A SUBUNIT MODEL FOR THE TROPOCOLLAGEN MACROMOLECULE

Self-assembly of biological structures

Images

Biochemical and physicochemical characterization of collagen-synthesizing polyribosomes

Synthesis of collagen on polyribosomes has been demonstrated in vitro in chick embryo corium by radioisotope incorporation, zone centrifugation through sucrose gradients, and analytical ultracentrifugation. Collagen synthesis was associated with polyribosomes ranging in size, as reflected by their sedimentation constants, from about 180S to approximately 1600S. Most of the newly formed collagen, hydroxyproline, was present on the largest polyribosome aggregates ( approximately 350-1600S), but small polyribosomes ( approximately 180-200S) also contained collagen. On the basis of the proline-(14)C/hydroxyproline-(14)C ratios and the disrupting effect of collagenase, the proposal is made that the 350-1600S polyribosomes from this tissue are involved predominantly in collagen synthesis. The large polyribosomes are disrupted extensively by collagenase but only partially by ribonuclease and trypsin. Therefore, it appears that they are stabilized by the interaction of newly forming collagen chains. Evidence is presented consistent with the hypothesis that these large polyribosomes are formed by the aggregation of small polyribosomes (180-200S) through the interaction of collagen polypeptides. It is suggested that these small polyribosomes might be involved in the synthesis of subunits of the collagen alpha chain.

Anisometric transport of ions and particles in anisotropic tissue spaces

The results of time-lapse measurements and electron microscopic observations on the diffusion of histological dyes, colloidal particles, and heavy metal salts in excised chicken breast tendon are reported. In all cases, the transport was found to be anisometric, the extent of the spreading being much greater parallel than perpendicular to the collagen fibers. The diffusion of colloidal gold was shown to be governed by a random diffusion process, with coefficients of 3 to 5 x 10(-7) and 1 to 2 x 10(-7) cm(2)/sec for the parallel and perpendicular directions, respectively; the anisotropy was attributed to steric hindrance. In the diffusion of uranyl nitrate, a sharp boundary appeared at the leading edge of the diffusate and advanced at a rate proportional to the square root of time. Electron micrographs showed uranyl nitrate clusters localized in space on the surface of the collagen fibrils and tightly bound to the polar amino acid regions of the macromolecule. A model was proposed involving diffusion with attrition, and predicted a sharp boundary advancing proportionally to the square root of time and to the 0.65 power of the initial diffusate concentration. Application of the model to the experimental results for uranyl nitrate gave a diffusion coefficient of 10 x 10(-7) and 4 x 10(-7) cm(2)/sec for the parallel and perpendicular directions, respectively, and a possible explanation of this large difference was advanced. The importance of anisometric transport in anisotropic tissues was indicated.

Polyribosomes and cisternal accumulations in root cells of radish

The zone of root hair formation of seedling radish roots, Raphanus sativus L., was studied by phase-contrast and electron microscopy. Localized dilations of the endoplasmic reticulum, which contained a moderately dense proteinaceous material, were found to be a common component of the cytoplasm in cells of the epidermis and cortex. The surfaces of these dilations were covered with polyribosomes in discrete coils commonly composed of 15 to 17 ribosomes. The function of these structures and the fate of the material accumulated in them are unknown. Their similarity to structures described in some types of animal cells is discussed.

Collagen: Structural Studies Based on the Cleavage of Methionyl Bonds

Eighteen peptides from the alpha1 fraction and eight from the alpha2 fraction of rat skin collagen cleaved with cyanogen bromide have been identified by chromatography on carboxymethyl cellulose. The isolated peptides show wide differences in amino acid composition, and their molecular weights vary from several thousand to about 23,000. The data indicate that there are two different chains in the alpha1 fraction, demonstrating the nonidentity of all three alpha-chains. The primary structure of each chain appears to be unique throughout its length.