Nuclear translocation ability of PLCζ
Another characteristic feature of PLCζ is the nuclear translocation ability. PLCζ-Venus expressed by injection of cRNA into mouse eggs is increased up to 3 h and attained a steady level at 4 – 5 h (18) (Fig. 3C). Expressed PLCζ distributes homogenously in the egg cytoplasm without special localization or membrane association. Interestingly, expressed PLCζ is accumulated into the PN formed at 5 – 6 h and continuously increased there (18) (Fig. 3C). This finding is consistent with earlier observation that the PN formed after fertilization has Ca2+ oscillation-inducing activity when introduced into a new unfertilized egg (23). Ca2+ oscillations after fertilization (10, 11) or after injection of PLCζ- encoding RNA (24) cease at about the time when the PN is formed (Fig. 2A), suggesting that sequestration of the sperm factor COIP may be the cause of the termination of Ca2+ oscillations. This idea is supported by the finding that Ca2+ oscillations do not stop but continue over 10 h when PN formation is prevented by injection of a lectin, WGA (25), or a point mutation is added to a putative nuclear translocation signal region of PLCζ (24). During early development, Ca2+ oscillations reappear in the 1-cell mouse embryo at the stage of nuclear envelope breakdown just before the first cell division (15 – 16 h after fertilization) and cease after the first cleavage (24) (Fig. 4). Correspondingly, when RNA encoding PLCζ-Venus is injected into the 1-cell embryo 6 h after fertilization in which the formed male and female pronuclei are visible, expressed PLCζ-Venus translolcates into the PN, and then accumulated PLCζ is liberated from the PN into the cytoplasm upon nuclear envelope breakdown (26) (Fig. 4). Subsequently, PLCζ- Venus translocates again into the nuclei of the 2-cell embryo after the first cleavage (24, 26) (Fig. 4). These results lead to the view that shuttling movement of PLCζ between the cytoplasm and nucleus may turn on and off a series of IP3-dependent [Ca2+]i rises in a cell cycle dependent manner.