Plant cell surface adhesion proteins

NaCl adapted tobacco cells develop tight zones of adhesion between the plasma membrane and cell wall, revealed by concave plasmolysis in osmotic solutions. Unadapted cells exhibit mostly convex plasmolysis and little adhesive character. Protoplasts isolated from the adapted cells retain the complementary adhesive character and adhere tightly to each other, whereas protoplasts from unadapted cells do not. The hexapeptide gly-arg-gly-asp-ser-pro, in which the arg-gly-asp represents the integrin-binding domain of several animal extracellular matrix proteins, specifically blocks adhesion of the protoplasts. Tobacco proteins immunologically related to human vitronectin were found in cell walls and membranes of unadapted and NaCl adapted cells, but the total extractable vitronectin-like protein was enriched in the adapted cells. Tobacco proteins immunologically related to human fibronectin were found in membranes and cell walls of NaCl adapted cells but not in those from unadapted cells. Similar to animal vitronectin, one of the tobacco vitronectin-like proteins (PVN1) was found to bind to glass surfaces and heparin. Glass bead affinity purified PVN1 has weak activity in the baby hamster kidney cell spreading assay. This plant adhesion protein is expressed in all tissues examined but is most abundant in roots and salt-adapted cultured cells. Immuno-gold labeling indicated that PVN1 is localized primarily in the inner layer of cell wall. Partial amino acid sequence of PVN1 revealed no similarity with vitronectin, but is instead highly homologous with the translational elongation factor-1$\alpha$ (EF-1$\alpha$). Immunological cross-reactivity between vitronectin-like proteins and EF-1$\alpha$ supports the hypothesis that PVN1 is related to EF-1$\alpha$. Cell surface arabinogalactan-proteins (AGPs) were also examined to ascertain their role in cell expansion and plasma membrane-cell wall adhesion. Unadapted cells contain a very high level of AGPs on the plasma membrane, which amount to 0.16 $\mu$g/$\mu$g membrane protein. In contrast, AGPs were virtually undetectable on the plasma membrane of NaCl adapted cells. These data are consistent with the proposed role of AGPs as lubricant and support the hypothesis that AGPs participate in cell expansion.