ENDOPROTEOLYTIC CLEAVAGE IN THE EXTRACELLULAR DOMAIN OF THE INTEGRAL PLASMA-MEMBRANE PROTEIN CE9 PRECEDES ITS REDISTRIBUTION FROM THE POSTERIOR TO THE ANTERIOR TAIL OF THE RAT SPERMATOZOON DURING EPIDIDYMAL MATURATION

Originally identified as a basolateral domain-specific integral plasma membrane protein of the rat hepatocyte, CE9 mRNA and protein were also detected at high levels in the testis of the rat by Northern and Western blotting and immunoprecipitation. CE9 proved to be a domain-specific integral plasma membrane protein of the rat spermatozoon: on testicular spermatozoa, it was concentrated within the posterior tail domain of the plasma membrane, whereas on vas deferens spermatozoa, CE9 was concentrated within the anterior tail domain. This change in the localization of CE9 was observed to take place in a progressive fashion during the passage of the spermatozoa from the caput epididymidis to the cauda epididymidis and was preceded by the specific endoproteolytic cleavage of CE9 in the proximal portion of the caput epididymidis. Amino-terminal amino acid microsequencing of CE9 immunoaffinity purified from epididymis suggested that the cleavage occurred on the carboxy-terminal side of arginine-74 in the primary sequence of CE9, resulting in the loss of approximately 40% of the amino acids in the extracellular domain of this transmembrane glycoprotein.