Sphingolipid and cholesterol-rich Triton X-100-insoluble membrane fragments (detergent-resistant membranes, DRMs) containing lipids in a state similar to the liquid-ordered phase can be isolated from mammalian cells, and probably exist as discrete domains or rafts in intact membranes. We postulated that proteins with a high affinity for such an ordered lipid environment might be targeted to rafts. Saturated acyl chains should prefer an extended conformation that would fit well in rafts. In contrast, prenyl groups, which are as hydrophobic as acyl chains but have a branched and bulky structure, should be excluded from rafts. Here, we showed that at least half of the proteins in Increasing evidence suggests that cholesterol and sphingolipid-rich lipid microdomains or rafts exist in eukaryotic cell membranes and have important functions there (1-3). These rafts are likely to be important in the structure and function of caveolae, plasma membrane invaginations that are implicated in signal transduction (4, 5), endocytosis (6), transcytosis across endothelial cells (7,8), and cholesterol trafficking (9 -11). However, rafts are not restricted to caveolae (2, 3, 12) and recent evidence suggests that they act in signal transduction in cells that lack distinct caveolae, such as T lymphocytes (13-16) and basophils (17)(18)(19). Rafts have also been implicated in protein and lipid sorting in the secretory and endocytic pathways (1, 20 -22).Cholesterol and sphingolipid-rich detergent-resistant membranes (DRMs) 1 can be isolated from mammalian cells (23). DRM lipids are in a state similar to the liquid-ordered (l o ) phase (3, 24 -26). The l o phase, which requires cholesterol to form, is favored by lipids like sphingolipids, whose long saturated acyl chains give them a high degree of order and a high acyl-chain melting temperature (3). Acyl chain order explains the detergent-insolubility of DRMs (3). We hypothesize that DRMs are an in vitro correlate of rafts in intact membranes. It is important to note that detergent insolubility can underestimate the association of proteins and lipids with the l o phase; some proteins and lipids that are in rafts can be solubilized (25). Nevertheless, DRM association provides a powerful tool for identifying molecules that are likely to have a high affinity for rafts. DRMs isolated from cells contain a number of proteins (27-29) which are undoubtedly crucial for the function of the domains in vivo. For this reason, it is important to determine how proteins associate with DRMs. Three DRM targeting signals have been defined. First, glycosylphosphatidylinositol (GPI)-anchored proteins are targeted to DRMs through acyl chain interactions (23)(24)(25)30). An N-terminal Met-Gly-Cys motif that is present in some Src family kinases and heterotrimeric G protein ␣ subunits, in which Gly is myristoylated and Cys is palmitoylated, can also serve as a DRM targeting signal (31,32). Third, dual palmitoylated Cys residues are required for raft association of the T cell adaptor protein LAT (15) and the neu...

The contrast observed in images of frozen-hydrated biological specimens prepared for electron cryo-microscopy falls significantly short of theoretical predictions. In addition to limits imposed by the current instrumentation, it is widely acknowledged that motion of the specimen during its exposure to the electron beam leads to significant blurring in the recorded images. We have studied the amount and direction of motion of virus particles suspended in thin vitrified ice layers across holes in perforated carbon films using exposure series. Our data show that the particle motion is correlated within patches of 0.3 – 0.5 μm, indicating that the whole ice layer is moving in a drum-like motion, with accompanying particle rotations of up to a few degrees. Support films with smaller holes, as well as lower electron dose rates tend to reduce beam-induced specimen motion, consistent with a mechanical effect. Finally, analysis of movies showing changes in the specimen during beam exposure show that the specimen moves significantly more at the start of an exposure than towards its end. We show how alignment and averaging of movie frames can be used to restore high-resolution detail in images affected by beam-induced motion.

Non-enveloped viruses of different types have evolved distinct mechanisms for penetrating a cellular membrane during infection. Rotavirus penetration appears to occur by a process resembling enveloped-virus fusion: membrane distortion linked to conformational changes in a viral protein. Evidence for such a mechanism comes from crystallographic analyses of fragments of VP4, the rotavirus-penetration protein, and infectivity analyses of structure-based VP4 mutants. We describe here the structure of an infectious rotavirus particle determined by electron cryomicroscopy (cryoEM) and single-particle analysis at about 4.3 Å resolution. The cryoEM image reconstruction permits a nearly complete trace of the VP4 polypeptide chain, including the positions of most side chains. It shows how the two subfragments of VP4 (VP8* and VP5*) retain their association after proteolytic cleavage, reveals multiple structural roles for the b-barrel domain of VP5*, and specifies interactions of VP4 with other capsid proteins. The virion model allows us to integrate structural and functional information into a coherent mechanism for rotavirus entry.