Phosphorylation of membrane proteins was studied in suspension culture
d pear (Pyrus communis L. cv, Passe Crassane) cells in response to a h
eat shock of 38 degrees C for 2 h. Protein kinase activity was assayed
in the membrane preparation, and was significantly reduced by heat tr
eatment of the cells. The presence of EGTA in the assay medium reduced
kinase activity about 60% in membranes from unheated cells and 40% in
membranes of heated cells. In vitro labelling of membrane preparation
s with [P-32]ATP for 60 s resulted in some bands more heavily labelled
in heated membranes than in unheated ones. When in vivo labelling of
proteins with P-32 was done during the second hour of heating, there w
ere differences in labelled membrane proteins, compared with membrane
proteins from unheated cells. Incorporation of [S-35]methionine during
the second hour of treatment resulted in the synthesis of a number of
heat shock proteins, two of which had the same M(r) as phosphorylated
proteins. These results indicate that membrane protein phosphorylatio
n may be involved in the response of cells to heat shock.