The thylakoid compartments of plant chloroplasts are a vital destination for copper. Copper is needed to form holo-plastocyanin, which must shuttle electrons between photosystems to convert light into biologically useful chemical energy. Copper can bind tightly to proteins, so it has been hypothesized that copper partitions onto ligand-exchange pathways to reach intracellular locations without inflicting damage en route. The copper metallochaperone Atx1 of chloroplast-related cyanobacteria ( PacS ͉ protein-protein interaction ͉ ScAtx1 ͉ metallochaperone P-type ATPase
Potassium channel-interacting proteins (KChIPs) are EF-hand calcium-binding proteins of the recoverin/neuronal calcium sensor 1 family that co-assemble with the pore-forming Kv4 ␣-subunits and thus control surface trafficking of the voltage-gated potassium channels mediating the neuronal I A and cardiac I to currents. Different from the other KChIPs, KChIP4a largely reduces surface expression of the Kv4 channel complexes. Using solution NMR we show that the unique N terminus of KChIP4a forms a 6-turn ␣-helix that is connected to the highly conserved core of the KChIP protein via a solvent-exposed linker. As identified by chemical shift changes, N-terminal ␣-helix and core domain of KChIP4a interact with each other through the same hydrophobic surface pocket that is involved in intermolecular interaction between the N-terminal helix of Kv4␣ and KChIP in Kv4-KChIP complexes. Electrophysiological recordings and biochemical interaction assays of complexes formed by wild-type and mutant Kv4␣ and KChIP4a proteins suggest that competition of these two helical domains for the surface groove is responsible for the reduced trafficking of Kv4-KChIP4a complexes to the plasma membrane. Surface expression of Kv4 complexes may thus be controlled by an autoinhibitory domain in the KChIP subunit.Potassium channel-interacting proteins (KChIPs) 2 (1) are cytoplasmic EF-hand proteins of the recoverin/neuronal calcium sensor 1 family that interact with the cytoplasmic N and C termini of the pore-forming ␣-subunits of Kv4 voltage-gated potassium (Kv) channels (Kv4␣) (2-5). Together with dipeptidyl-aminopeptidase-like proteins (DPP6 and DPP10) (6, 7), type II transmembrane proteins, KChIPs are integral components of these heteromultimeric Kv channels in brain (KChIPs1-4) and heart (KChIP2) where they act as important regulators of excitability by mediating the somatodendritic A-type current I A in neurons (8) and the transient outward current I to in cardiac myocytes (9).KChIPs are Ca 2ϩ -binding proteins with a conserved core region of ϳ180 amino acids and highly variable N termini (ϳ35-100 residues). The core region contains 4 EF-hand motifs comprising 8 helices that are preceded and followed by an additional helix, respectively (5, 10). EF-hands 3 and 4 are canonical Ca 2ϩ binding sites in all KChIP isoforms, whereas EF-hand 1 is non-functional because of the lack of key Ca 2ϩ coordinating residues. EF-hand 2 in KChIPs2-4 prefers Mg 2ϩ over Ca 2ϩ because of a Glu to Asp exchange in the last bidentate Ca 2ϩ ligand, in KChIP1 EF-hand 2 does not bind divalents as the second and third Ca 2ϩ -coordinating residues are substituted by Thr.Within Kv4 channel complexes, KChIP proteins exert two distinct functional effects observed in heterologous expression systems: first, inactivation gating is markedly altered, and second, surface expression of the Kv4 complexes is largely increased (1). For the latter effect, KChIPs are thought to promote the release of channel complexes from the endoplasmic reticulum (ER) and/or from retention in the Go...
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