A Membrane-Assisted Mechanism for the Release of Ceramide from the CERT START Domain

Ceramide transfer
protein CERT is the mediator of nonvesicular
transfer of ceramide from the ER to Golgi. In CERT, START is the domain
responsible for the binding and transport of ceramide. A wealth of
structural data has revealed a helix-grip fold surrounding a large
hydrophobic cavity holding the ceramide. Yet, little is known about
the mechanisms by which START releases the ceramide through the polar
region and into the packed environment of cellular membranes. As such
events do not lend themselves easily to experimental investigations,
we used multiple unbiased microsecond-long molecular simulations.
We propose a membrane-assisted mechanism in which the membrane acts
as an allosteric effector initiating the release of ceramide and where
the passage of the ceramide acyl chains is facilitated by the intercalation
of a single phosphatidylcholine lipid in the cavity, practically greasing
the ceramide way out. We verify using free energy calculation and
experimental lipidomics data that CERT forms stable complexes with
phosphatidylcholine lipids, in addition to ceramide, thus providing
validation for the proposed mechanism.