Transporters account for 10% of the human genome. They regulate the flux of different molecules such as sugars, amino acids, ions, lipids, vitamins, and catabolites through plasma and subcellular membranes
[1]. In addition to their physiological roles, many transporters are involved in drug absorption, distribution, metabolism, and excretion (ADME) processes. Moreover, many drugs can interact with membrane transporters, leading to side effects
[2]. Indeed, about 60% of FDA-approved drug targets are membrane proteins, including a growing number of transporters
[3][4]. A well-established functional classification categorizes transport proteins in channels and permeases. Channels catalyze the transport of ions down their electrochemical gradient, with a turnover rate up to 10
8 s
−1. Permeases are involved in the transport of many different molecules with a lower turnover rate. On the basis of the transport driving force, permeases can be subdivided in primary and secondary active transporters
[5]. The first use ATP hydrolysis to drive the transport of molecules against their concentration gradient. The secondary active transporters retrieve the driving force from the concentration gradient of the transported substrates and are grouped into symporters, antiporters, and uniporters. The uniporters catalyze the substrate movement in a thermodynamically favorable direction; the symporters or antiporters catalyze co-transport of more molecules in the same or in the opposite direction, respectively
[5][6]. Among the transporters, the SoLute Carrier (SLC) superfamily currently includes 458 proteins grouped into 65 families
[7][8]. Member proteins within each family share at least 20–25% sequence similarity with at least one other member of the family
[7][9]. The number of members within the SLC families is heterogeneous, ranging from only one (SLC32, SLC40, SLC48, SLC50, SLC53, SLC61, SLC62, and SLC64) to 53 members for the largest family: SLC25, the mitochondrial carrier family
[7]. The members of this superfamily act as secondary active transporters
[10]. Regarding the molecular mechanism of transport, the secondary active transporters use the “alternating access” transport mechanism in which the ligand binding site is available on only one side of the membrane at a time; then, as a consequence of the substrate interaction, a protein conformational change does occur and the substrate is transported to the other side of the membrane
[11]. Three major types of alternating access mechanisms have been described for SLC transporters: the rocker-switch, the gated pore and the elevator mechanism
[10]. Analyzing hydropathy plots, the SLC transporters are predicted to present between 1–16 transmembrane domains (TMD), although in more than 80% of the members the number of TMD is between 7 and 12
[12]. Despite their role in human pathophysiology and the efforts employed in fulfilling knowledge of them, about 30% of the SLC members are still “orphans”, with no structural and functional data available
[7][13]. The reason for this gap in information lies in the difficulty in expressing, purifying, and assaying the function of this kind of proteins
[14]. These difficulties arise from the hydrophobic nature of the TMD and, in some cases, from very large (about 100 amino acids) hydrophilic moieties which connect TMDs
[15][16]. Different expression hosts, either prokaryotic or eukaryotic, have been adopted with success allowing structure/function studies on SLC transporters such as,
E. coli [17],
L. lactis [18],
S. cerevisiae [19],
P. pastoris [20],
S. frugiperda [21], HEK293F
[22]. However, less than 10% of the human SLCs structure has been solved
[23]. Due to very low cultivation costs, fast growth, easy handiness, and absence of toxicity,
Escherichia coli is the most used host for protein over-expression. In particular, BL21(DE3) and its derivative strains are the most commonly used for heterologous membrane protein production
[23]. Even though widely exploited for protein over-expression, in the case of human membrane transporters,
E. coli could be ineffective, leading to the inclusion of a bodies formation from which it is difficult to recover the target protein in an active folded form
[24][25]. On the other side, expression in insect or mammalian cell systems is characterized by high cultivation costs, slow growth, and needs special equipment, often resulting in low yield. A good middle ground is represented by the yeast expression system
[26] which combines the advantages of ease of manipulation, fast growth, low cost of production, eukaryotic protein processing
[27] (which can facilitate the expression of correctly-folded eukaryotic proteins)
[28], heterodimer formation, and prosthetic group association to the recombinant proteins
[29]. Moreover, the localization of an expressed membrane transporter in the yeast membrane can facilitate its recovery in a folded state
[30]. Indeed, in this milieu, the human membrane proteins can find a more similar phospholipid/sterol composition with respect to the bacterial hosts
[31]; furthermore, the possibility of increasing culture volume to obtain large quantities of folded protein
[31][32] can be exploited either for functional or structural studies
[20][33][34]. Indeed, yeast and especially
P. pastoris can reach extremely high biomass, thus obviating the need to achieve extremely high levels of overexpression that could result in the aggregation and denaturation of membrane proteins.
