Control of membrane furrowing during cellularization by slam, a regulator of membrane insertion. T. Lecuit , E. Wieschaus. Molec Biology Dept, Princeton Univ, Princeton, NJ.

   Cleavage of the blastoderm called cellularization is a specialized form of cytokinesis triggered by 2 key developmental transitions: entry into G phase and induction of the zygotic transcription. An acto-myosin hexagonal network assists in the stepwise invagination of the membrane between the nuclei, first slow, then fast. The membrane surface increases about 30-fold, resulting in the formation of a polarized epithelium. We showed that cellularization involves the rapid mobilization of a cytoplasmic reservoir from the ER and the Golgi. New membrane populations are inserted in a precise sequence first apically (slow phase) then apico-laterally (fast phase) such that polarity arises progressively as the membrane grows. Similar results suggest the importance of membrane growth per se during cytokinesis. A few genes only are zygotically required to control a machinery maternally provided. One of them, slow as molasses (slam) is essential for membrane growth in the slow phase of invagination, but not for the following rapid acceleration of furrowing during fast phase. In slam mutant embryos, the lateral transfer of apical membrane that normally takes place as new membrane is apically inserted is blocked. In addition, the transmembrane protein Neurotactin is normally exported from the Golgi but accumulates in an apical compartment instead of being inserted into the recipient apical membrane. These results suggest that slam controls a key aspect of membrane trafficking during membrane invagination. We cloned slam. It encodes a new 138kD protein with distant homology to a phosphatase domain. Consistent with having a key regulatory role, slam transcripts are rapidly induced at the onset of cellularization but rapidly disappear at the end of slow phase. Further characterization of the protein localization and function will be presented.