Determining the Lipid-Binding Specificity of SMP Domains: An ERMES Subunit as a Case Study.

Membrane contact sites between the endoplasmic reticulum (ER) and mitochondria function as a central hub for the exchange of phospholipids and calcium. The yeast Endoplasmic Reticulum-Mitochondrion Encounter Structure (ERMES) complex is composed of five subunits that tether the ER and mitochondria. Three ERMES subunits (i.e., Mdm12, Mmm1, and Mdm34) contain the synaptotagmin-like mitochondrial lipid-binding protein (SMP) domain. The SMP domain belongs to the tubular lipid-binding protein (TULIP) superfamily, which consists of ubiquitous lipid scavenging and transfer proteins. Herein, we describe the methods for expression and purification of recombinant Mdm12, a bona fide SMP-containing protein, together with the subsequent identification of its bound phospholipids by high-performance thin-layer chromatography (HPTLC) and the characterization of its lipid exchange and transfer functions using lipid displacement and liposome flotation in vitro assays with liposomes as model biological membranes. These methods can be applied to the study and characterization of novel lipid-binding and lipid-transfer proteins.