Fig. 7. ß-dependent cell-surface expression of PS3 indicates that ß makes a heterodimer with PS3. Embryos were labelled with antibodies against the PS2 subunit (green) and the PS3 subunit (magenta). (A) The PS2 subunit is expressed in the somatic and visceral muscles, and localizes to sites of attachment between adjacent muscles in wild-type stage 16 embryos (arrows). The PS3 subunit is expressed in the midgut endoderm and is localized cortically (A'). (B) In stage 16 embryos lacking both maternal and zygotic ßPS, PS2 failed to be transported to the surface of muscle cells and remained trapped in the endoplasmic reticulum (arrow). However, the plasma membrane expression of PS3 was retained in midgut endodermal cells (B'). (C) In the absence of both ß and zygotic ßPS, the PS3 subunit failed to be transported to the surface of endodermal cells (C').