suggest the uniqueness of such molecules among mammalian orders. In contrast, conservation of func- tion would suggest molecules involved in binding and fusion may be quite similar among species. Evidence has accumulated that fertilin, a sperm plasma membrane protein, is involved in sperm–oocyte binding in the mouse as well as in the guinea pig (Pri- makoff et al., 1987; Blobel et al., 1992; Myles et al., 1994). The fertilin β molecule has also been identified in the monkey, rabbit, human, rat and bull (Perry et al., 1995; Hardy & Holland, 1996; Jury et al., 1997; McLaughlin et al., 1997; Waters & White, 1997), although there is little experimental evidence for its function during fertilisa- tion in these species. A second sperm plasma membrane protein, the DE protein originally identified as a fertili- sation-related protein in the rat, has also been reported in the mouse (Rochwerger et al., 1992; Rochwerger & Cuasnicu, 1992; Cohen et al., 1996). These observations Introduction Sperm–oocyte binding and fusion is an essential part of fertilisation and involves interaction between the membranes of the two gametes. Sperm make initial contact with the oocyte plasma membrane by the inner acrosomal membrane (Koehler et al., 1982; Talbot & Chacon, 1982). Subsequent binding and fusion occur between the equatorial segment and post-acrosomal region of the sperm plasma membrane and the oocyte plasma membrane (Yanagimachi, 1994). We still know little about the molecules involved in binding and fusion. Species- or order-specific fertilisation would 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 Zygote 8 (May), pp 181–187. © 2000 Cambridge University Press Printed in the United Kingdom All correspondence to: Trish Berger, Department of Animal Science, University of California, Davis, CA 95616–8521, USA. e-mail: tberger@ucdavis.edu. Tel: +1 (530) 752 1267. Fax: +1 (530) 752 0175. Binding of porcine sperm plasma membrane proteins to sheep, hamster and mouse oocyte plasma membrane Elizabeth Ward and Trish Berger Department of Animal Science, University of California, Davis, California, USA Date submitted: 25.11.99. Date accepted: 20.12.99 Summary Four porcine sperm plasma membrane proteins were previously identified as putative ligands for the oocyte plasma membrane. The present study examined the binding of these proteins and two additional porcine sperm membrane proteins to oocytes from sheep, mice and hamsters as a first step in assessing potential conservation of these putative sperm ligands across species and across mammalian orders. Plasma membrane vesicles were isolated from porcine sperm, solubilised, and the proteins separated by one-dimensional gel electrophoresis. The 7, 27, 39 and 62 kDa porcine sperm protein bands demonstrat- ing predominant binding of the porcine oocyte plasma membrane on ligand blots, a 90 kDa protein band demonstrating minor binding, and a 97 kDa protein band that did not bind the oocyte plasma membrane probe were electroeluted. Proteins were biotinylated, and incubated with zona-free oocytes. Bound biotinylated protein was labelled with fluorescent avidin and the oocytes examined with a confocal microscope. The 7 kDa, 27 kDa and the 39 kDa proteins bound to the sheep oocytes but not to a majority of the hamster or mouse oocytes. The 62 kDa protein bound to sheep oocytes and mouse oocytes but not to a majority of the hamster oocytes. The 90 kDa protein bound to oocytes from all three species. The 97 kDa protein, which did not recognise the porcine oocyte probe on a Western ligand blot, did not bind to oocytes from any species and served as a negative control. These observations are consistent with signif- icant conservation of molecule and function among species within the same mammalian order. Hence, one species may be a good model for other species from the same order. Only limited conservation of binding activity of porcine sperm plasma membrane proteins to rodent oocytes was observed, suggest- ing a greater divergence either in molecular structure or in function among species from different orders. Keywords: Fertilisation, Oolemma, Ovine, Species-specific, Sperm–oocyte