2. Yeasts as a System for Heterologous Expression of Human SLC Transporters
A similar cellular architecture with respect to higher eukaryotes, sharing most of the metabolic and cellular pathways, and its simplicity in molecular and genetic manipulations make yeast particularly suited to produce human proteins for structure-function studies
[35][36][37]. Indeed, yeast can perform many eukaryote-specific post-translational modifications such as proteolytic digestion, disulfide bridge formation, and some type of glycosylation
[38]. However, differently from other eukaryotes, no trimming of mannose residues occurs in yeast
[39]. Several yeast strains have been developed in which glycan remodeling has been induced, specifically deleting genes coding for glycosyltransferases and expressing the genes that carry out mammalian-specific glycosylation
[40][41][42][43][44]. Indeed, DNA can be introduced or removed into/from the yeast chromosomes, enabling the creation of unique protein expression strains
[45][46][47]. This “genetic” approach is largely used in yeast by exploiting homologous recombination: yeast transformation triggers the recombination between yeast sequences carried on the plasmid and homologous sequences in the yeast genome. This approach can serve different aims (
Figure 1).
Figure 1. Uses of the homologous recombination in yeast.
2.1. The Homologous Recombination
The observation that linear DNA fragments can efficiently promote recombination in
S. cerevisiae led to the development of several methods for DNA manipulation in yeast
[48]. Transformation has been used to clone genes by genetic complementation
[49], to clone functional chromosomal components such as origins of replication
[50], centromeres
[51], and telomeres
[52], and to clone functional suppressors acting as dominant negative mutants
[53]. Recombination-based DNA manipulation methods also include integrative DNA transformation, which is used to induce gene deletion
[54], and allele rescue which involves the transplacement of a mutation from the chromosome onto a plasmid-derived copy of that gene (
Table 1)
[55]. Indeed, homologous recombination in
Saccharomyces cerevisiae can be considered as an easy and highly efficient cloning alternative. Yeast recombination cloning allows in a single step the assembly of multiple DNA fragments, deriving for example from many PCR reactions
[56] (
Figure 1). It is extremely efficient, requiring only 29 nucleotides of overlapping sequences that can be added to the synthesized oligonucleotides. Starting from these observations, by exploiting homologous recombination a yeast cloning cassette was used to point out a versatile method called any-gene-any-plasmid (AGAP) allowing the cloning of any gene (or combination of DNA fragments) into any vector in a single step without the need for ligase or a molecular cloning kit
[57]. Towards the aim of obtaining the expression of a membrane transporter different aspects must be taken into account to transform yeast with the gene of interest (
Figure 2).
Figure 2. Workflow of a SLC production in yeast for structural and functional studies.
