Inhibition of thigmostimulated cell differentiation with RGD-peptides in Uromyces germlings

Abstract

Germlings of the plant pathogenic fungus Uromyces appendiculatus sense and respond to topographical signals of various substrata by undergoing a cell differentiation process that culminates in a structure termed an appressorium. In some cell systems, recognition and mediation of extracellular signals is via transmembrane glycoproteins known as integrins that often exhibit specific affinities to the tripepride sequence Arg-Gly- Asp (RGD) found in several extracellular matrix components. Germlings grown on substrata inductive for appressorium formation in the presence of buffered synthetic peptides containing the amino acid sequence RGD, e.g., RGD, RGDS, GRGD, and GRGDGSPK (0.5-2.0 mM), were inhibited from developing appressoria. Two non-RGD peptides (GGGG and RGES) as well as two RGD peptides (GRGDS and RGDSPASSKP) did not inhibit appressorium formation. Germling growth was not significantly affected by any of the peptides. Furthermore, 0.5 μm diameter micropipettes that are normally inductive for appressorium formation when positioned between the germling apex and the substratum did not induce appressorium formation when coated with the RGD peptide. Silanized micropipettes left uncoated or coated with RGES were inductive for appressorium formation. Those observations lead to the hypothesis that an integrin-like protein may be involved in the process of signaling for initiation of appressorium formation in Uromyces. An RGDSPC-affinity column was used to isolate proteins from Uromyces germlings with affinity to the RGD sequence. Elution with RGD or EDTA, but not with RGES, yielded at least 12 proteins of which one protein (95 kDa) expressed affinity on immunoblots to two different antibodies of β1-integrin; one to the carboxyl-terminus of a synthetic peptide of integrin from chicken, and the other from the amino terminus of integrin from human placenta.