Cellular and molecular approaches to understanding primary adhesion in Enteromorpha: an overview

The attachment of motile spores of the green alga Enteromorpha to the substratum is an active process involving an irreversible commitment to adhesion and the secretion of an adhesive. This paper provides an overview of the spore adhesion processes and outlines the results of an experimental approach towards the molecular characterisation of the adhesive, based on the use of monoclonal antibody (mAb) technology. Hybridomas were produced to settled spores displaying secreted adhesive. Candidates producing mAbs to putative adhesive were selected using a range of criteria based on cellular localisation, time of secretion and functional inhibition of adhesion. MAb Ent 6 immunolabelled fibrillar material which was secreted during the early stages of adhesion and low (nM) concentrations of this mAb, or its F(ab)₂ fragments, strongly inhibited the attachment of zoospores. A related antibody (Ent 1) also labelled the spore adhesive apparatus, but the antigen appeared to be secreted later during the adhesion process and was predominantly associated with the developing cell wall. Ent 1 also inhibited settlement in spore adhesion assays but the effect was most pronounced at later time points which suggests that this antigen does not have a role in the earliest stages of adhesion. Immunolocalisation showed that both antigens were absent from the cytoplasm or organelles of vegetative tissue but labelled the vegetative cell wall, suggesting a relationship between cell wall components and materials involved in primary adhesion. Both mAbs labelled the Golgi region of settled spores, suggesting continued synthesis of both antigens after adhesion. Both mAbs recognised a 110 kDa N‐linked polydisperse and heterogeneous glycoprotein in extracts of swimming spores under denaturing conditions. In native form the antigens behaved as high molecular weight aggregates (M_r>1.3 × 10⁶). The antigens became progressively insoluble after zoospore attachment. Taken together, the data suggest that the two antibodies recognise closely related, polydisperse, self‐aggregating cell wall glycoproteins in which there is some structural variation to suit alternative roles in primary adhesion and cell wall formation. The two mAbs Ent 1 and Ent 6 partially discriminate between these structural and functional variants. A model for zoospore adhesion is discussed in which adhesion is viewed as an extension of cell wall synthesis, with cross‐links between glycoproteins and other cell wall matrix components providing a strong physical continuum between the cell and the adhesive at the substratum interface.