Proteomic study of microsomal proteins reveals a key role for Arabidopsis annexin 1 in mediating heat stress-induced increase in intracellular calcium levels.

To understand the early signaling steps in the response of plant cells to increased environmental temperature, 2-D difference gel electrophoresis was used to study the proteins in microsomes of Arabidopsis seedlings that are regulated early during heat stress. Using mass spectrometry, 19 microsomal proteins that showed an altered expression level within 5 min after heat treatment were identified. Among these proteins, annexin 1 (AtANN1) was one of those up-regulated rapidly after heat-shock treatment. Functional studies show loss-of-function mutants for AtANN1 and its close homolog AtANN2 were more sensitive to heat-shock treatment, whereas plants overexpressing AtANN1 showed more resistance to this treatment. Correspondingly, the heat-induced expression of heat-shock proteins and heat-shock factors is inhibited in ann1/ann2 double mutant, and the heat-activated increase in cytoplasmic calcium concentration ([Ca(2+)]cyt) is greatly impaired in the ann1 mutant and almost undetectable in ann1/ann2 double mutant. Taken together these results suggest that AtANN1 is important in regulating the heat-induced increase in [Ca(2+)]cyt and in the response of Arabidopsis seedlings to heat stress.