Characterization and localization of a mouse egg cortical granule antigen prior to and following fertilization or egg activation.

Immunological approaches were used to characterize an antigen that is present within the cortical granules of mouse oocytes and eggs. Immunoelectron microscopy shows a specific localization of the antigen to the cortical granules in the cortex of mouse oocytes and eggs. Following in vitro fertilization, the antigen is present in the perivitelline space and is associated with the zona pellucida. No cortical granules and very little antigen are detected in the two-cell embryo. This antiserum detects a protein of Mr = 75,000 (p75) following immunostaining of egg proteins on Western blots, or immunoprecipitation of metabolically labeled oocyte proteins or radio-iodinated egg proteins. p75 is also present in exudates obtained from A23187-treated eggs, as detected by either radio-iodination of the released egg proteins, or maturation and ionophore activation of metabolically labeled oocytes. Two-dimensional gel electrophoresis of radio-iodinated egg proteins reveals four species of p75 with pIs between 4.9 and 5.3, whereas only the most basic form of p75 is detected in metabolically labeled oocytes. Multiple forms of the radio-iodinated p75 are present in the exudate of ionophore-treated eggs. p75 displays a greater electrophoretic mobility under nonreducing conditions, indicating the presence of intramolecular disulfide bonds, a common characteristic of secreted proteins. We conclude that p75 is synthesized in oocytes, modified and packaged into cortical granules, and released from eggs following fertilization or activation.