Chemical structure and function of glycosphingolipids of Sphingomonas spp and their distribution among members of the alpha-4 subclass of Proteobacteria.

Sphingomonas spp are phylogenetically placed in the alpha-4 subclass of Proteobacteria. They have glycosphingolipids (GSL) in their membranes instead of lipopolysaccharide (LPS) as in other Gram-negative bacteria. S. paucimobilis, the type species of the genus, has GSL-1, which contains only glucuronic acid (GlcA) as a sugar moiety, and GSL-4A, which contains a tetrasaccharide including GlcA. GSL-1 and GSL-4A form the outer membrane of S. paucimobilis with outer membrane proteins and phospholipids. In the outer membrane, GSLs are assumed to locate and function as does the LPS of other Gram-negative bacteria. Sphingomonas spp closely related to the type species contain both GSL-1 and the oligosaccharide-type GSL such as GSL-4A, but other Sphingomonas spp and other genera in the alpha-4 subclass of Proteobacteria contain only GSL-1. Structural variations of fatty acids and dihydrosphingosines in the GSL-1 are presented.