Growth stimulation by serum in Entamoeba histolytica is associated with protein tyrosine dephosphorylation

Very little protein tyrosine phosphorylation was observed in growing (expon ential-phase) Entamoeba histolytica cells by immunoblotting and quantitativ e immunofluorescence. After 1 h of serum deprivation, two proteins (42 and 38 kDa in SDS-PAGE) were tyrosine phosphorylated and two more proteins (96 and 63 kDa) also showed tyrosine phosphorylation when examined after 4 h of serum deprivation. Intense enhancements of anti-phosphotyrosine immunofluo rescence levels were observed during this period of serum withdrawal. Membr ane-associated tyrosine kinase activity reached a peak (3.5-fold increase) 1 h after serum deprivation and decreased thereafter reaching a basal level by 2 h of serum deprivation. Interestingly, tyrosine kinase activities rem ained unaffected by serum stimulation (2-60 min) of serum-deprived cells. A lso, during this period of serum stimulation tyrosine phosphorylated protei ns of serum-deprived cells were dephosphorylated. Tyrosine phosphatase acti vities were suppressed during serum deprivation and on serum addition to se rum-deprived cells tyrosine phosphatase activities increased significantly. Our data attest that protein tyrosine phosphorylation was associated with growth inhibition of E. histolylica and serum stimulation of E. histolytica produced tyrosine phosphatase activation and protein tyrosine dephosphoryl ation. (C) 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.