The role of charged residues in independent glycine receptor folding domains for intermolecular interactions and ion channel function

Abstract

Glycine receptor (GlyR) truncations in the intracellular TM3-4 loop, documented in patients suffering from hyperekplexia and in the mouse mutant oscillator, lead to non-functionality of GlyRs. The missing part that contains the TM3-4 loop, TM4 and C-terminal sequences is essential for pentameric receptor arrangements. In vitro co-expressions of GlyRα1-truncated N-domains and C-domains were able to restore ion channel function. An ionic interaction between both domains was hypothesized as the underlying mechanism. Here, we analysed the proposed ionic interaction between GlyR N- and C-domains using C-terminal constructs with either positively or negatively charged N-termini. Charged residues at the N-terminus of the C-domain did interfere with receptor surface expression and ion channel function. In particular, presence of negatively charged residues at the N-terminus led to significantly decreased ion channel function. Presence of positive charges resulted in reduced maximal currents possibly as a result of repulsion of both domains. If the C-domain was tagged by a myc-epitope, low maximal current amplitudes were detected. Intrinsic charges of the myc-epitope and charged N-terminal ends of the C-domain most probably induce intramolecular interactions. These interactions might hinder the close proximity of C-domains and N-domains, which is a prerequisite for functional ion channel configurations. The remaining basic subdomains close to TM3 and 4 were sufficient for domain complementation and functional ion channel formation. Thus, these basic subdomains forming α-helical elements or an intracellular portal represent attractants for incoming negatively charged chloride ions and interact with the phospholipids thereby stabilizing the GlyR in a conformation that allows ion channel opening.

The intracellular domain of homomeric glycine receptors modulates agonist efficacy

“…25 and 1.7% in WT, p < 0.005, respectively; Table 1). Maximum glycine responses for the human α1 GlyR ΔICD were significantly smaller (p<0.005) than for WT, suggesting that constructs with a short ICD either have a low expression (15) or have a toxic effect for cells that express them at high levels. Experiments with single-channel recordings proved to be challenging with the human α1 GlyR ΔICD constructs, because in the majority of patches no channel activity was detected.…”