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Fig. 7. Sec5 localization and function during oocyte development. (A) At stage 2,
Sec5 is present on all egg chamber membranes at low levels, but concentrates
between the posterior follicle cells and the oocyte, particularly at the
apical membrane of the polar cells. Throughout the egg chamber, Sec5 is
required to preserve the integrity of the growing cells. Sec5 in the posterior
follicle cells and oocyte is essential to position the oocyte at the posterior
of the egg chamber. (B) By stage 5, Sec5 concentrates also along the anterior
oocyte membrane, at a time when this membrane grows disproportionately to the
nurse cells. The enrichment of Sec5 in the polar cells persists, and is
probably required there to maintain the posterior position of the oocyte. (C)
At stage 8, Yolkless rapidly inserts in the oocyte membrane in a
Sec5-dependent manner. At this time, Sec5 is present along the oocyte
membrane. Sec5 is also moderately enriched at the anterior rim of the oocyte,
and is required there for the initial trafficking of Gurken. (D) By stage 10,
Sec5 is concentrated only at the anterior rim of the oocyte membrane. At this
time, the majority of Gurken is trafficked to the dorsoanterior membrane to
signal to the EGFR in the adjacent follicle cells. The enrichment of Sec5
along all the anterior rim of the oocyte indicates that Sec5 alone cannot
specify to which region of that domain Gurken is targeted.
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