Using surface biotinylation assays in HeLa cells, we demonstrate that CNIH-2 can escape from the cycle within the early secretory pathway between ER and Golgi compartments. If assembled with GluA subunits, CNIH-2 is transported to the cell surface. In line with these observations from heterologous expression systems, both endogenous and over-expressed CNIH-2 is readily detected in the plasma membrane of dissociated hippocampal neurons that express native AMPARs (Fig. 6). Moreover, electrophysiological recordings indicate that over-expressed CNIH-2 is able to modify AMPAR gating in these neurons. Thus, we propose that the interaction with GluA subunits let CNIH-2 evolve from an ER cargo exporter into a bona fide auxiliary subunit of AMPARs. Unlike other cargo, GluA subunits are not disengaged from CNIH-2 during early anterograde traffic, but stay together for surface transport. This might be a novel property in the evolutionary diversification of the mammalian cornichon family of proteins. In this respect, it is noteworthy that CNIH-1, which has the highest sequence homology to the cornichon orthologues in yeast and drosophila, was not found as a constituent of native AMPARs [22].
