No abstract

‘Checkpoint’ controls ensure that the events of the cell cycle are completed in an orderly fashion. For example, such controls delay mitosis until DNA synthesis and repair of radiation‐induced DNA damage are complete. The rad series of radiosensitive fission yeast mutants was examined to identify strains deficient for the DNA damage‐responsive checkpoint control. Five were identified. A characterization of one (rad1‐1) and the wild‐type is presented. The rad1‐1 mutant does not arrest after irradiation, is sensitive to killing by radiation and is not arrested by hydroxyurea, and thus is also deficient for the DNA synthesis‐responsive checkpoint control. The radiosensitivity of the rad1‐1 mutant was greatly reduced when irradiated and maintained for 6 h in a non‐dividing (density inhibited) state, demonstrating that rad1‐1 is repair proficient and radiosensitive only through failure to delay. The checkpoint controls for which rad1 is required appear to regulate G2‐M progression through the activity of cdc2, here implicated in this role by the coincidence of the radiation transition point and the cdc2 execution point.

We

The Src family of tyrosine protein kinases represent an expanding class of closely related intracellular enzymes that participate in the signal transduction pathways of a variety of surface receptors. One of the more surprising aspects of the information relating Src protein kinases to receptor signaling is the apparent diversity of receptor types with which the Src-related enzymes are reported to interact physically or functionally. Traditional biochemical and genetic approaches have yielded much information regarding the interactions between the Src tyrosine protein kinases and other cellular proteins in defined cell types, and emerging technologies, most notably homologous recombination in embryonal stem cells to achieve gene "knockouts," are providing new insights into the participation of the Src-related gene products in signal transduction and development.

The protein tyrosine phosphatase CD45 is a critical component of the T cell antigen receptor (TCR) signaling pathway, acting as a positive regulator of Src family protein tyrosine kinases (PTKs) such as Lck. Most CD45‐deficient human and murine T cell lines are unable to signal through their TCRs. However, there is a CD45‐deficient cell line that can signal through its TCR. We have studied this cell line to identify a TCR signaling pathway that is independent of CD45 regulation. In the course of these experiments, we found that the Syk PTK, but not the ZAP‐70 PTK, is able to mediate TCR signaling independently of CD45 and of Lck. For this function, Syk requires functional kinase and SH2 domains, as well as intact phosphorylation sites in the regulatory loop of its kinase domain. Thus, differential expression of Syk is likely to explain the paradoxical phenotypes of different CD45‐deficient T cells. Finally, these results suggest differences in activation requirements between two closely related PTK family members, Syk and ZAP‐70. The differential activities of these two kinases suggest that they may play distinct, rather than completely redundant, roles in lymphocyte signaling.