CELL-SURFACE GLYCOPROTEINS AND PLACE-DEPENDENT SUPPRESSION OF CELL PROLIFERATION

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Evidence is accumulating that cell surface-associated glycoproteins have important roles in regulating primary processes involved in the growth and development of both plants and animals. Edelman (1988) has coined the term "Topobiology" to encompass the range of place-dependent interactions mediated by certain cell surface-associated glycoproteins that are associated with the plasma membrane (CAMs) or excreted into the extracellular matrix (SAMs). These molecules have been implicated in mediating cell-cell recognition/adhesion, cell differentiation, and cell division, among other processes, in animals. We are extending the concept of topobiology to include plant development. Although there are certain to be specific differences in the regulatory molecules operating at the cell surfaces of plants and animals, it seems possible that the mechanisms underlying similar developmental processes in eukaryotic organisms will be similar. In this context, we have begun to explore the possibility that certain glycoproteins, arabinogalactan proteins (ACPs), associated with plant cell surfaces have some functions analogous to the CAMs and/or SAMs in animal development. We are focusing our efforts on the place-dependent control of cell proliferation because of its pivotal role in normal and abnormal development of both plants and animals. In plants, AGPs comprise a large family of hydroxyproline-containing glycoproteins associated with both the plasma membranes and the extracellular matices. We have obtained evidence that AGPs may be involved in suppressing cell division in plants. When plants of one of our "model" species, Gymnocolea inflata, are cultured under one set of conditions, cell division in specific leaf primordia is suppressed. When cultured under a second set of conditions, cell division in these primordia is "desuppressed". Correlated with the desuppression of cell division is the disappearance of "specific" AGPs having buoyant densities between 1.66 and 1.64. Do these AGPs have a role in suppressing a place-dependent suppression of cell division? We will attempt to answer this question by use of CsC1 density gradients and HPLC to isolate AGP fractions that change with suppressed/desuppressed cell division and immunocytochemical procedures to determine if specific AGP moieties are associated with the place-dependent suppression of cell division.