Pinosomes help a virus escape

In This Issue In This Issue Microtubules get swept up in the excitement of mitosis icrotubules (MTs) undergo a dramatic suctioning at the onset of mitosis, according to a report by Rusan et al. on page 997. The process places MTs in the vicinity of the growing mitotic spindle and may prevent nonspindle MTs from interfering with chromosome segregation. Visualizing mitotic MT rearrangement has not been easy, because the process is so rapid. The new confocal videos of living cells expressing GFP-tubulin catch prophase in a way that imaging of fixed or microinjected cells never did. Rearrangement began in prophase with the depolymerization of cytosolic MTs and the formation of bundles and foci. M COPs are held up by Sec16p n page 1029, Supek et al. illustrate how a peripheral membrane protein organizes a coat protein complex involved in secretory vesicle formation. The protein in question, yeast Sec16p, is an ER resident required in vivo for COPII-dependent vesicle budding. In vitro, Sec16p is not necessary for budding from lipo-somes reconstituted with pure cytosolic COPII proteins. However, this in vitro reaction depends on a nonhydrolyzable form of GTP, probably because the COPII coat falls apart when Sar1p (the initiator of coat assembly) hydrolyzes GTP. Until now, the function of Sec16p in liposome budding could not be tested, because the protein was difficult to purify. Supek et al. report conditions that stabilize Sec16p and have purified enough protein for in vitro studies. Microsomal membranes stripped of endogenous Sec16p were stimulated in vesicle budding by the purified protein, but only in the presence of hydrolyzable GTP. Thus, the in vivo function of Sec16p may be either to slow GTP hydrolysis during budding or to stabilize the COPII coat even after GTP is hydrolyzed by Sar1p. The authors used liposomes to show that Sec16p recruited Sar1p-GTP and another coat component, Sec23/24p, to membranes. However, this interaction did not inhibit GTP hydrolysis. Thus, the function of Sec16p appears to be to tether the COPII coat on membranes. ᭿ O More vesicles bud from liposomes when Sec16p is present (bottom). Pinosomes help a virus escape any virus species are taken into their host via receptor-mediated endocytosis, after which the virus particle is temporarily contained within endosomes. For genome propagation, most viruses must escape into the cytosol. New results from Meier et al. (page 1119) explain how Adenovirus type 2 (Ad2), one of the most rapidly escaping …