain drastically reduces surface expression of functional channels (Deutsch, 2002, Minor et al., 2000 and Zerangue et al., 2000). Thus, whereas the T1 domain acquires its

tertiary structure shortly after emerging from the ribosomal exit tunnel (Kosolapov et al., 2004) and enables interactions among Kv subunits still attached to ribosomes (Lu et al., 2001a), there are other subunit interactions that mediate channel assembly. This notion is echoed by the finding of reduced surface expression of GluA2 lacking its amino Afatinib supplier terminal domain (ATD) for dimerization

(Kumar et al., 2011). Dimerization of dimers is another common theme for Kv channels (Tu and Deutsch, 1999) and glutamate receptors (Kumar et al., 2011). Studies of Kv channel biogenesis illustrate how a monomeric channel subunit first acquires secondary structure within the ribosome and then folds into a membrane protein in the ER. Most of the helical segments that span the membrane or reside in sequences connecting transmembrane segments adopt their compact structures within a permissive vestibule in the ribosomal tunnel near the exit port (Tu and Deutsch, 2010). This is thought to be followed by concerted insertion of the VSD, linker, and PD (Sato et al., 2002 and Tu et al., 2000). Remarkably, despite the fact that many parts of the PD make subunit-subunit interactions within a fully assembled channel, the PD portion of a single Kv subunit appears to be able to adopt its tertiary fold in the absence of other Kv subunits (Gajewski

et al., 2011). This finding suggests that each subunit takes on a fairly mature appearance prior to tetramerization and raises questions about what happens to the polar elements of the transmembrane portions, which face the ion-conducting portions of the selectivity filter and GBA3 central pore in the fully assembled channels, while they are waiting to interact with the other three members required to make a functioning pore. The exposure of polar residues within the transmembrane domain is likely to facilitate ER retention or retrieval of monomeric channel subunits and partially folded or assembled channel complexes via ER quality control machineries involving proteins like Rer1 (Sato et al., 2003 and Sato et al., 2004), as shown for muscle acetylcholine receptor subunits (Valkova et al., 2011). Inefficient folding as demonstrated for CFTR chloride channels and squid KVs (Liu et al., 2001 and Ward et al.