A multimethod approach was used to characterize unicellular green algae that were traditionally assigned to the genus Chlorella Beijerinck and to resolve their phylogenetic relationships within the Chlorophyta. Biochemical, physiological, and ultrastructural characters, together with molecular data such as DNA base composition and DNA hybridization values, were compared with a molecular phylogeny based on complete 18S rRNA sequences. Our results show that Chlorella taxa are dispersed over two classes of chlorophytes, the Trebouxiophyceae and the Chlorophyceae. We propose that only four species should be kept in the genus Chlorella (Chlorophyta, Trebouxiophyceae)

To investigate the function of c-Jun during skin development and skin tumor formation, we conditionally inactivated c-jun in the epidermis. Mice lacking c-jun in keratinocytes (c-jun(Deltaep)) develop normal skin but express reduced levels of EGFR in the eyelids, leading to open eyes at birth, as observed in EGFR null mice. Primary keratinocytes from c-jun(Deltaep) mice proliferate poorly, show increased differentiation, and form prominent cortical actin bundles, most likely because of decreased expression of EGFR and its ligand HB-EGF. In the absence of c-Jun, tumor-prone K5-SOS-F transgenic mice develop smaller papillomas, with reduced expression of EGFR in basal keratinocytes. Thus, using three experimental systems, we show that EGFR and HB-EGF are regulated by c-Jun, which controls eyelid development, keratinocyte proliferation, and skin tumor formation.

The protein-tyrosine phosphatase SHP-1 is a negative regulator of multiple signal transduction pathways. We observed that SHP-1 effectively antagonized Srcdependent phosphorylations in HEK293 cells. This occurred by dephosphorylation of Src substrates, because Src activity was unaffected in the presence of SHP-1. One reason for efficient dephosphorylation was activation of SHP-1 by Src. Recombinant SHP-1 had elevated activity subsequent to phosphorylation by Src in vitro, and SHP-1 variants with mutated phosphorylation sites in the C terminus, SHP-1 Y538F, and SHP-1 Y538F,Y566F were less active toward Src-generated phosphoproteins in intact cells. A second reason for efficient dephosphorylation is the substrate selectivity of SHP-1. Pull-down experiments with different GST-SHP-1 fusion proteins revealed efficient interaction of Src-generated phosphoproteins with the SHP-1 catalytic domain rather than with the SH2 domains. Phosphopeptides that correspond to good Src substrates were efficiently dephosphorylated by SHP-1 in vitro. Phosphorylated "optimal Src substrate" AEEEIpYGEFEA (where pY is phosphotyrosine) and a phosphopeptide corresponding to a recently identified Src phosphorylation site in p120 catenin, DDLDpY 296 GMMSD, were excellent SHP-1 substrates. Docking of these phosphopeptides into the catalytic domain of SHP-1 by molecular modeling was consistent with the biochemical data and explains the efficient interaction. Acidic residues N-terminal of the phosphotyrosine seem to be of major importance for efficient substrate interaction. Residues C-terminal of the phosphotyrosine probably contribute to the substrate selectivity of SHP-1. We propose that activation of SHP-1 by Src and complementary substrate specificities of SHP-1 and Src may lead to very transient Src signals in the presence of SHP-1. The SH21 domain PTP SHP-1 regulates multiple signal transduction events by dephosphorylation (1-3). These comprise signaling of cytokine receptors such as the erythropoietin receptor (4), and the interleukin-3 receptor (5), and of receptor tyrosine kinases such as c-Kit (6 -8), the colony-stimulating factor-1 receptor (9, 10), and the epithelial kinase Ros (11). SHP-1 modulates also the function of immunoreceptors (12), and cytoplasmic tyrosine kinases such as Lck (13,14). In these and many other cases, SHP-1 regulates signaling in a negative manner. In other pathways, SHP-1 may also exert a positive function. Thus, a role of SHP-1 for differentiation of glia cells (15), and for Ras-dependent activation of mitogen-activated protein kinase (16) have been reported. Also, SHP-1 has the capacity to activate Src kinase by dephosphorylation of the inhibitory phosphotyrosine in the Src C terminus and may thus stimulate Src-dependent phosphorylations in certain cell types (17). SHP-1 can translocate into the nucleus, however, its nuclear substrates are still elusive (18 -21). Among the recently identified substrates of SHP-1 is p120 ctn (22), a cytoplasmic protein that possesses armadillo-like repeats, participates in ...

Activation of the SH2 domain-possessing protein-tyrosine phosphatase SHP-1 by acidic phospholipids as phosphatidic acid (PA) has been described earlier and suggested to participate in regulation of SHP-1 activity toward cellular substrates. The mechanism of this activation is poorly understood. Direct binding of phosphatidic acid to recombinant SHP-1 could be demonstrated by measuring the extent of [14C]PA binding in a chromatographic assay, by measuring the extent of binding of SHP-1 to PA-coated ELISA plates or silica beads (TRANSIL), and by spectroscopic assays employing fluorescently labeled PA liposomes. In addition to PA, phosphatidylinositol 3,4,5-trisphosphate (PIP3), dipalmitoylphosphatidylglycerol, phosphatidylinositol 4,5-bisphosphate, and phosphatidylserine (PS) were found to bind to SHP-1, albeit to a lesser extent. A high-affinity binding site for PA and PIP3 was mapped to the 41 C-terminal amino acids of SHP-1. This site was absent from the related protein-tyrosine phosphatase SHP-2 and conferred activation of SHP-1 by PA toward two different substrates at low lipid concentrations. A SHP-1 mutant missing this binding site could, however, still be activated toward phosphorylated myelin basic protein as a substrate at high PA concentrations. This activation is likely to be mediated by a second, low-affinity binding site for PA in the N-terminal part of SHP-1 within the SH2 domains. High-affinity phospholipid binding to the C-terminus of SHP-1 may present a specific mechanism of regulating activity and/or cellular localization.