Insulin signaling in the yeast Saccharomyces cerevisiae. 2. Interaction of human insulin with a putative binding protein

Insulin-like and mimetic molecules from non-mammalian organisms: potential relevance for drug discovery

Insulin was first discovered in extracts of vertebrate pancreas during a focused search for a therapy for diabetes. Subsequent efforts to discover and isolate a similar active principle from yeast and plants driven by the hope to identify insulin-like/mimetic molecules with critical advantages in the pharmacokinetic profile and expenditure of production compared to authentic human insulin were not successful. As a consequence, it has generally been assumed that hormones evolved exclusively during course of the evolution of vertebrate endocrine organs, implying a rather recent origin. Concomitantly, the existence and physiological role of vertebrate hormones in lower multi- and unicellular eukaryotes have remained a rather controversial subject over decades, albeit there is some evidence that hormones and hormone-binding proteins resembling those of vertebrates are expressed in fungi and yeast. Past and recent findings on the existence of insulin-like and mimetic materials, such as the glucose tolerance factor, in lower eukaryotes, in particular Neurospora crassa and yeast, will be presented. These data provide further evidence for the provocative view that the evolutionary roots of the vertebrate endocrine system may be far more ancient than is generally believed and that the identification and characterisation of insulin-like/mimetic molecules from lower eukaryotes may be useful for future drug discovery efforts.

Insulin receptor kinase-independent signaling via tyrosine phosphorylation of phosphatase PHLPP1

Most insulin responses correlate well with insulin receptor (IR) Tyr kinase activation; however, critical exceptions to this concept have been presented. Specific IR mutants and stimulatory IR antibodies demonstrate a lack of correlation between IR kinase activity and specific insulin responses in numerous independent studies. IR conformation changes in response to insulin observed with various IR antibodies define an IR kinase-independent signal that alters the C-terminus. IR-related receptors in lower eukaryotes that lack a Tyr kinase point to an alternative mechanism of IR signaling earlier in evolution. However, the implied IR kinase-independent signaling mechanism remained obscure at the molecular level. Here we begin to define the molecular basis of an IR-dependent but IR kinase-independent insulin signal that is equally transmitted by a kinase-inactive mutant IR. This insulin signal results in Tyr phosphorylation and catalytic activation of phosphatase PHLPP1 via a PI 3-kinase-independent, wortmannin-insensitive signaling pathway. Dimerized SH2B1/PSM is a critical activator of the IR kinase and the resulting established insulin signal. In contrast it is an inhibitor of the IR kinase-independent insulin signal and disruption of SH2B1/PSM dimer binding to IR potentiates this signal. Dephosphorylation of Akt2 by PHLPP1 provides an alternative, SH2B1/PSM-regulated insulin-signaling pathway from IR to Akt2 of opposite polarity and distinct from the established PI 3-kinase-dependent signaling pathway via IRS proteins. In combination, both pathways should allow the opposing regulation of Akt2 activity at two phosphorylation sites to specifically define the insulin signal in the background of interfering Akt-regulating signals, such as those controlling cell proliferation and survival.

GPI-anchored protein cleavage in the regulation of transmembrane signals

The structure of covalently-linked glycosylphosphatidylinositol (GPI) anchors of membrane proteins displayed on the cell surface is described. Evidence of how the GPI-anchors are sorted into membrane rafts in the plasma membrane is reviewed. Proteins are released by hydrolysis of the linkage to the GPI anchor and phospholipases from different sources involved in this process are characterised. The regulation of protein conformation and function resulting from phospholipase cleavage of the GPI anchor is discussed in the context of its role in signal transduction by insulin. In this signalling system, re-distribution of critical membrane components, including GPI-anchored proteins and non-receptor tyrosine kinases, between different raft domains appears to play a central role in the signal transduction pathway.

Ligand-receptor interactions: a new method for determining the binding parameters

A new experimental procedure and new plot coordinates that allow determination of the binding parameters of ligand-acceptor interaction have been proposed. Instead of titration of a constant concentration of receptors with changing concentrations of ligand, as requested by the well-known methods of Klotz and Scatchard, a series of sequential dilutions of the reacting ligand-receptor mixture is suggested. This allows the application of a new coordinate system that transforms the binding isotherms into straight lines. The case of one acceptor with two classes of receptors with different binding constants is also considered briefly, where the correspondent graphs are nonlinear. It is suggested that in some cases this approach can be a simple and convenient substitute of the broadly used methods of Klotz and Scatchard.

Stable plasma membrane expression of the soluble domain of the human insulin receptor in yeast

The soluble cytoplasmic kinase domain of the human insulin receptor was N-terminally equipped with either an N-acetylation or a dual-acylation motif (MGC box, to allow myristoylation/palmitoylation) and expressed in yeast cells under the control of the inducible CUP1 promoter. Although the cellular concentration was about the same in both instances (reflecting similar stability against proteolysis), only the myristoylated protein was capable of autophosphorylation to a significant extent and was active to phosphorylate endogenous yeast proteins at tyrosine residues in vivo. Cellular subfractionation showed that the insulin receptor was associated with plasma membranes, from where it was not extractable with high salt or alkali, but a significant fraction was also localized in the nuclear fraction. The myristoylated protein is absent from the cytoplasm. No effect of expression of either the acetylated or the myristoylated version on growth and respiration on various carbon sources was detected, suggesting a failure of the active insulin receptor kinase domain to couple to yeast (glucose) signalling cascades.