Cargo selection and export from the endoplasmic reticulum is mediated by the COPII coat machinery that includes the small GTPase Sar1 and the Sec23/24 and Sec13/31 complexes. We have analyzed the sequential events regulated by purified Sar1 and COPII coat complexes during synchronized export of cargo from the ER in vitro. We find that activation of Sar1 alone, in the absence of other cytosolic components, leads to the formation of ER-derived tubular domains that resemble ER transitional elements that initiate cargo selection. These Sar1-generated tubular domains were shown to be transient, functional intermediates in ER to Golgi transport in vitro. By following cargo export in live cells, we show that ER export in vivo is also characterized by the formation of dynamic tubular structures. Our results demonstrate an unanticipated and novel role for Sar1 in linking cargo selection with ER morphogenesis through the generation of transitional tubular ER export sites.

The role of dynamin GTPases in the regulation of receptor-mediated endocytosis is well established. Here, we present new evidence that the ubiquitously expressed isoform dynamin-2 (dyn2) can also function in a signal transduction pathway(s). A ≤5-fold increase of dyn2 relative to endogenous levels activates the transcription factor p53 and induces apoptosis, as demonstrated by reduced cell proliferation, DNA fragmentation, and caspase-3 activation. Dyn2-triggered apoptosis occurs only in dividing cells and is p53 dependent. A mutant defective in GTP binding does not trigger apoptosis, indicating that increased levels of dyn2·GTP, rather than protein levels per se, are required to transduce signals that activate p53. A truncated dyn2 lacking the COOH-terminal proline/arginine-rich domain (PRD), which interacts with many SH3 domain-containing partners implicated in both endocytosis and signal transduction, triggers apoptosis even more potently than the wild-type. This observation provides additional support for the importance of the NH 2 -terminal GTPase domain for the apoptotic phenotype. All described effects are dyn2-specific because >200-fold overexpression of dyn1, the 70% identical neuronal isoform, has no effect. Our data suggest that dyn2 can act as a signal transducing GTPase affecting transcriptional regulation.