A β4 integrin-like protein co-localises with a phosphotyrosine containing protein in the oomycete Achlya bisexualis: Inhibition of tyrosine phosphorylation slows tip growth

We present immunocytochemical data that indicate the presence of, and a close association between β4 integrin-like proteins and proteins containing phosphorylated tyrosine residues in the oomycete Achlya bisexualis. When hyphae were plasmolysed, these proteins were present in wall-membrane attachment sites where there was also F-actin. A combination of immunoblots, ELISA, and a coupled enzyme assay suggest that phosphorylation may occur by both autophosphorylation and through the action of a tyrosine kinase. Tyrphostins, which are inhibitors of tyrosine kinases, abolished the anti-phosphotyrosine staining, inhibited the kinase activity, slowed tip growth and affected the organisation of the actin cytoskeleton, in a dose-dependent manner. By analogy with the integrins and associated kinases of the metazoa we suggest that these proteins could contribute to the process of tip growth by providing a means of bidirectional signaling between the cell wall and the cytoplasm.