R.D.B. Fraser

Chain conformation in the collagen molecule.

Abstract Quantitative X-ray diffraction data have been collected from stretched kangaroo tail tendon and used to test models for the conformation of the polypeptide chains in the collagen molecule. The magnitude of the unit twist of the molecular helix was estimated to be 107.1 ° ± 0.6 °, which is close to the value expected for a helix with ten units in three turns. The intensity data were used to carry out a linked-atom least-squares refinement of models based on two possible interchain hydrogen bonding schemes suggested by Rich & Crick (1955, 1961). No stereochemically acceptable solution could be found for the hydrogen bonding scheme of model I, but a stereochemically satisfactory solution was found for the scheme of model II which gave a crystallographic R factor of 0.272.

Molecular conformation and packing in collagen fibrils

New X-ray diffraction data have been collected from specimens of tendon collagen stained with phosphotungstic acid. Measurements of the positions of the Bragg reflections associated with the crystalline lattice provide, for the first time, a complete description of the unit cell. A strong band of intensity in the molecular transform associated with the pitch of the molecular helix can be identified and a detailed analysis of the intensities and positions of the Bragg reflections in this band has been carried out. The principal conclusions are that the portions of the collagen molecule that contribute to these reflections have a common direction; that they have a length very much less than that of a complete molecule; that the paths of the individual portions through the crystal are incompatible with a completely straight molecule, and that the molecule is therefore crimped. No evidence was obtained for a second series of Bragg reflections attributable to a second set of molecular portions linking the first set, and it is concluded that the linking set is more mobile and subject to positional variation from cell to cell. The most plausible explanation of our finding is that the first set corresponds to the portions of the molecules in the overlap zone and the second set to the portions in the gap zone. A detailed analysis of the Bragg reflections in the strong band of intensity associated with the pitch of the molecular helix has provided information about the relative azimuthal orientations and the lateral positions in the unit cell of the five molecular segments in the overlap zone. None of the existing models for fibril structure accounts satisfactorily for all the results obtained in the present studies and alternative models are developed and tested.

Structure of α-keratin: Structural implication of the amino acid sequences of the type I and type II chain segments

Abstract Two amino acid sequences from potentially helical fragments of low-sulphur proteins from α-keratin have been analysed computationally and periods 9.4 and 28 residues long noted in the axial disposition of charged residues. Ionic interactions between chains have also been calculated and these indicate a preference for the helical fragments to aggregate in parallel with zero shift between chains in a manner essentially identical to that found for α-tropomyosin.

Structure of the α-keratin microfibril

Quantitative measurements of the intensity of the meridional reflections in the X-ray-diffraction pattern of α -keratin are shown to be consistent with a microfibril structure in which a surface lattice with an axially projected period around 200 A is subject to a periodic interruption with an axially projected period of 470 A. Taken in conjunction with recent evidence on the chemical structure of α-keratin and other inter-mediate filaments this finding enables an elaboration to be made of a model proposed earlier by RDB Fraser, TP MacRae, & E Suzuki (3. Mol. Biol.108, 435–452, 1976.) for the α-helical framework of the microfibrii. The disposition and connectivity of the helical segments suggested here provides a straightforward explanation of a number of recent physicochemical and electron-microscopical observations on intermediate filaments and provides a starting point for the development of models for the framework of other intermediate filaments.

Intermediate filament structure: 1. Analysis of IF protein sequence data

Abstract Amino acid sequence data for intermediate filament proteins have been analysed with a view to identifying structurally invariant segments and determining their likely secondary structure. The sequences in these segments have also been analysed for periodic distributions of particular types of residue. The results support the classification of intermediate filament proteins into three main groups and also reinforce the concept of a molecular structure with a central domain of coiled-coil segments, together with essentially non-helical N-terminal and C-terminal domains of variable size and composition. Regions exhibiting the greatest homology between the three types of IF chain are identified and significant variation in charged residue disposition along the length of individual chains is noted. The conservation in all IF protein chains of specific sites of coiled-coil rope interruption are discussed in terms of the probable molecular structure. Stabilizing ionic interactions between coiled-coil chain segments have been investigated quantitatively as a function of the relative chain stagger. In all cases and calculations favour ropes in which the constituent chains are in-register and parallel rather than antiparallel.

The coiled-coil model of α-keratin structure

Quantitative measurements of the X-ray diffraction pattern of dry porcupine quill tip are reported for both low- and high-angle regions. There is a considerable measure of agreement with the intensity transform calculated for the coiled-coil rope model proposed by Crick but precise comparison is handicapped by interference effects due to macromolecular organization. A method of allowing for these effects is devised for the equator and good agreement obtained with the observed pattern.

Disulphide bonding in α-keratin

Abstract Epidermal appendages such as hair and nail contain a complex mixture of proteins known as α-keratin. Their chemical inertness, and many of their physical characteristics, are governed by the high content of disulphide linkages between the protein chains. Sufficient data on the amino acid sequences of the constituent proteins are now available to provide insights into the nature and distrubution of these linkages in the three-dimensional structure of the α-keratin complex. From stereochemical considerations constraints on the formation of disulphide linkages in and between two-strand coiled-coil ropes were identified. In earlier studies certain staggers between rod domain segments were shown to be favoured on the basis of ionic interactions and in the present studies one of these was also found to have a high potential for disulphide bond formation.

The structure of feather keratin

Abstract When feather rachis is pressed in steam the keratin molecules are partially denatured and the x-ray diffraction pattern, which is greatly simplified, indicates that the microfibrils have a helical structure with four units per turn. Quantitative infra-red and x-ray data have been collected which suggest that this simplified pattern originates from about one third of the material in the form of a central core or framework which is resistant to denaturing agents. A model for this core consisting of a helical array of β-crystallites is shown to give quantitative agreement with the observed x-ray pattern over a wide range of angles. Each crystallite contains two sheets symmetrically disposed about the fibre axis and the sheets are distorted to conform to a ruled surface of opposite sense to the primitive helix.

Poly-L-alanylglycine

The sequential polymer poly- L -alanylglycine has been synthesized and shown to form a β-structure similar to that found in Bombyx mori silk fibroin. The chain arrangement is such that all the glycyl residues project on one side of the sheet and the alanyl residues on the other. The sheets are packed in pairs with the glycyl surfaces in contact. Structure factor calculations suggest that the inter-sheet distance across the glycyl-glycyl contact is significantly greater than in the model for silk fibroin proposed by Marsh, Corey & Pauling (1955) .

X-ray diffraction patterns of α-fibrous proteins

A method is described which enables allowance to be made for the effect of surrounding material on the Fourier transform of a coiled coil. Using this method, distributions of intensity on the equator and near-equatorial layer lines are calculated for the wet and dry states, and compared with the observed distributions in oyster muscle, porcupine quill, rabbit psoas muscle and tropomyosin.