Nuclear distribution of proliferating cell nuclear antigen (PCNA) in fertilized eggs of the starfish Asterina pectinifera.

Previous studies (Nomura et al. (1991) Dev. Biol. 143, 289-296 (1993) Dev. Biol. 159, 288-297) determined the time of DNA replication period (S phase) in starfish eggs fertilized either during or after oocyte maturation. Here proliferating cell nuclear antigen (PCNA) localized within nuclei of starfish eggs was detected with an anti-PCNA human antiserum. Using a confocal laser scanning microscope, a three-dimensional structure of the PCNA region was analyzed. In eggs fertilized during maturation, PCNA started to localize within the nuclei at the same time as the initiation of the first S phase. During the S phase, the distribution of localized PCNA in a three-dimensional view coincided with the chromatin distribution. After the S phase, PCNA remained localized within the nuclei, but its distribution no longer coincided with the chromatin distribution. In eggs fertilized after maturation, however, PCNA started to localize within the female pronuclei about 10 minutes ahead of the first S phase. Localized PCNA occupied only a limited region of the nuclei without diffusing over the whole nuclear area. Chromatin distributed around the peripheral region of the nuclei mostly outside the PCNA region. When the first S phase was initiated, the chromatin distribution became coincident with the PCNA region. Later behavior of PCNA was the same as that of the eggs fertilized during maturation. The precocious localization of PCNA in those eggs fertilized after maturation simply demonstrates that the 'postactivation process' for preparing DNA replication is triggered by fertilization and PCNA localization and S phase are sequentially initiated with a time-lapse. On the other hand, the simultaneous occurrence of them seen in those eggs fertilized during maturation indicates that the postactivation process must be going on in parallel with the maturation process.