Which factor can alter the structure and selectivity of the filter in cation channels?

The factor that significantly alters the structure and selectivity of the filter in cation channels is the changes to the primary sequence of the reentrant loop. The reentrant loop plays a critical role in determining the specificity and permeability of the channel. This part of the protein structure creates the filter that differentiates between various ions, impacting which ions can pass through and at what rates.

Modifications to the amino acid sequence within the reentrant loop can lead to changes in the size and charge of the filter, directly influencing the channel's ion conduction properties. For instance, a substitution of a charged amino acid with a neutral one, or vice versa, can either enhance or diminish the selectivity of the channel, thereby altering its physiological functionality.

While factors such as temperature, ion concentration, and chemical inhibitors can also influence the activity and function of ion channels, they do so typically by modifying existing properties or functionalities rather than by directly altering the structural components of the channel itself. The primary sequence of the reentrant loop directly affects its physical and chemical characteristics, which is why it is the most pertinent factor in this context.