PHYSIOLOGICAL RELEVANCE OF THE MEMBRANE ATTACK COMPLEX INHIBITORY PROTEIN CD59 IN HUMAN SEMINAL PLASMA - CD59 IS PRESENT ON EXTRACELLULAR ORGANELLES (PROSTASOMES), BINDS CELL-MEMBRANES, AND INHIBITS COMPLEMENT-MEDIATED LYSIS

We demonstrate here that CD59, an inhibitor of the membrane attack com plex (MAC) of the complement system, is present in cell-free seminal p lasma (SP) at a concentration of at least 20 mug/ml. Analyses by sodiu m dodecyl sulfate-polyacrylamide gel electrophoresis, Western blotting , and Edman degradation indicated that this protein, SP CD59, was simi lar, if not identical, to CD59 isolated from erythrocyte (E) membranes (E CD59). Like purified E CD59, SP CD59 also possesses a glycosyl pho sphatidyl inositol (GPI) anchor and incorporates into the membranes of heterologous cells where it inhibits lysis by the human MAC. This phe nomenon could be demonstrated not only if cells were incubated with pu rified SP CD59 but also if unfractionated SP were used. Further, CD59 in unfractionated SP bound to washed spermatozoa, increasing their mem brane content of the protein. The mechanism by which this protein reta ins its GPI anchor while apparently present in the fluid phase is of i nterest and was further investigated. Using the techniques of high-spe ed centrifugation, fast performance liquid chromatography fractionatio n, and electron microscopy, we found that all detectable SP CD59 was a ssociated with vesicular extracellular organelles. These organelles, n amed ''prostasomes,'' were previously known to be present in SP and to interact with spermatozoa, although their function was uncertain. Int eraction of heterologous E with prostasomes rendered the cells more re sistant to lysis by human MACs. We propose that these organelles repre sent a pool of CD59 from which protein lost from spermatozoa, perhaps as a result of low level complement attack or of normal membrane turno ver, can be replenished.