Table 1. Yeast strains and genotypes.
| Host |
Strain |
Genotype |
Feature |
References |
| P. pastoris |
KM71H |
aox1::ARG4, arg4 |
Strain with MutS phenotype |
[58] |
| P. pastoris |
SMD1168H |
pep4 |
Strain without protease A activity |
[59] |
| S. cerevisiae |
PAP1500 |
MATα ura3-52 trp1::GAL10-GAL4 lys2-801 leu2Δ1 his3Δ200pep4::HIS3prb1Δ1.6Rcan1GAL |
Overexpression of the Gal4 transcription factor |
[60] |
| S. cerevisiae |
AB11c |
ena1-4Δnhx1Δnha1Δ |
Deletion of endogenous cation/proton antiporters and pumps |
[61] |
| S. cerevisiae |
MSY6210 |
MAT α leu2-3,112 ura3-52 his3200 trp1-901lys2-801suc2-9 smf1::HIS3,smf2::KANR |
Deletion of Mg2+ transporters |
[62] |
| S. cerevisiae |
MSY6211 |
MAT a leu2-3,112 ura3-52 his3200 trp1-901 ade2-101 suc2-9 smf3::LEU2 |
Deletion of Mg2+ transporters |
[62] |
| P. pastoris |
GS115 |
his4 |
Deletion of histidinol dehydrogenase |
[63] |
| S. cerevisiae |
EBY.S7 |
MATα hxt1-17Δgal2Δagt1Δstl1Δleu2-3,112 ura3-52 trp1-289 his3-Δ1 MAL2–8c SUC2 hxtΔfgy1 |
Deletion of hexose transporters |
[64] |
| S. cerevisiae |
EBY.F4–1 |
MATα hxt1-17Δgal2Δagt1Δstl1Δleu2-3,112 ura3-52 trp1-289 his3-Δ1 MAL2–8c SUC2 hxtΔfgy1 fgy41 |
Deletion of hexose transporters |
[64] |
| S. cerevisiae |
EBY.VW4000 |
MATa leu2-3,112 ura3-52 trp1-289 his3-1 MAL2-8c SUC2 Δhxt1-17 Δgal2 Δstl1::loxP Δagt1::loxP Δmph2::loxP Δmph3::loxP |
Deletion of hexose transporters |
[65] |
| S. cerevisiae |
SDY.022 |
MATa leu2-3,112 ura3-52 trp1-289 his3-∆1 MAL2-8C SUC2 ∆hxt1-17 ∆gal2 ∆agt1 ∆stl1 fgy1-1 erg4::kanMX |
Deletion of hexose transporters |
[66] |
| S. cerevisiae |
BJ5457 |
MATα ura3-52 trp1 lys2-801 leu2-Δ1 his3-Δ200 pep4:HIS3 prb1-delta1.6R can1 GAL |
Protease deficient |
[67] |
| S. cerevisiae |
RE700A |
MATa hxt1::HIS3::hxt4 hxt5::LEU2 hxt2::HIS3hxt3::LEU2::hxt6 hxt7::HIS3 |
Deletion of hexose transporters |
[68] |
| S. cerevisiae |
BY4742 |
MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0 |
Minimize homologous recombination |
[59] |
| S. cerevisiae |
BY4742 GEV |
MATa, (PGAL10+gal1)Δ::loxP, leu2Δ0::PACT1-GEV-NatMX, gal4Δ::LEU2, HAP1+ |
Minimize homologous recombination |
[69] |
| P. pastoris |
SMD1168H |
pep4 |
Protease A deficiency |
[59] |
| S. cerevisiae |
FAB158 |
MATa his3- Δ200 leu2- Δ1 lys2-801 trp1- Δ1 ade2-101 ura3-52 tat2 Δ::HIS3 |
Deletion of tryptophan transporter |
[70] |
| S. cerevisiae |
TMY203 |
