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J P Chalcroft

Publications and source records attributed to J P Chalcroft.

8 recordsLinked to original sources
PubMed1987

Structure of the porin from a bacterial stalk.

The stalks (hyphae) of a prosthecate bacterium, directly sampled from the water surface of a hot pond, show extended regular patterns on their envelope in the electron microscope. Image processing revealed a structure of the crystalline complexes which is very similar to the gross morphology of the Escherichia coli porins OmpC and OmpF. The natural two-dimensional crystal of the outer membrane protein has p3 symmetry and a lattice constant of 7.95 nm. The three-dimensional structure of the stalk porin has been determined to an almost isotropic resolution of 1.7 nm. The reconstruction revealed a complex network of channels within the membrane matrix with a triplet of pores merging into a common outlet, similar to the structure of the E. coli porin OmpF in reconstituted membranes. In addition, a blindly ending pore exists which appears to be connected to the continuous pores via small channels. The significance of the regularly arrayed porin cylinders with respect to the shape and function of the stalks is discussed.

Bacterial Outer Membrane Proteins↗
PubMed1985

Considerations for the quantitative analysis of coated reliefs.

The contrast visible in a coated relief specimen viewed by transmission electron microscopy is generated by the angular difference between the direction of contrast coat evaporation and the direction from which the relief is viewed. This contrast can often be exploited for the computer reconstruction of specimen relief from optical density data if the contrast angle is known. If an additional backing coat is present, the relief must be viewed along the direction of backing coat evaporation, and may therefore require reorientation by the use of a rotate/tilt holder. Despite attainment of correct orientation, undercutting - multiple intersections of imaging electrons with different regions of contrast coat - may yet prevent valid reconstruction. Although the danger of experiencing undercutting is high, "vertical" coating, in conjunction with minimal tilting, could be used when maximum relief information is required from a unique specimen. Bidirectional contrasting may also be useful for increasing the information capacity of relief coats.

Computers↗
PubMed1970

An interpretation of liver cell membrane and junction structure based on observation of freeze-fracture replicas of both sides of the fracture.

A modification of the freeze-fracturing technique to permit observation of replicas of both sides of the fracture is described. It has been used to study mouse liver cell membrane structure. Membranes break to give two faces with three-dimensional complementarity, although there is some small-scale mismatching which is discussed. Since the two distinctive sets of membrane faces are complementary sets, they cannot be the two outside surfaces. In particular, structures (such as particles) seen on these faces are within the membrane. It is not possible from this work to say precisely where the fracture plane goes with respect to a plasma membrane, only that it must be close to the interface between membrane and cytoplasm, or at that interface. Models, consistent with the appearance of the matching replicas, are derived for three regions of the plasma membrane: (a) The nonjunctional plasma membrane, which contains many scattered particles. Except for these particles, the otherwise flat fracture face is not at variance with a bimolecular leaflet structure. (b) Gap junctions. Each of the two membranes comprising a gap junction contains a close-packed array of particles. (c) Tight junctions. Here membranes have ridges within them.

Animals↗