Aim: 

The targeting and stoichiometry of ion channels is of critical importance to regulate cellular functions in excitable and non-excitable cells. A number of intrinsic sequence motifs have been identified in inwardly rectifying potassium channels type 2 (Kir2.x) that determine the functional stoichiometry of Kir2.x heteromers and their targeting into basolateral membrane domains. However, the identity of specific adaptor proteins, which may bind to these motifs, has remained elusive.

Methods: 

A yeast two-hybrid screen of a mouse brain derived cDNA library was performed with a fragment of the Kir2.1 c-terminus, which contains a previously identified trafficking determinant, as bait protein. Interactions were validated by co-immunoprecipitation. The cellular distribution of Kir2.x channels was assessed by confocal and light microscopy.

Results: 

The yeast two-hybrid screen identified multiple putative Kir2.1 interacting proteins, including the adaptor protein complex 4 (AP-4). Co-immunoprecipitation confirmed the interaction of Kir2.1 with AP-4. Kir2.1 and AP-4 localise in the trans-Golgi network. The functional impact of this interaction on Kir2.1 and Kir2.x ion channels is under current investigation employing a RNAi approach.

Conclusion: 

Intrinsic protein sequence motifs control the expression of Kir2.x ion channels on the cell surface of neurons. AP-4 is ubiquitously expressed, but its function as a signal-recognition adaptor is less well established. Here we report the identification of a novel binding partner of Kir2.1, that may provide evidence for the mechanism underlying protein motif dependent trafficking of Kir2.x ion channels.