Membrane fusion

Sequential action of two GTPases to promote vacuole docking and fusion. Eitzen, G. et al. EMBO J. 19, 6713?6720 (2000) [PubMed]

Vacuole fusion in Saccharomyces cerevisiae can be dissected into three steps: priming, docking and fusion. The Rab protein Ypt7p regulates vacuole fusion after the priming step ? it binds in its GTP-bound form to the HOPS complex, which docks the membranes together. Ypt7p is then no longer needed for fusion to proceed. Surprisingly, fusion requires another, unknown GTPase that might regulate the calcium release necessary for fusion.

Cell death

Wee1-regulated apoptosis mediated by the Crk adaptor protein in Xenopus egg extracts. Smith, J. J. et al. J. Cell Biol. 151, 1391?1400 (2000) [PubMed]

The protein kinase Wee1 inhibits the cell-cycle kinase Cdc2, but Smith and colleagues now find a new function for it. They identified Wee1 in a screen for proteins that interact with the adaptor protein Crk, which is needed for apoptotic signalling in Xenopus egg extracts. Wee1, like Crk, is needed for apoptosis in this system and addition of Wee1 markedly accelerates apoptosis. By contrast, other Cdc2 inhibitors have no effect on apoptosis in this system.

Development

Wasp , the Drosophila Wiskott?Aldrich syndrome gene homologue, is required for cell fate decisions mediated by Notch signalling. Ben-Yaacov, S. et al. J. Cell Biol. 152, 1?136 (2001) [PubMed]

WASP, first identified because its mutation causes Wiskott?Aldrich syndrome, is involved in translating signals into cell movement. Ben-Yaacov and colleagues now identify a single WASP-like gene in Drosophila and find that cell-fate decisions are disrupted in WASP mutants: the phenotype is enhanced by intoducing a weak mutant of the Notch receptor into the WASP mutants, and suppressed when the Notch pathway is elevated, indicating that WASP might be involved in Notch-dependent cell-fate decisions.

Neurotransmission

Complexins regulate a late step in Ca2+-dependent neurotransmitter release. Reim, K. et al. Cell 104 , 71?81 (2001) [Contents page]

Complexins bind to SNARE complexes in the brain. This study establishes their function as positive regulators of neurotransmission, acting at or after the Ca2+-dependent step. Whereas complexin-I knockout mice show no phenotype, complexin-I and -II double deletion mutants die a few hours after birth. Spontaneous neurotransmitter release is normal in these mice, but they fail to respond to action potentials. Complexins are unlikely to be calcium sensors, but probably interact with the calcium sensor (which might or might not be synaptotagmin).