A recent study on the human zinc-activated channel (hZAC) has unveiled important insights related to its structural configuration and activation mechanism, significantly enhancing our comprehension of this unique class of ion channels. Predominantly activated by zinc ions and protons, hZAC plays a pivotal role as part of the mammalian cysteine-loop receptor (CLR) family, which contributes to rapid neurotransmission across synapses and is implicated in various neurological disorders.
Using advanced cryo-electron microscopy techniques, researchers were able to detail the high-resolution structures of hZAC under three different conditions: without ligands, with zinc ions, and under acidic conditions (pH 4.0). The results reveal hZAC forms symmetrical homo-pentamers with distinct features, including a central ion-conduction pore, which facilitates the passage of cations.
The structure consists of two domains: the extracellular domain (ECD) and the transmembrane domain (TMD). The TMD comprises four membrane-spanning helices, resembling more closely with the structures of certain other cation-selective CLRs such as glycine receptors than with typical cation-permeable ones. A noteworthy finding is the presence of the C-terminal tail (C-tail) of hZAC, which forms unique disulfide bonds and engages with adjacent protein loops, contributing to intersubunit stabilization.
Importantly, when zinc ions bind to hZAC, they induce significant conformational shifts, most noticeably altering the cys-loop region. This suggests the cys-loop acts as an unprecedented binding site, fundamentally differing between hZAC and other CLRs where traditional orthosteric sites exist. Upon zinc binding, the hZAC channel transitions from a closed state to one ready for ion conduction.
Through experiments showing how deletion of the C-tail alters channel activity, the researchers postulate the C-tail serves as both stabilizing and inhibitory, preventing erroneous activation by maintaining the ECD's position. Functional assays indicated constitutive activity for C-tail deleted mutants and confirmed the necessity of the C-tail for proper channel regulation.
The binding of zinc was shown to occur at two specific sites—Zn1 and Zn2—each contributing differently to structural shifts. The insights gained from this study not only highlight the distinct activation mechanisms of hZAC but also suggest potential pathophysiological roles it may play, particularly within neurological contexts and the treatment of disorders where receptor dysfunction is implicated.
This research offers valuable information for developing drugs targeting zinc-activated channels, potentially opening new therapeutic avenues for controlling synaptic activities disrupted by diseases such as schizophrenia or Alzheimer's disease.