Observation of a membrane fusion intermediate structure.

We report the observation of a phase of phospholipid that contains a structure similar to the commonly postulated interbilayer state that is crucial to membrane fusion. The widely accepted model for membrane fusion suggests that there is an intermediate state in which the two contacting monolayers become continuous via an hourglass-shaped structure called a stalk. Many efforts have been made to estimate the free energy for such a state in order to understand the functionality of membrane fusion proteins and to define key parameters in energy estimates. The observation of the stalk structure supports the stalk hypothesis for membrane fusion and enables the measurement of these parameters experimentally.