Properties of the TRPML3 Channel Pore and Its Stable Expansion by the Varitint-Waddler-causing Mutation

TRPML3 is a H⁺-regulated Ca²⁺ channel that shuttles between intracellular compartments and the plasma membrane. The A419P mutation causes the varitint-waddler phenotype as a result of gain-of-function (GOF). The mechanism by which A419P leads to GOF is not known. Here, we show that the TRPML3 pore is dynamic when conducting Ca²⁺ to change its conductance and permeability, which appears to be mediated by trapping Ca²⁺ within the pore. The pore properties can be restored by strong depolarization or by conducting Na⁺ through the pore. The A419P mutation results in expanded channel pore with altered permeability that limits modulation of the pore by Ca²⁺. This effect is specific for the A419P mutation and is not reproduced by other GOF mutations, including A419G, H283A, and proline mutations in the fifth transmembrane domain. These findings describe a novel mode of a transient receptor potential channel behavior and suggest that pore expansion by the A419P mutation may contribute to the varitint-waddler phenotype.