Localization of polysialoglycoprotein as a major glycoprotein component in cortical alveoli of the unfertilized eggs of Salmo gairdneri

Abstract

Polysialoglycoprotein (PSGP, 200 kDa), first isolated by S. Inoue and M. Iwasaki in 1978 ( Biochem, Biophys. Res. Commun. 83, 1018–1023) from unfertilized eggs of rainbow trout, has been shown to comprise a unique class of glycoproteins associated with the exocytosis of cortical alveoli. In 1986, 200-kDa PSGP was shown to undergo proteolytic depolymerization to 9-kDa PSGP on egg fertilization (activation) and there was an indication that 200-kDa PSGP may possibly be a component of cortical alveoli ( J. Biol. Chem. 261, 5256–5261). In this paper we present evidence demonstrating that PSGP is actually a component of cortical alveolus. First, a cortical alveolus-rich fraction (CA fraction) was obtained by low-speed centrifugation of the homogenate of unfertilized eggs of rainbow trout. The 200-kDa PSGP was found to be a major component extractable with buffered saline from the CA fraction by chemical analysis of isolated materials. Treatment of the eggs to induce parthenogenetic activation resulted in all cases in the loss of both cortical alveoli and PSGP in the CA fraction. Second, perivitelline space fluid was isolated from the activated eggs of rainbow trout and analyzed, and 9-kDa PSGP was confirmed to be present as a major proteinaceous component. Third, following incubation of the eggs in water for activation, the time course of the appearance of 9-kDa PSGP and the breakdown of 200-kDa PSGP was observed. The formation of 9-kDa PSGP was detected in the eggs after 1 min of incubation and its level rose rapidly, attaining a maximum at 7 min after incubation. During this period, there was a concomitant fall in the level of 200-kDa PSGP. This formation and rapid increase in 9-kDa PSGP correspond directly to the time course of cortical alveolus exocytosis in activated chum salmon eggs recently studied by scanning electron microscopy.