MATa his3- Δ200 leu2- Δ1 lys2-801 trp1- Δ1 ade2-101 ura3-52 tat1 Δ::kanMX4 tat2Δ::LEU2 |
Deletion of tryptophan transporters |
[70] |
| S. cerevisiae |
FAY18A |
MATa his3- Δ200 leu2- Δ1 lys2-801 trp1- Δ1 ade2-101 ura3-52 HPG1-1, Rsp5P514T |
Deletion of Rsp5 ubiquitin ligase |
[70] |
| S. cerevisiae |
XPY1263a |
MATa thi3Δ::natMX thi7D::kanMX |
Deletion of thiamine transporter |
[71] |
| S. cerevisiae |
BY4741mp |
MATa; his3Δ1; leu2Δ0; met15Δ0; ura3Δ0; mir1Δ; pic2Δ |
Deletion of phosphate and copper transporter |
[72] |
| S. cerevisiae |
BY4741 pic2Δ |
MATa, leu2,met15, ura3, his3, PIC2::KANMX |
Deletion of copper transporter |
[73] |
| S. cerevisiae |
WB-12 |
MATα ade2-1 trp1-1 ura3-1 can1-100 aac1::LEU2 aac2::HIS3 |
Deletion of adenine nucleotide carriers |
[74] |
| S. cerevisiae |
JL1-3Δ2 |
Matα leu2-3,112 his3-11,15 ade2-1 trp1-1 ura3-1can1-100 anc1::LEU2 Δ anc2::HIS3 anc3::URA3 |
Deletion of adenine nucleotide carriers |
[75] |
| S. cerevisiae |
W303-B1 |
Mata; ade2-1; his3-11, -15; leu2-3, -112; ura3-1; canR; cyr+ |
Poor leucine uptake |
[76] |
| S. cerevisiae |
W303-1A |
MATa: ade2-2; trp1-1; can1-100; leu2-3, 112; his 3-11, 15; ura3-1 |
Poor leucine uptake |
[77] |
| S. cerevisiae |
W303 |
MATa/MATα (leu2-3,112 trp1-1 can1-100 ura3-1 ade2-1 his3-11,15) [phi+] |
Poor leucine uptake |
[78] |
| S. cerevisiae |
W303 Wagc1Δ |
Mat a/Mat a, ura3-1/ura3-1, trp1- Δ2/trp1- Δ2, leu2-3,112/leu2-3,112, his3-11/his3-11, ade2-1/ade2-1, can1-100/can1-100 |
Poor leucine uptake; deletion of AGC1 carrier |
[79] |
| S. cerevisiae |
PW001 |
BY4741 agc1∆::URA3 |
Deletion of AGC1 carrier |
[80] |
| S. cerevisiae |
PW002 |
BY4741 agc1∆::HIS3 |
Deletion of AGC1 carrier |
[80] |
| S. cerevisiae |
ΔArg11 Y02386 |
MATa; his3Δ1; leu2Δ0; met15Δ0; ura3D0; YOR130c::KanMX4 |
Deletion of ornithine transporter 1 |
[81] |
| S. cerevisiae |
ΔAnt1 BJ1991 |
MATa, leu2, trp1, ura3-251, prb1-1122, and pep4-3 |
Deletion of ANT1 carrier |
[82] |
| S. cerevisiae |
TCY119 |
MAT α ura3–52 leu2–3, 112 trp1-Δ1 ade2 his3-Δ1::hisG aac1-Δ1::hisG aac2-Δ1::kanMX6 aac3-Δ1::hisG [r+, TRP1] |
Deletion of AAC1, AAC2 and AAC3 carriers |
[83] |
| S. cerevisiae |
W303 |
his3-11,15; ade2-1; leu2-3,112; ura3-1; trp1-1; can1-100; RIM2/RIM2::kanMX |
Poor leucine uptake; deletion of pyrimidine nucleotide carrier |
[84] |
| S. cerevisiae |
ST9352 |
MATa, aro10Δ, pdc5Δ, pTEF1->ARO7, pPGK1->ARO4, pTEF1->FjTAL, pTEF1->HsSLC25A44 |
