Extracellular components implicated in the stationary organization of the actin cytoskeleton in mesophyll cells of Vallisneria

In mesophyll cells of Vallisneria gigantea Graebner, an aquatic angiosperm, the association of the plasma membrane with the cell wall at the end wall has been reported to be indispensable for the mechanism that maintains the stationary organization of the bundles of microfilaments (MFs) [Masuda et al. (1991) Protoplasma 162: 151]. To identify putative extracellular components that might play a crucial role in this mechanism, we examined the effects of two exogenously applied synthetic hexapeptides, GRGDSP and ARYDEI, which include an RGD and an RYD motif, respectively. The RGD motif is known as a recognition site in molecules required for adhesion to the substratum at sites of focal contacts. Within 24 h, both peptides (at concentrations of 1-15 mM) induced extremely abnormal patterns of cytoplasmic streaming, as well as the striking disruption of the arrangement of bundles of MFs. GRGESP and ARYEEI peptides, used as controls, had no detectable effects. Immunofluorescence microscopy revealed that polyclonal antibodies against the ARYDEI peptide bound to the cell walls of mesophyll cells while a preimmune serum did not. Western blotting analysis demonstrated that the antibodies recognized polypeptides of 54 kDa and 27 kDa in an extract of total proteins from the leaves of Vallisneria. The results suggest that some extracellular protein(s), with a conserved RGD or RYD motif in its amino acid sequence, might be involved in the maintenance of the stationary organization of the bundles of MFs.