Sphingomyelinase action on lipid monolayers: mechanism and features of the enzymatic generation of ceramide-enriched domains

MARIA L. FANANI; LUISINA DE TULLIO; STEFFEN HARTER; BRUNO MAGGIO

Buzios, Rio de Janeiro

Congreso; VII Iberoamerican Congress of Biophysics; 2009

Latin American federation of Biophysical Societies (LAFEBS)

Sphingomyelinase (SMase)-induced ceramide (Cer)-enriched domains in a lipid monolayer are shown to result from an out-of-equilibrium situation. This is induced by a change of composition caused by the enzymatic production of Cer in a sphingomyelin (SM) monolayer that leads to a fast SM/Cer demixing into a liquid-condensed (LC) Cer-enriched and a liquid-expanded (LE) SM-enriched phases. Labelled-SMase is preferably localized in the LE, SM-enriched, phase and it is not concentrated at the domain boundaries. A novel mechanism is proposed for the action of SMase where the enzyme is homogeneously active over the LE surface, which can also explain the regulatory effect of the presence of pre-existent domains on the enzyme activity. The morphological evolution of Cer-enriched domains shows domain shape annealing from branched to rounded shapes after quenching of SMase activity by EDTA. The fast phase separation causes a transient compositional overshoot within the LC phase that implies an increased out-of-equilibrium enrichment of Cer into LC domains. As a consequence, higher intradomain repulsion leads to transient branched structures that relax to rounded shapes by decreasing the proportion of Cer in the domain while approaching to equilibrium values. The fast action of SMase can be taken as a compositional perturbation that brings about important consequences for the structural dynamics.