Summary Glucagon plays an important role in glucose homeostasis by regulating hepatic glucose output in both normo- and hypo-glycemic conditions. In this study, we created and characterized α-cell specific insulin receptor knockout (αIRKO) mice to directly explore the role of insulin signaling in the regulation of glucagon secretion in vivo. Adult male αIRKO mice exhibited mild glucose intolerance, hyperglycemia and hyperglucagonemia in the fed state, and enhanced glucagon secretion in response to L-Arginine stimulation. Hyperinsulinemic-hypoglycemic clamp studies revealed an enhanced glucagon secretory response and an abnormal norepinephrine response to hypoglycemia in αIRKO mice. The mutants also exhibited an age-dependent increase in β-cell mass. Furthermore, siRNA-mediated knockdown of insulin receptor in glucagon-secreting InR1G cells promoted enhanced glucagon secretion and complemented our in vivo findings. Together, these data indicate a significant role for intra-islet insulin signaling in the regulation of α-cell function in both normo- and hypo-glycemic conditions.

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Spa2p is a nonessential protein that regulates yeast cell polarity. It localizes early to the presumptive bud site and remains at sites of growth throughout the cell cycle. To understand how Spa2p localization is regulated and to gain insight into its molecular function in cell polarity, we used a coimmunoprecipitation strategy followed by tandem mass spectrometry analysis to identify proteins that associate with Spa2p in vivo. We identified Myo1p, Myo2p, Pan1p, and the protein encoded by YFR016c as proteins that interact with Spa2p. Strikingly, all of these proteins are involved in cell polarity and/or actin function. Here we focus on the functional significance of the interactions of Spa2p with Myo2p and Myo1p. We find that localization of Spa2GFP to sites of polarized growth depends on functional Myo2p but not on Myo1p. We also find that Spa2p, like Myo2p, cosediments with F-actin in an ATP-sensitive manner. We hypothesize that Spa2p associates with actin via a direct or indirect interaction with Myo2p and that Spa2p may be involved in mediating polarized localization of polarity proteins via Myo2p. In addition, we observe an enhanced cell-separation defect in a myo1spa2 strain at 37 degrees C. This provides further evidence that Spa2p is involved in cytokinesis and cell wall morphogenesis.

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