SUMMARY The Src-homology 2 (SH2) domain is a protein interaction domain that directs myriad phosphotyrosine (pY) signaling pathways. Genome-wide screening of human SH2 domains reveals that ≈90% of SH2 domains bind plasma membrane lipids and many have high phosphoinositide specificity. They bind lipids using surface cationic patches separate from pY-binding pockets, thus binding lipids and the pY motif independently. The patches form grooves for specific lipid headgroup recognition or flat surfaces for non-specific membrane binding and both types of interaction is important for cellular function and regulation of SH2 domain-containing proteins. Cellular studies with ZAP70 showed that multiple lipids bind its C-terminal SH2 domain in a spatiotemporally specific manner and thereby exert exquisite spatiotemporal control over its protein binding and signaling activities in T cells. Collectively, these studies reveal how lipids control SH2 domain-mediated cellular protein-protein interaction networks and suggest a new strategy for therapeutic modulation of pY signaling pathways.

SUMMARY In organellogenesis of the chloroplast from endosymbiotic cyanobacterium, the establishment of protein targeting mechanisms to the chloroplast should have been pivotal. However, it is still mysterious how these mechanisms were established and how they work in plant cells. Here, we show that AKR2A, the cytosolic targeting factor for chloroplast outer membrane (COM) proteins, evolved from the ankyrin repeat domain (ARD) of the host cell by stepwise extensions of its N-terminal domain, and two lipids monogalactosyldiacylglycerol (MGDG) and phosphatidylglycerol (PG) of the endosymbiont were selected to function as the AKR2A receptor. Structural analysis, molecular modeling and mutational analysis of the ARD identified two adjacent sites for coincidental and synergistic binding of MGDG and PG. Based on these findings, we propose that the targeting mechanism of COM proteins was established using components from both the endosymbiont and host cell through a modification of the protein-protein interacting ARD into a lipid binding domain.

Abstract. Chlorella vulgaris, a unicellular microalgae, exerts various biological effects; however their effect on proliferation signaling pathways in normal cells has not been studied. We investigated the effect of hot water extracts of Chlorella vulgaris (CVE) on cell proliferation and related signaling pathways in rat intestinal epithelial cells (IEC-6). CVE increased the expression of insulin-like growth factor-I receptor (IGF-IR) and the phosphorylation of focal adhesion kinase (FAK) and Src. In addition, CVE induced activation of the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K)/Akt pathways. We verified the increased phosphorylation of extracellular-signal-related kinase (ERK) and Akt and the increased expression of the PI3K regulatory subunit p85. CVE also influenced the canonical Wnt pathway through increased expression of the nuclear β-catenin, cyclin D1. Tyr-397 of FAK mediates interactions with Src homology 2 (SH2) domains in a number of other signaling proteins, including PI3K, PLC-γ, Shc, Grb7, Src and Nck2. Because CVE induced FAK activation, FAK may affect the Wnt pathway. Addition of a FAK inhibitor decreased the expression of nuclear β-catenin, cyclin D1 and c-myc, and increased the expression of cytosolic β-catenin. We conclude that CVE stimulated proliferation of IEC-6 cells via the MAPK, PI3K/Akt and canonical Wnt pathways, and that this affected the canonical Wnt pathway.

3-Phosphoinositides are ubiquitous cellular lipids that play pivotal regulatory roles in health and disease. Generation of 3-phosphoinositides are driven by three families of phosphoinositide 3-kinases (PI3K) but the mechanisms underlying their regulation and cross-talk are not fully understood. Among 3-phosphoinositides, phosphatidylinositol-3,5‐bisphosphate (PI(3,5)P2) remains the least understood species in terms of its spatiotemporal dynamics and physiological function due to the lack of specific probes. By means of spatiotemporally resolved in situ quantitative imaging of PI(3,5)P2 using a newly developed ratiometric PI(3,5)P2 sensor we demonstrate that a special pool of PI(3,5)P2 is generated on lysosomes and late endosomes in response to growth factor stimulation. This PI(3,5)P2 pool, the formation of which is mediated by Class II PI3KC2β and PIKFyve, plays a crucial role in terminating the activity of growth factor-stimulated Class I PI3K, one of the most frequently mutated proteins in cancer, via specific interaction with its regulatory p85 subunit. Cancer-causing mutations of Class I PI3K inhibit the p85-PI(3,5)P2 interaction and thereby induce sustained activation of Class I PI3K. Our results unravel a hitherto unknown tight regulatory interplay between Class I and II PI3Ks mediated by PI(3,5)P2, which may be important for controlling the strength of PI3K-mediated growth factor signaling. These results also suggest a new therapeutic possibility of treating cancer patients with p85 mutations.

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