Beactica (Sweden)

Analysis of ligand-induced structural changes in proteins is challenging due to the lack of experimental methods suited for detection and characterisation of both ligand binding and induced structural changes. We have explored biosensors with different detection principles to study interactions between ligands and acetylcholine binding proteins (AChBPs), soluble homologues of Cys-loop ligand gated ion channels (LGICs) that undergo similar structural changes as LGICs upon ligand binding. X-ray crystallography was used to identify binding sites and establish if the detected conformational changes involved small changes in loop C or major structural changes in the pentamer associated with ion channel opening. Experiments were initially focused on ligands exhibiting complex surface plasmon resonance (SPR) biosensor sensorgrams or detected by second harmonic generation (SHG) biosensor analysis. Surface acoustic wave (SAW) and SHG biosensors confirmed that complexities in SPR data were indeed due to ligand-induced conformational changes. Grating coupled interferometry (GCI) biosensor sensorgrams were less complex, despite similar detection principles. switchSENSE biosensor analysis revealed that ligands resulted in either a compaction or expansion of the protein structure. X-ray crystallography of the protein-ligand complexes was only successful for 7 out of 12 ligands, despite nM-μM affinities. Crystals were not obtained for the two compounds shown by SHG analysis to induce large structural changes, while electron densities were not seen in the structures for some ligands. The work presented herein shows that several biosensor technologies have a unique capability to detect and discriminate binding and ligand induced conformational changes in proteins, also when interactions are rapid, weak and structural changes are small. However, they are complementary and provide different information.

The A kinase-anchoring protein AKAP79/150 is a postsynaptic scaffold molecule and a key regulator of signaling events. At the postsynapse it coordinates phosphorylation and dephosphorylation of receptors via anchoring kinases and phosphatases near their substrates. Interactions between AKAP79 and two Ca(2+) -binding proteins caldendrin and calmodulin have been investigated here. Calmodulin is a known interaction partner of AKAP79/150 that has been shown to regulate activity of the kinase PKC in a Ca(2+) -dependent manner. Pull-down experiments and surface plasmon resonance biosensor analyses have been used here to demonstrate that AKAP79 can also interact with caldendrin, a neuronal calcium-binding protein implicated in regulation of Ca(2+) -influx and release. We demonstrate that calmodulin and caldendrin compete for a partially overlapping binding site on AKAP79 and that their binding is differentially dependent on calcium. Therefore, this competition is regulated by calcium levels. Moreover, both proteins have different binding characteristics suggesting that the two proteins might play complementary roles. The postsynaptic enrichment, the complex binding mechanism, and the competition with calmodulin, makes caldendrin an interesting novel player in the signaling toolkit of the AKAP interactome.