Characterization of rat Glut4 glucose transporter expressed in the yeast Saccharomyces cerevisiae: comparison with Glut1 glucose transporter

Shr3p mediates specific COPII coatomer-cargo interactions required for the packaging of amino acid permeases into ER-derived transport vesicles

The SHR3 gene of Saccharomyces cerevisiae encodes an integral membrane component of the endoplasmic reticulum (ER) with four membrane-spanning segments and a hydrophilic, cytoplasmically oriented carboxyl-terminal domain. Mutations in SHR3 specifically impede the transport of all 18 members of the amino acid permease (aap) gene family away from the ER. Shr3p does not itself exit the ER. Aaps fully integrate into the ER membrane and fold properly independently of Shr3p. Shr3p physically associates with the general aap Gap1p but not Sec61p, Gal2p, or Pma1p in a complex that can be purified from N-dodecylmaltoside-solubilized membranes. Pulse-chase experiments indicate that the Shr3p-Gap1p association is transient, a reflection of the exit of Gap1p from the ER. The ER-derived vesicle COPII coatomer components Sec13p, Sec23p, Sec24p, and Sec31p but not Sar1p bind Shr3p via interactions with its carboxyl-terminal domain. The mutant shr3-23p, a nonfunctional membrane-associated protein, is unable to associate with aaps but retains the capacity to bind COPII components. The overexpression of either Shr3p or shr3-23p partially suppresses the temperature-sensitive sec12-1 allele. These results are consistent with a model in which Shr3p acts as a packaging chaperone that initiates ER-derived transport vesicle formation in the proximity of aaps by facilitating the membrane association and assembly of COPII coatomer components.

Characterization of glucose transport activity reconstituted from heart and other tissues

We examined several aspects of glucose transport reconstituted in liposomes, with emphasis on transporters of rat heart (mostly GLUT4) compared to those of human erythrocytes (GLUT1), and on effects of agents that modulate transport in intact cells. Several types of samples gave higher reconstituted activity using liposomes of egg lipids rather than soybean lipids. Diacylglycerol, proposed to activate transporters directly as part of the mechanism of insulin action, increased the intrinsic activity of heart transporters by only 25%, but increased the size of the reconstituted liposomes by 90%. The dipeptide Cbz-Gly-Phe-NH2 inhibited GLUT4 with a Ki of 0.2 mM, compared to 2.5 mM for GLUT1, which explains its preferential inhibition of insulin-stimulated glucose transport in adipocytes. Verapamil, which inhibits insulin- and hypoxia-stimulated glucose transport in muscle, had no effect on reconstituted transporters. Heart transporters had a higher Km for glucose uptake (13.4) than did GLUT1 (1.6 mM), in agreement with a recent study of GLUT1 and GLUT4 expressed in yeast and reconstituted in liposomes. Transporters reconstituted from heart and adipocytes were 40-70% inactivated by external trypsin, suggesting the presence of trypsin-sensitive sites on the cytoplasmic domain of GLUT4. NaCl and KCl both reduced reconstituted transport activity, but KCl had a much smaller effect on the size of the liposomes.

Tryptophan 388 in putative transmembrane segment 10 of the rat glucose transporter Glut1 is essential for glucose transport

The molecular mechanism of substrate recognition in membrane transport is not well understood. Two amino acid residues, Tyr446 and Trp455 in transmembrane segment 10 (TM10), have been shown to be important for galactose recognition by the yeast Gal2 transporter; Tyr446 was found to be essential in that its replacement by any of the other 19 amino acids abolished transport activity (Kasahara, M., Shimoda, E., and Maeda, M. (1997) J. Biol. Chem. 272, 16721-16724). The Glut1 glucose transporter of animal cells belongs to the same Glut transporter family as does Gal2 and thus might be expected to show a similar mechanism of substrate recognition. The role of the two amino acids, Phe379 and Trp388, in rat Glut1 corresponding to Tyr446 and Trp455 of Gal2 was therefore studied. Phe379 and Trp388 were individually replaced with each of the other 19 amino acids, and the mutant Glut1 transporters were expressed in yeast. The expression level of most mutants was similar to that of the wild-type Glut1, as revealed by immunoblot analysis. Glucose transport activity was assessed by reconstituting a crude membrane fraction of the yeast cells in liposomes. No significant glucose transport activity was observed with any of Trp388 mutants, whereas the Phe379 mutants showed reduced or no activity. These results indicate that the two aromatic amino acids in TM10 of Glut1 are important for glucose transport. However, unlike Gal2, the residue at the cytoplasmic end of TM10 (Trp388, corresponding to Trp455 of Gal2), rather than that in the middle of TM10 (Phe379, corresponding to Tyr446 of Gal2), is essential for transport activity.

Cysteine-scanning mutagenesis of flanking regions at the boundary between external loop I or IV and transmembrane segment II or VII in the GLUT1 glucose transporter

To investigate local secondary structure of GLUT1, site-directed and cysteine-scanning mutagenesis were employed to probe p-chloromercuribenzenesulfonate sensitivity of flanking regions at the boundary of external loops (ELs) and transmembrane segments (TMs) and to check the compatibility of two alternative membrane topology models with the experimental data. In the Cys-less GLUT1, single serine residues located in external loops adjacent to putative transmembrane segments were replaced with cysteine. Transport activities of the cysteine-replacement mutants were comparable to that of the nonmutated Cys-less GLUT1. Only the cysteine residues inserted into the first or fourth EL contributed to transport inhibition by p-chloromercuribenzenesulfonate (pCMBS). Dependent on the pCMBS sensitivity of these residues, cysteine-scanning mutagenesis of flanking regions was performed, including EL I-TM II and TM VII-EL IV, respectively. Of the 27 amino acids changed, the majority of cysteine-substitution mutants displayed transport activities comparable to that of Cys-less GLUT1. Irreplaceable amino acids were Phe-72, Gly-286, Asp-288, Tyr-292, and Tyr-293. The pCMBS sensitivity of loop residues decreased when the distance between inserted thiol groups and the putative transmembrane limit increased. The mutants T62C, T63C, T295C, and I297C even exhibited transport stimulation after pCMBS treatment. Regarding putative membrane-harbored residues, a few thiol groups were involved in pCMBS-induced transport inhibition. Drawn on a helix wheel, these pCMBS-sensitive cysteine residues lie on the same facial half of the helix, shown for TM II and TM VII. With respect to EL-TM boundaries, the experimental data are consistent with the local secondary structure predicted from hydropathy profiles. Conversely, certain data obtained by pCMBS-sensitivity scanning are not consistent with either of the two recently published alternative GLUT1 topology models.