Extracellular Components Implicated in the Stationary Organization of the Actin Cytoskeleton in Mesophyll Cells of Vallisneria

In mesophyll cells of Vallisneria gigantea Graebner, an aquaticangiosperm, the association of the plasma membrane with thecell wall at the end wall has been reported to be indispensablefor the mechanism that maintains the stationary organizationof the bundles of microfilaments (MFs) [Masuda et al. (1991)Protoplasma 162: 151]. To identify putative extracellular componentsthat might play a crucial role in this mechanism, we examinedthe 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 requiredfor adhesion to the substratum at sites of focal contacts. Within24 h, both peptides (at concentrations of 1–15 mM) inducedextremely abnormal patterns of cytoplasmic streaming, as wellas the striking disruption of the arrangement of bundles ofMFs. GRGESP and ARYEEI peptides, used as controls, had no detectableeffects. Immunofluorescence microscopy revealed that polyclonalantibodies against the ARYDEI peptide bound to the cell wallsof mesophyll cells while a preimmune serum did not. Westernblotting analysis demonstrated that the antibodies recognizedpolypeptides of 54 kDa and 27 kDa in an extract of total proteinsfrom the leaves of Vallisneria. The results suggest that someextracellular protein(s), with a conserved RGD or RYD motifin its amino acid sequence, might be involved in the maintenanceof the stationary organization of the bundles of MFs. (Received August 13, 1996; Accepted January 23, 1997)