Protein probes to visualize sphingomyelin and ceramide phosphoethanolamine.

Sphingomyelin (SM) is a major sphingolipid in mammalian cells whereas its analog, ceramide phosphoethanolamine (CPE) is found in trace amounts in mammalian cells and in larger amounts in invertebrates such as insect cells like Drosophila melanogaster. To visualize endogenous SM or CPE, we need specific probes able to recognize the chemical structure of the lipid, rather than its physical property. A limited number of proteins is known to specifically and strongly bind SM or CPE. These proteins are either toxins produced by non-mammalian organisms, subunits or fragments of toxins or a protein that has similar structure to a toxin. These proteins labeled with small fluorophore (e.g. Alexa Fluor) or conjugated to fluorescent proteins (e.g. mCherry) or other types of markers (e.g. I, maltose-binding protein) are used to detect SM or CPE. Here we summarize the characteristics of specific SM-binding proteins, lysenin and equinatoxin II; CPE- and SM/cholesterol (Chol) binding aegerolysin proteins, pleurotolysin A, ostreolysin and erylysin A and SM/Chol-binding protein, nakanori. Then we give examples of their applications including their limitations related not only to their lipid specificity and binding constants, but also to the lipid organization in the membrane.