Requirement of Src kinase Lyn for induction of DNA synthesis by granulocyte colony-stimulating factor

Treatment of cells with granulocyte colony-stimulating factor (G-CSF) leads to tyrosine phosphorylation of cellular proteins. G-CSF stimulates both the activation of protein tyrosine kinases Lyn, Jak1, and Jak2 and the association of these enzymes with the G-CSF receptor. Wild-type, lyn-deficient, and syk-deficient chicken B lymphocyte cell lines were transfected with the human G-CSF receptor, and stable transfectants were studied. G-CSF-dependent tyrosyl phosphorylation of Jak1 and Jak2 occurred in all three cell lines. Wild-type and syk-deficient transfectants responded to G-CSF in a dose-responsive fashion with increased thymidine incorporation, but none of the clones of lyn-deficient transfectants did. Ectopic expression of Lyn, but not that of c-Src, in the lyn-deficient cells restored their mitogenic responsiveness to G-CSF. Ectopic expression in wild-type cells of the kinase-inactive form of Lyn, but not of the kinase-inactive form of Jak2, inhibited thymidine incorporation in response to G-CSF. These studies show that the absence of Lyn results in the loss of mitogenic signaling in the G-CSF signaling pathway and that activation of Jak1 or Jak2 is not sufficient to cause mitogenesis.     

Osmoprotective compounds in the Plumbaginaceae: a natural experiment in metabolic engineering of stress tolerance

In common with other zwitterionic quarternary ammonium compounds (QACs), glycine betaine acts as an osmoprotectant in plants, bacteria, and animals, with its accumulation in the cytoplasm reducing adverse effects of salinity and drought. For this reason, the glycine betaine biosynthesis pathway has become a target for genetic engineering of stress tolerance in crop plants. Besides glycine betaine, several other QAC osmoprotectants have been reported to accumulate among flowering plants, although little is known about their distribution, evolution, or adaptive value. We show here that various taxa of the highly stress-tolerant family Plumbaginaceae have evolved four QACs, which supplement or replace glycine betaine-namely, choline O-sulfate and the betaines of beta-alanine, proline, and hydroxyproline. Evidence from bacterial bioassays demonstrates that these QACs function no better than glycine betaine as osmoprotectants. However, the distribution of QACs among diverse members of the Plumbaginaceae adapted to different types of habitat indicates that different QACs could have selective advantages in particular stress environments. Specifically, choline O-sulfate can function in sulfate detoxification as well as in osmoprotection, beta-alanine betaine may be superior to glycine betaine in hypoxic saline conditions, and proline-derived betaines may be beneficial in chronically dry environments. We conclude that the evolution of osmoprotectant diversity within the Plumbaginaceae suggests additional possibilities to explore in the metabolic engineering of stress tolerance in crops.