Doorkeeper Ring (IMAGE) Johannes Gutenberg Universitaet Mainz Caption A doorkeeper ring: The extracytosolic/lumenal domain of the human TRPML2 ion channel, whose structure the present study elucidated for the first time, functions as a pH-dependent calcium controller. It regulates the function of the channel in different cellular compartments. Ill./©: Kerstin K. Viet Credit Ill./©: Kerstin K. Viet Usage Restrictions This image may only be used in connection with the press release "An ion channel with a doorkeeper: The pH of calcium ions controls ion channel opening", issued by Johannes Gutenberg University Mainz on June 25, 2019. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.