Targeting and membrane insertion into the endoplasmic reticulum membrane of Saccharomyces cerevisiae essential protein Rot1

Copper inhibits protein maturation in the secretory pathway by targeting the Sec61 translocon in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, proteins destined for secretion utilize the post-translational translocon machinery to gain entry into the endoplasmic reticulum. These proteins then mature by undergoing a number of post-translational modifications in different compartments of the secretory pathway. While these modifications have been well established for many proteins, to date only a few studies have been conducted regarding the conditions and factors affecting maturation of these proteins before entering into the endoplasmic reticulum. Here, using immunoblotting, microscopy, and spot test assays, we show that excess copper inhibits the Sec61 translocon function and causes accumulation of two well-known post-translationally translocated proteins, Gas1 (glycophospholipid-anchored surface protein) and CPY (carboxypeptidase Y), in the cytosol. We further show that the copper-sensitive phenotype of sec61-deficient yeast cells is ameliorated by restoring the levels of SEC61 through plasmid transformation. Furthermore, screening of translocation-defective Sec61 mutants revealed that sec61-22, bearing L80M, V134I, M248V, and L342S mutations, is resistant to copper, suggesting that copper might be inflicting toxicity through one of these residues. In conclusion, these findings imply that copper-mediated accumulation of post-translationally translocated proteins is due to the inhibition of Sec61.

Tales of tails in transporters

Cell nutrition, detoxification, signalling, homeostasis and response to drugs, processes related to cell growth, differentiation and survival are all mediated by plasma membrane (PM) proteins called transporters. Despite their distinct fine structures, mechanism of function, energetic requirements, kinetics and substrate specificities, all transporters are characterized by a main hydrophobic body embedded in the PM as a series of tightly packed, often intertwined, α-helices that traverse the lipid bilayer in a zigzag mode, connected with intracellular or extracellular loops and hydrophilic N- and C-termini. Whereas longstanding genetic, biochemical and biophysical evidence suggests that specific transmembrane segments, and also their connecting loops, are responsible for substrate recognition and transport dynamics, emerging evidence also reveals the functional importance of transporter N- and C-termini, in respect to transport catalysis, substrate specificity, subcellular expression, stability and signalling. This review highlights selected prototypic examples of transporters in which their termini play important roles in their functioning.

A transmembrane serine residue in the Rot1 protein is essential for yeast cell viability

Polar residues are present in TM (transmembrane) helices and may influence the folding or association of membrane proteins. In the present study, we use an in vivo approach to analyse the functional and structural roles for amino acids in membrane-spanning motifs using the Rot1 (reversal of Tor2 lethality 1) protein as a model. Rot1 is an essential membrane protein in Saccharomyces cerevisiae and it contains a single TM domain. An alanine insertion scanning analysis of this TM helix revealed that the integrity of the central domain is essential for protein function. We identified a critical serine residue inside the helix that plays an essential role in maintaining cell viability in S. cerevisiae. Replacement of the serine residue at position 250 with a broad variety of amino acids did not affect protein targeting and location, but completely disrupted protein function causing cell death. Interestingly, substitution of the serine residue by threonine resulted in sustained cell viability, demonstrating that the hydroxy group of the TM serine side chain plays a critical role in protein function. The results of the present study indicate that Rot1 needs the TM Ser250 to interact with other membrane components and exert its functional role, avoiding exposure of the serine hydrogen-bonding group at the lipid-exposed surface.

Molecular basis of the functional distinction between Cln1 and Cln2 cyclins

Cln1 and Cln2 are very similar but not identical cyclins. In this work, we tried to describe the molecular basis of the functional distinction between Cln1 and Cln2. We constructed chimeric cyclins containing different fragments of Cln1 and Cln2 and performed several functional analysis that make it possible to distinguish between Cln1 or Cln2. We identified that region between amino acids 225 and 299 of Cln2 is not only necessary but also sufficient to confer Cln2 specific functionality compared with Cln1. We also studied Cln1 and Cln2 subcellular localization identifying additional differences between them. Both cyclins are distributed between the nucleus and the cytoplasm, but Cln1 shows stronger nuclear accumulation. Nuclear import of both cyclins is mediated by the classical nuclear import pathway and by sequences in the N-terminal end of the proteins. For Cln2, but not for Cln1, a nuclear export mechanism mediated by karyopherin Msn5 has been identified. Strikingly, Cln2 export depends on a Msn5-dependent NES between amino acids 225 and 299. In fact, the introduction of this region confers to Cln1 an export mechanism dependent on Msn5; importantly, this causes the gain of Cln2-specific cytosolic functions and the impairment of nuclear function. In short, a region from Cln2 controlling an Msn5-dependent nuclear export mechanism confers a specific functionality to Cln2 compared with Cln1.