MAL11 and MAL61 encode the inducible high-affinity maltose transporter of Saccharomyces cerevisiae

Abstract

We have investigated the transport of maltose in a genetically defined maltose-fermenting strain of Saccharomyces cerevisiae carrying the MALI locus. Two kinetically different systems were identified: a high-affinity transporter with a Km of 4 mM and a low-affinity transporter with a Km of 70 to 80 mM. The high-affinity maltose transporter is maltose inducible and is encoded by the MALlI (and/or MAL61) (1,5,7,12,17,18). Each locus is a complex locus consisting of three genes essential for maltose fermentation (5,7,17,18). GENE 1 has been suggested to encode maltose permease based on genetic analysis of mutations in MAL61, the GENE 1 located at the MAL6 locus (4). This suggestion is strongly supported by sequence analysis of the MAL61 gene, which indicates that the product of MAL61 is an integral membrane protein and belongs to a family of sugar transporters including the GLUT] human glucose transporter'(8). GENE 2 encodes maltase and GENE 3 encodes the MAL activator, a positive trans-acting factor regulating the transcription of GENE 1 and GENE 2 (3, 10).Early biochemical studies in baker's Saccharomyces strains revealed one maltose transporter with a Km of 4 mM (13). Later studies on genetically undefined strains of Saccharomyces cerevisiae indicated the apparent presence of at least two forms of the maltose transporter: a low-affinity transporter with a Km of 70 mM, and a high-affinity transporter with a Km of 4 mM (2). Given the polygenic nature of the MAL loci, we wished to explore the possibility that these two kinetically distinct transporters could be the products of different MAL loci or that at least one of them could be the product of GENE 1. Therefore, the kinetics of maltose uptake was studied in genetically well-characterized strains carrying only a single copy of either the MAL]I or the MAL61 gene, GENE 1 of the MAL] and MAL6 loci, respectively.The method used to measure the uptake of radiolabeled maltose was as'described in Kruckeberg and Bisson (16) with slight modifications (22). Yeast strains were grown on YP medium (1% yeast extract, 2% peptone; Difco Laboratories) supplemented with the appropriate sugar as indicated. A 0.1 M tartaric acid-Tris (pH 4.2) buffer was used to resuspend the cells for assaying maltose uptake. We determined the initial velocity of maltose uptake over a range of substrate concentrations from 0.2 to 200 mM and plotted the data as velocity (V) versus velocity/substrate concentrations (VIS), using the Eadie-Hofstee transformation (11,15).The kinetics of maltose uptake were studied in a maltose- Fig. 2A

Glc7–Reg1 Phosphatase Signals to Yck1,2 Casein Kinase 1 to Regulate Transport Activity and Glucose-Induced Inactivation of Saccharomyces Maltose Permease

“…Maltose transport is measured as the uptake of 1 mm 14 C-labeled maltose as described by Cheng and Michels (1991) and Medintz et al (1996). Assays were done in duplicate on at least three independent transformants.…”

Overexpression of Mal61p in Saccharomyces cerevisiae and characterization of maltose transport in artificial membranes

“…Maltose transport has been described as a symport reaction in which one proton is taken up per molecule of maltose (42,51). Maltose transport follows biphasic kinetics with apparent affinity constants (K m app ) of 4 and 70 mM (6,10). The low-affinity transport activity may be caused by nonspecific binding of maltose to cell wall and/or plasma membrane components (3) or, alternatively, by passive diffusion of the solute across the plasma membrane at the high concentrations of sugars used (19).…”

Catabolite inactivation of the yeast maltose transporter occurs in the vacuole after internalization by endocytosis

“…Maltose uptake was measured as described previously (2), with 4 mM labeled maltose (0.5 Ci/mol) and incubation for 15 s at 20ЊC. At this concentration only the high-affinity component of the maltose transport system is determined (5,9). We have focused our attention exclusively on this component because the appearance of a low-affinity uptake seems to be due to an experimental artifact (2).…”