Phospholipid methyltransferase asymmetry in synaptosomal membranes

Area of Science:

• Neuroscience
• Biochemistry
• Cell Biology

Background:

• Phosphatidylcholine synthesis is crucial for cell membranes.
• Methylation of phosphatidylethanolamine is a key pathway.

Purpose of the Study:

• To elucidate the sequential methylation of phosphatidylethanolamine to phosphatidylcholine in rat brain synaptosomes.
• To determine the localization and orientation of the involved methyltransferases.
• To propose a mechanism for phospholipid transport across the synaptosomal membrane.

Main Methods:

• Comparison of methylation rates in intact versus lysed synaptosomes.
• Analysis of S-adenosyl-L-methionine accumulation.
• Trypsin digestion studies on intact and lysed synaptosomes.
• Enzymatic hydrolysis using Phospholipase C.

Main Results:

• The first methylation occurs on the cytoplasmic side, while the second enzyme faces the external surface.
• Synaptosomes accumulate S-adenosyl-L-methionine, indicating its role in the pathway.
• Methylated phosphatidylcholine is localized to the external layer of the synaptosomal plasma membrane.

Conclusions:

• The sequential methylation pathway involves two distinct methyltransferases with specific membrane localizations.
• A model for enzyme-mediated phospholipid flip-flop from the cytoplasmic to the outer surface of the synaptosomal plasma membrane is proposed.