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Kröger Group - Adhesion
Molecular Analysis of Diatom Adhesion
Benthic, pennate diatoms are well known for their adhesion strength to natural and man-made surfaces. Much of the research on diatom adhesion has been driven by the need to develop anti-fouling surfaces that cannot be colonized by diatoms. On the other hand, diatom adhesives are highly attractive as a versatile bonding agent that adheres strongly to both hydrophobic and hydrophilic surfaces under water. Diatom adhesion is accomplished through an adhesion/motility complex that includes an intracellular bundle of actin fibers and on the extracellular side mucilage strands that project through a slit in the cell wall (raphe). The mucilage strands attach the cell to the surface, providing the traction for gliding movement of the cell through an as yet unidentified connection of the mucilage strands to the actin bundle (Fig. 1). As diatoms glide across a substratum they leave behind a trail of the adhesive mucilage that can be easily visualized using the organic dyes Stains All and Alcian Blue, or fluorescently labeled lectins (i.e., carbohydrate binding proteins) and monoclonal antibodies that were raised against extracellular polymeric substances (EPS) from diatoms (Fig. 1A). These data suggest that diatom trails contain acidic glycans that are part of polysaccharide molecules and/or glycoproteins. Previous attempts to characterize the chemical composition of the diatom adhesive mucilage have been inconclusive as the methods employed were not able to distinguish between the non-adhesive organic cell covering and the adhesive mucilage that project from the raphe and becomes deposited as trails. In the proposed project we will utilize biochemical and molecular genetic techniques to identify diatom adhesion molecules, characterize their molecular structures, and analyze their adhesive properties.
Figure 1. Diatom adhesion and motility (A) Immunofluorescence labelling of C. australis trails using monoclonal antibody StF.H4. (J. L. Lind, K. Heimann, E. A. Miller, C. vanVliet, N. J. Hoogenraad, R. Wetherbee (1997) Substratum adhesion and gliding in a diatom are mediated by extracellular proteoglycans. Planta, 203, 213-221.) (B) Longitudinal section in girdle view at the raphe of Navicula cuspidata. Mucilage strands projecting from the raphe contact the substratum. (from: L. A. Edgar, J. D. Pickettheaps (1983) The Mechanism of Diatom Locomotion .1. An Ultrastructural-Study of the Motility Apparatus. Proceedings of the Royal Society of London Series B-Biological Sciences, 218, 331). (C) Model for diatom gliding. The adhesive mucilage is processed and packaged into vesicles within the Golgi complex and transported to the plasma membrane at the region immediately beneath the central nodule along actin filaments, powered by the molecular motor myosin. Once the vesicles are secreted the adhesive mucilage forms strands that extend through the raphe slit attaching the cell the substratum. At their proximal ends the mucilage strands are linked via a continuum of molecules, including transmembrane connector molecule, to the myosin motor, which generates force along the underlying actin cytoskeleton. The reward translocation of the myosin-adhesion complexes results in forward movement of the cell.