Protein translocation pulls the plug

Protein translocation pulls the plug embrane-bound compartments present an immediate problem: proteins need to get into and across those membranes. Now, we have the first picture of how that process works, thanks to the structure determined and colleagues. They find that a single SecY complex from Methanococcus jannaschii forms an hourglass-shaped pore with a basal plug that probably swings out The complex was not an easy target: the structure came only after five years of experiments with proteins from ten different organisms. The effort was worthwhile. " In where people had models before the structure have any ideas of how this would work, " says Rapoport. " We were shocked because we didn't expect the pore to be in one complex. " Earlier EM experiments had suggested that a large pore formed in a gap between four associated complexes. But more recent EM data are consistent with this supposed pore being only an indentation, and the new structure clearly suggests a path for nascent proteins through a single complex. Translocation begins, according to cross-linking data, when the signal sequence of the nascent protein inserts between n retrospect it seems obvious. If you need to uncoat a vesicle only after the vesicle has formed, use the spherical shape of the vesicle as a trigger for uncoating. and colleagues have evidence for this mechanism. Morphology was not the first suspect as a trigger for uncoating Golgi transport vesicles. Lipid metabolism was a possibility, and Antonny and others experimented in vitro with the effect of different lipid mixtures on uncoating. But in vivo evidence for changes in lipid composition during vesicle budding was lacking. The different in vitro lipid compositions did result in different uncoating dynamics, however. Antonny suspected that some of the in vitro mixtures were mimicking a distorted lipid arrangement seen during I transmembrane domains (TM) 2b and 7 of the main, channel-forming ␣-subunit. This probably has two effects. First, a plug formed by TM2a swings ‫ف‬ 22 Å out of the way, revealing a narrow, central constriction delimited by a ring of six hydro-phobic residues. Second, the two pseudosymmetric halves of the channel are pried open a little to widen the pore ring. Further separation of the two halves should allow release of membrane-spanning domains of the nascent protein. The plug and pore ring are Rapoport's candidates for forming the tight seal that prevents passage of molecules other than the …