CLIPs to the rescue

Suicide attempt linked to breakdown he Golgi apparatus breaks down temporarily to segregate into daughter cells during mitosis, but during apoptosis the organelle fragments irreversibly. On page 637, Chiu et al. report that cleavage of a critical tethering protein during apoptosis not only drives the fragmentation of the Golgi apparatus, but also appears to help propagate the apoptotic signal. The work adds to mounting evidence that the Golgi apparatus is a perpetrator as well as a victim of apoptosis. The authors found that the tethering protein p115, which is essential for T A COOH-terminal fragment (right) but not full-length (left) p115 induces apoptosis. maintaining normal Golgi apparatus architecture, is selectively cleaved during apoptosis. The apoptotic proteases caspase-3 and caspase-8 cleave the protein in vitro, and a stable cell line expressing a cleavage-resistant form of p115 exhibits a delay in Golgi fragmentation during apoptosis. Expressing the COOH-terminal cleavage fragment of p115 in cells is sufficient to induce fragmentation, and, surprisingly, this fragment also translocates to the nucleus, where it induces apoptosis. The data suggest that the p115 COOH-terminal fragment serves two functions during apoptosis, both disrupting the Golgi apparatus as a dominant–negative inhibitor of p115, and sending a signal to the nucleus to induce other apoptotic processes. ᭿ CLIPs to the rescue icrotubules grow persistently from the centrosome to the cell margin, and then enter a state of dynamic instability, repeatedly growing and shrinking over short distances to probe the margin of the cell. This process is thought to be critical for rapid sensing of cell shape changes. On page 589, Komarova et al. demonstrate that the cytoplasmic linker proteins (CLIPs) are important and highly specific regulators of dynamic instability. The work helps explain why microtubules normally probe only the cell margin, rather than growing and shrinking over greater distances. The authors found that expression of a truncated form of the CLIP protein CLIP-170 acts in a dominant–negative fashion in cultured mammalian cells, knocking endogenous CLIPs off their normal perches on the plus ends of microtubules. Though this has no effect on the rate of microtubule growth or shortening, it significantly reduces the frequency with which shrinking microtubules are rescued, or switched from shortening to growth. When rescue is inhibited, the microtubules still grow persistently from the centrosome to the cell margin; but once shortening begins, the microtubules shrink persistently back toward the centrosome, probing the entire cytoplasm. The results suggest …