BKAP, ein neu entdecktes Protein, das mit dem C-Terminus des calcium-und spannungsabhängigen Kaliumkanals der Ratte interagiert (English shop)

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Understanding the physiological functions of ion channels fundamentally requires knowledge of the identity and function of the proteins that physically interact with the ion channel. This work characterises a newly discovered protein that interacts with the intracellular C-terminus of the rat Ca2+- and voltage-dependent potassium channel (BK channel) in the yeast two-hybrid system, and was therefore named BK-associated protein (BKAP). Interaction studies with recombinant proteins confirm that BKAP binds directly and specifically to C-terminal fragments of the BK channel. The position of the interaction domain was narrowed down to the 76 C-terminal amino acids of the channel protein. BKAP mRNA is expressed in all tissues examined, particularly strongly in testis and brain. The expression patterns of BKAP and BK in the adult rat brain show a remarkable similarity. BKAP transcripts occur in at least 7 splice variants in the rat brain. The variant BKAP-A consists of 361 amino acids, and the protein transiently expressed in mammalian cells (COS-7) has an apparent molecular weight of 44 kDa, consistent with BKAP from rat brain. BKAP-A overexpressed in COS-7 cells is located predominantly in the nucleus, but also at the plasma membrane, where it is colocalised with the BK channel. In hippocampal neurons, however, BKAP was detected only in the nucleus and not at the plasma membrane, and in the membrane fraction from rat brain, in which the BK channel is strongly enriched, BKAP was not detectable. These observations are consistent with the similarity of the amino acid sequence of BKAP to a single-stranded DNA binding protein (ssDP), but are unexpected with regard to the protein’s interaction with the BK channel. The C-terminus of the BK channel contains a classical nuclear localisation signal that is functional in mammalian cells (COS-7). In the presence of coexpressed BKAP-A, C-terminal BK fragments accumulate to a greater extent in the nucleus of COS-7 cells. The hypothesis that the C-terminus of the BK channel might be proteolytically cleaved in vivo and, together with BKAP, might exert a regulatory function in the nucleus is discussed.