Question

What properties of ions and their respective transmembrane channels make K+ channels selective for a specific ion? (Hint: focus on pore and the selectivity filter).

Answer 1

The carbonyl oxygens are strongly electro-negative and cation-attractive. The filter can accommodate potassium ions at 4 sites usually labelled S1 to S4 starting at the extracellular side. In addition, one ion can bind in the cavity at a site called SC or one or more ions at the extracellular side at more or less well-defined sites called S0 or Sext. Several different occupancies of these sites are possible.

The selectivity filter of K+ channels is responsible for their exquisite ion selectivity. This region is also responsible for C-type inactivation, a regulatory process in many voltage-dependent K+ channels. Although the functional properties of inactivated channels have been known for decades, the first potential glimpse of their structure emerged from crystal structures of a constricted selectivity filter in an open channel. However, recent studies challenged the suggestion that the constricted selectivity filter is the inactivated structure, leaving open the question of what the inactivated structure looks like.