No abstract

The preprotein translocase of the outer membrane of mitochondria (TOM complex) facilitates the recognition, insertion, and translocation of nuclear-encoded mitochondrial preproteins. We have purified the TOM complex from Neurospora crassa and analyzed its composition and functional properties. The TOM complex contains a cation-selective high-conductance channel. Upon reconstitution into liposomes, it mediates integration of proteins into and translocation across the lipid bilayer. TOM complex particles have a diameter of about 138 A, as revealed by electron microscopy and image analysis; they contain two or three centers of stain-filled openings, which we interpret as pores with an apparent diameter of about 20 A. We conclude that the structure reported here represents the protein-conducting channel of the mitochondrial outer membrane.

Abstract. Neurofilaments, assembled from NF-L, NF-M, and NF-H subunits, are the most abundant structural elements in myelinated axons. Although all three subunits contain a central, t~-helical rod domain thought to mediate filament assembly, only NF-L selfassembles into 10-nm filaments in vitro. To explore the roles of the central rod, the NH2-terminal head and the COOH-terminal tail domain in filament assembly, full-length, headless, tailless, and rod only fragments of mouse NF-L were expressed in bacteria, purified, and their structure and assembly properties examined by conventional and scanning transmission electron microscopy (TEM and STEM). These experiments revealed that in vitro assembly of NF-L into bona fide 10-nm filaments requires both end domains: whereas the NHE-terminal head domain promotes lateral association of protofilaments into protofibrils and ultimately 10-nm filaments, the COOH-terminal tail domain controls lateral assembly of protofilaments so that it terminates at the 10-nm filament level. Hence, the two end domains of NF-L have antagonistic effects on the lateral association of pmtofilaments into higher-order structures, with the effect of the COOH-terminal tail domain being dominant over that of the NH:-terminal head domain. Consideration of the 21-nm axial beading commonly observed with 10-nm filaments, the approximate 21-nm axial periodicity measured on paracrystals, and recent cross-linking data combine to support a molecular model for intermediate filament architecture in which the 44--46-nm long dimer rods overlap by 1-3-nm head-to-tail, whereas laterally they align antiparallel both unstaggered and approximately half-staggered.

No abstract

By making use o f DIC video microscopy to monitor microtubule motility we have studied the effect o f several M APs (MAP2, M AP2c, tau) on m icrotubulekinesin interactions and microtubule gliding. Of the three M APs tested, M AP2 interferes most strongly with kinesin-dependent microtubule motility.

Aluminum fluoride may be used both to stabilize microtubules and to induce strong binding o f kin esin, thus circumventing the need for taxol and AMP-PNP in kinesin preparations.

We have compared the structures of the filamentous assembly products of different recombinant NF–L polypeptides as seen by the scanning force microscope (SFM) in buffer solution with those obtained by transmission electron microscopy (TEM) of dehydrated specimens. For SFM the filaments were immobilized on a glass surface by photocrosslinking, whereas for TEM the filaments were negatively stained and air-dried. Regarding their length and overall shapes, there is an excellent agreement between the SFM and the TEM data. However, the significant discrepancy of the filament width measured by these two microscopies illustrates the effect of the tip shape with the SFM on width measurements.