Intrinsically Disordered Linker and Plasma Membrane‐Binding Motif Sort Ist2 and Ssy1 to Junctions

Abstract

Membrane junctions or contact sites are close associations of lipid bilayers of heterologous organelles. Ist2 is an endoplasmic reticulum (ER)-resident transmembrane protein that mediates associations between the plasma membrane (PM) and the cortical ER (cER) in baker's yeast. We asked the question what structure in Ist2 bridges the up to 30 nm distance between the PM and the cER and we noted that the region spacing the transmembrane domain from the cortical sorting signal interacting with the PM is predicted to be intrinsically disordered (ID). In Ssy1, a protein that was not previously described to reside at membrane junctions, we recognized a domain organization similar to that in Ist2. We found that the localization of both Ist2 and Ssy1 at the cell periphery depends on the presence of a PM-binding domain, an ID linker region of sufficient length and a transmembrane domain that most probably resides in the ER. We show for the first time that an ID amino acid domain bridges adjacent heterologous membranes. The length and flexibility of ID domains make them uniquely eligible for spanning large distances, and we suggest that this domain structure occurs more frequently in proteins that mediate the formation of membrane contact sites.

Relating sequence encoded information to form and function of intrinsically disordered proteins

“…The overall theme presented in this discussion is summarized in Figure 5. Long disordered linkers that belong either to the R2 or R3 region of the diagram-of-states can help localize proteins to the junction between the endoplasmic reticulum and plasma membrane [62]. C-terminal disordered tails of E.coli single stranded DNA binding proteins belong to region R1 and these tails engender positive cooperativity in single stranded DNA binding.…”

TMEM110 regulates the maintenance and remodeling of mammalian ER–plasma membrane junctions competent for STIM–ORAI signaling

“…Ist2 is an ER membrane protein in Saccharomyces cerevisiae that acts as an ER-plasma membrane tether (9,10). Yeast lack STIM-ORAI proteins and ER store-dependent calcium signaling, and the Ist2 homologs in mammals-the TMEM16 family-are plasma membrane Cl − channels or phospholipid scramblases (26) that have lost the long unstructured cytoplasmic region and polybasic C-terminal tail essential to the tethering function in the yeast protein (27,28). Thus, there is no reason to expect that Ist2 would interact with STIM, ORAI, or mammalian proteins dedicated to STIM-ORAI signaling.…”

A Plasma Membrane Association Module in Yeast Amino Acid Transporters

“…Often these sequences contain an amphipathic ␣-helix, which has the ability to associate with the membrane, e.g. as shown for Ist2 and Ssy1 in S cerevisiae (8,29). Here, we report on a membrane association module comprising a palmitoylated sequence (PM asseq ) present in the C terminus of 10 yeast AAPs.…”

“…The most investigated sequence in yeast, which at the same time represents a rather complex association signal, is that of Ist2, the cortical ER protein involved in ER-PM tethering (8,29). The association of Ist2 with the PM requires one amphipathic helix followed by a clus-ter of positively charged residues in the C terminus (30).…”