No abstract

Bed conditions (micro‐relief, textural associations and packing structural arrangements) in the gravel‐bed channel of Seale's Brook are shown to be closely interrelated; various categories are identified and related to mode of bed material transport and deposition. Entrainment of bed material, commonly treated as a simple function of particle weight and channel hydraulics, is also shown to be strongly affected by varying and variable bed conditions. In particular, the classic concept of competence appears to be of restricted utility in such channels; resistance of bed material to fluid drag and to particle impact is augmented, over large parts of the channel bed, by its interlocking structure, made possible by the wide range in particle calibre, and by the characteristic disc and blade shapes of the slate debris. Particle mobility, as indicated by distance of travel of labelled bed material, is only partly a function of particle weight; indeed, although particle mobility decreases from small pebbles to large cobbles, it also decreases for the finest bed material (very small pebbles). This appears to be explicable, partly in terms of the ease of entrainment (and duration of travel), and, partly in relation to the ease of transport of material over an uneven channel bed surface. Particle mobility is greatest for material in open and infilled structures and smallest for sediment in tight structural arrangements. Local bed slope also exerts an influence on the probability of particle entrainment and on particle mobility. The findings emphasize the need for combining sedimentological and engineering approaches to bed material transport in coarse‐bedded channels, and, at the same time, illustrate some of the reasons for the existence of indeterminacy in the modelling of bed‐material transporting processes.

ABSTRACT. Hydrometric and sediment data collected by Environment Canada in the Mackenzie Basin during the period 1974-94 have been analyzed to produce detailed estimates of sediment inputs to the Mackenzie Delta, based largely on sediment rating equations. The mean annual sediment supply to the delta is determined as 128 million tonnes (Mt), of which about 4 Mt is sandy bed material moved in by the Mackenzie River itself. Virtually all of this sediment (more than 99%) is supplied to the delta during the May-October period, the peak months being May (27%), June (36%), and July (19%). About 17% of the fine-sediment load is supplied by the Peel River; the rest is delivered by the Mackenzie. The largest single contributor to the Mackenzie River wash load (103 Mt) is the Liard River (41 Mt). The preliminary estimate of the contribution of the other west-bank tributaries, in combination, is about 36 Mt, though this figure is probably too low. The precision of these estimates using the sediment rating approach (compared to time-integration during months with reasonable sampling frequency) is about 10% for the mean monthly sediment loads and about 5% for the mean annual sediment load during the 1974-94 period. The absolute accuracy of sediment load estimates is more difficult to assess because published flow data for delta inflow stations are acknowledged to be much less reliable for the spring breakup period than for other times of the year.

No abstract

Although the S. aureus and human proteins have unrelated amino acid sequences, secondary structure composition, and cation requirements for effective ligand binding, both proteins bind at multiple sites within one collagen molecule, with the sites in collagen varying in their affinity for the adherence molecule. We propose that (i) these evolutionarily dissimilar adherence proteins recognize collagen via similar mechanisms, (ii) the multisite, multiclass protein/ligand interactions observed in these two systems result from a binding-site trench, and (iii) this unusual binding mechanism may be thematic for proteins binding extended, rigid ligands that contain repeating structural motifs.Collagen polypeptides are largely composed of repeats of the GPX tripeptide and associate to form triple-helical monomers. These monomers combine into macroscopic fibers. Prokaryotic and eukaryotic cells bind collagen via receptors on their cell surfaces (1-6). We now hypothesize that to accommodate such an unusually shaped ligand, the collagen-binding surface proteins of these cells must adopt an atypical binding-site structure.Bacterial pathogens utilize this interaction as a means of adherence to collagenous host tissues. Some Staphylococcus aureus strains express an adhesin, Cna, 1 of the MSCRAMM class that binds collagen (1, 7-13). Cna from S. aureus FDA 574 is depicted in Fig. 1a: it contains two major domains, A and B, in addition to features characteristic of cell-surface proteins on Gram-positive bacteria (11). The collagen-binding site has been localized within the Cna A domain (12). Binding analyses demonstrate that (i) a synthetic peptide mimicking a short sequence of the A domain can inhibit collagen binding to S. aureus (8); (ii) the A domain/collagen interaction involves more than one affinity class and multiple sites of contact within a single collagen molecule (8, 10); and (iii) the B domain does not alter the collagen binding ability of the A domain (13).The crystal structure of a truncated form of the Cna A domain reveals a binding-site "trench" on one face of the protein. In molecular modeling studies, this trench was found to accommodate a triple-helical peptide that mimics the collagen structure. Symersky et al. (7) noted that this trench complemented well the structure of a collagen triple helix and binding studies of site-specific mutants of the S. aureus Cna truncate revealed that (i) no single residue or area within the trench was responsible for collagen binding, but rather, a number of contacts contributed to the protein/collagen interaction and (ii) this binding-domain truncate bound to multiple sites along a collagen molecule. The affinity of Cna for an individual site within collagen may be the consequence of the number of "good" and "bad" contacts within the binding trench.Binding of eukaryotic cells to collagen serves not only as a mechanism of tissue adherence, but also may induce a complex signaling cascade in the cell. Attachment of eukaryotic cells to the extracellular matrix is primarily medi...