Isolation and characterization of low density detergent-insoluble membrane (LD-DIM) fraction from sea urchin sperm

The low density detergent-insoluble membrane (LD-DIM) fraction was obtained by a sucrose density gradient centrifugation from sperm of three sea urchin species, Hemicentrotus pulcherrimus, Strongylocentrotus purpuratus, and Anthocidaris crassispina. These LD-DIM preparations were characterized by enriched glycosphingolipids (GSL) including gangliosides and sulfatide (SLF), having more than 50% of the total amount of GSL present in these sperm. Interestingly, a minor component of H. pulcherrimus sperm (HO3S-->8Neu5Aca2-->8Neu5Ac alpha 2-->6Glc beta 1-->Cer) was shown to be even more enriched in the LD-DIM as revealed by using monoclonal antibody (mAb.3G9) specific to this ganglioside. In addition to the GSL, phosphatidylserine (PS) and diacylglycerol (DG) were enriched in the LD-DIM. On the other hand, cholesterol (CL) and sphingomyelin (SP) were not so enriched, which contrasted with the LD-DIM from Madin-Darby canine kidney (MDCK) cells, where CL and SM were reported to be abundant. Because mammalian somatic cell-derived DIMs have been proposed to be associated with functional signal transduction, it seems possible that the ganglioside-enriched LD-DIM in sea urchin sperm can participate in binding to eggs and the subsequent egg activation process. To our knowledge this is the first chemical characterization of the LD-DM fraction of a gametic cell. (C) 1999 Academic Press.