Structural elucidation of novel phosphocholine-containing glycosylinositol-phosphoceramides in filamentous fungi and their induction of cell death of cultured rice cells

Abstract

Novel ZGLs (zwitterionic glycosphingolipids) have been found in and extracted from the mycelia of filamentous fungi (Acremonium sp.) isolated from soil. Five ZGLs (ZGL1–ZGL5) were structurally elucidated by sugar compositional analysis, methylation analysis, periodate oxidation, matrix-assisted laser-desorption ionization–time-of-flight MS, 1H-NMR spectroscopy and fast-atom bombardment MS. Their chemical structures were as follows: GlcN(α1-2)Ins1-P-1Cer (ZGL1), Man(α1-6)GlcN(α1-2)Ins1-P-1Cer (ZGL2), Man(α1-6)Man(α1-6)GlcN(α1-2)Ins1-P-1Cer (ZGL3), PC→6Man(α1-6)GlcN(α1-2)Ins1-P-1Cer (ZGL4), and PC→6Man(α1-6)Man(α1-6)GlcN(α1-2)Ins1-P-1Cer (ZGL5) (where Cer is ceramide and PC is phosphocholine). In addition, one acidic glycosphingolipid, which was the precursor of ZGLs, was also characterized as inositol-phosphoceramide. The core structure of the ZGLs, GlcN(α1-2)Ins1-P, is rather different from those found in other fungi, such as Man(α1-2)Ins1-P and Man(α1-6)Ins1-P. Interestingly, the terminal mannose residue of ZGL4 and ZGL5 was modified further with a PC group. The presence of PC-containing glycosylinositol-phosphoceramides has not been reported previously in any organism. The ceramide constituents of both ZGLs and acidic glycosphingolipid were essentially the same, and consisted of a 4-hydroxyoctadecasphinganine (phytosphingosine) as the sole sphingoid base and 2-hydroxytetracosanoic acid (>90%) as the major fatty acid. ZGLs were found to cause cell death in suspensions of cultured rice cells. The cell death-inducing activity of ZGLs is probably due to the characteristic glycan moiety of Man(α1-6)GlcN, and PC-containing ZGLs had high activity. This study is the first to demonstrate that fungal glycosylinositol-phosphoceramides induce cell death in cultured rice cells.