Interaction among Btn1p, Btn2p, and Ist2p Reveals Potential Interplay among the Vacuole, Amino Acid Levels, and Ion Homeostasis in the Yeast Saccharomyces cerevisiae

Abstract

Btn2p, a novel cytosolic coiled-coil protein in Saccharomyces cerevisiae, was previously shown to interact with and to be necessary for the correct localization of Rhb1p, a regulator of arginine uptake, and Yif1p, a Golgi protein. We now report the biochemical and physical interactions of Btn2p with Ist2p, a plasma membrane protein that is thought to have a function in salt tolerance. A deletion in Btn2p (btn2⌬ strains) results in a failure to correctly localize Ist2p, and strains lacking Btn2p and Ist2p (btn2⌬ ist2⌬ strains) are unable to grow in the presence of 0.5 or 1.0 M NaCl. Btn2p was originally identified as being up-regulated in a btn1⌬ strain, which lacks the vacuolar-lysosomal membrane protein, Btn1p, and serves as a model for Batten disease. This up-regulation of Btn2p was shown to contribute to the maintenance of a stable vacuolar pH in the btn1⌬ strain. Btn1p was subsequently shown to be required for the optimal transport of arginine into the vacuole. Interestingly, btn1⌬ ist2⌬ strains are also unable to grow in the presence of 0.5 or 1.0 M NaCl, and ist2⌬ suppresses the vacuolar arginine transport defect in btn1⌬ strains. Although further investigation is required, we speculate that altered vacuolar arginine transport in btn1⌬ strains represents a mechanism for maintaining or balancing cellular ion homeostasis. Btn2p interacts with at least three proteins that are seemingly involved in different biological functions in different subcellular locations. Due to these multiple interactions, we conclude that Btn2p may play a regulatory role across the cell in response to alterations in the intracellular environment that may be caused by changes in amino acid levels or pH, a disruption in protein trafficking, or imbalances in ion homeostasis resulting from either genetic or environmental manipulation.

Btn2, a Hook1 Ortholog and Potential Batten Disease-Related Protein, Mediates Late Endosome-Golgi Protein Sorting in Yeast

“…Thus, we predict that changes in ion, pH, or amino acid homeostasis, seen in the absence of Btn2, result from the improper recycling of cargo proteins within the endomembrane transport system. If so, this would preclude a general role in the regulation of cell homeostasis, as predicted by Kim et al (43). Furthermore, we predict that Batten disease/JNCL in humans may originate from the failed recycling of cargo proteins to their proper sites of action.…”

“…Previous studies have shown that Btn2 interacts with a wide variety of proteins (i.e., Rhb1, Yif1, and Ist2) (13,14,43) purportedly involved in numerous processes relevant to cell growth and homeostasis (i.e., ion, pH, or amino acid balance). However, the precise role of Btn2 in intracellular membrane transport has remained obscure.…”

High Resolution 1H NMR-based Metabolomics Indicates a Neurotransmitter Cycling Deficit in Cerebral Tissue from a Mouse Model of Batten Disease

“…Moreover, cationic imbalances have been documented in a yeast model of Batten disease whereby a defect in the transport of the basic amino acids lysine and arginine into the vacuole has been identified (40). Such a transport defect at the lysosomal membrane in neuronal cells may also warrant compensatory changes in osmolyte distribution (41). Perturbations in NAA concentration also discriminated between wild type and Cln3 cerebral tissue, showing an increase in 1-month-old mice and decreasing in 2-, 3-, and 6-month-old mice.…”

A Knock-In Reporter Model of Batten Disease

“…Studies in yeast indicate roles in maintenance of vacuolar arginine levels, pH, and/or ion homeostasis (Pearce et al, 1999;Chattopadhyay et al, 2000;Kim et al, 2003Kim et al, , 2005. A recent report indicates that, similar to yeast cells, arginine transport into lysosomes is defective in JNCL patient-derived lymphoblasts (Ramirez-Montealegre and .…”