Alteration of aromatic amino acid metabolism |
[85] |
| S. cerevisiae |
CEN.PK113-7D ndt1Δndt2Δ |
MATa MAL2-8c SUC2 ndt1Δndt2Δ |
Deletion of NAD transporter |
[86] |
| S. cerevisiae |
BY4742 ndt1Δndt2Δ |
MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0, ndt1Δndt2Δ |
Deletion of NAD transporter |
[87] |
| S. cerevisiae |
FOH33 |
MATα MAL2-8c SUC2 his3Δ ura3-52 hfd1Δ pox1Δ faa1Δ faa4Δ adh6Δ::kanMX gal80Δ gal1/10/7Δ::(GAL7p-MmCAR-ADH1t)+(GAL3p-npgA-FBA1t); pAOH9 |
Deletion of alcohol dehydrogenase |
[88] |
| S. cerevisiae |
KY114 |
MATα, gal, ura3-52, trp1, lys2, ade2, his d2000 |
Deletion of thymidine transport |
[89] |
| S. cerevisiae |
BY4742-YBR021W |
MATα, his3, leu2, lys2, ura3, ΔFUR4 |
Deletion of uridine permease |
[90] |
| S. cerevisiae |
DY150 |
Mata ade2-1 his3-11 leu2-3,112 trp1-1 ura3-52 can1-100(oc) |
Deletion of Fe2+/Mn2+ transporter |
[91] |
| S. cerevisiae |
YPL1 |
MATα fui1 Δ:: HIS3, ura3-52, lys2-801, HIS3 Δ |
Deletion of uridine permease |
[92] |
| S. cerevisiae |
W303-Δpep4 |
leu2-3,112 trp1-1 can1-100 ura3-1 ade2-1 hwas3-11,15 Δ pep4 MATα |
Deletion of vacuolar endopeptidase Pep4 |
[93] |
| S. cerevisiae |
W303-1A |
MATa ade2-1, can1-100, cyh2, his3-11,15, leu1-c, leu2-3,112, trp1-1, ura3-1 |
No growth in absence of adenine |
[94] |
| S. cerevisiae |
FGY217 |
MATα, ura3-52, lys2 Δ 201 and pep4 Δ |
Deletion of vacuolar endopeptidase Pep4 |
[95] |
| S. cerevisiae |
BY4743 |
Mata/α his3Δ1/ his3Δ1 leu2Δ0/ leu2Δ0 lys2Δ0/+ met15Δ0/+ ura3Δ0/ ura3Δ0 |
Essential genes deletion |
[96] |
| S. cerevisiae |
BY4743 Δpmr1::KanMX |
Mata/α his3Δ1/ his3Δ1 leu2Δ0/ leu2Δ0 lys2Δ0/+ met15Δ0/+ ura3Δ0/ ura3Δ0, Δpmr1::KanMX |
Deletion of essential genes and Pmr1 Ca2+ and Mn2+ transporter |
[97] |
| S. cerevisiae |
ctr1 |
MATa ura3 lys2 ade2 trp1 his3 leu2 Dctr1::LEU2 |
Deletion of CTR1 transporter |
[98] |
| S. cerevisiae |
YPH499 |
MATa ura3-52 lys2-801ade2-101 trp1-Δ63 his3-Δ200 leu2-Δ1 |
|
[99] |
| S. cerevisiae |
YPH500 |
MAT α/ura3–52/lys2–801/ade2–101/trp1-Δ63/his3-Δ200/leu2-Δ1 |
|
[100] |
| S. cerevisiae |
STY50 |
MATa,his4-401,leu2-3,-112,trp1-1,ura3-52,HOL1-1,suc2::LEU2 |
Deletion of invertase 2 |
[101] |
| P. pastoris |
CBS7435 |
Wild type |
|
[102] |
| S. cerevisiae |
YPH501 |
MATa/MATα ura3-52/ura3-52 lys2-801/lys2-801 ade2-101/ade2-101 trp1-Δ63/ trp1-Δ63 his3- Δ200/his3- Δ200 Ieu2- Δ1/leu2- Δ1 |
|
[100] |
| S. cerevisiae |
CB001L |
MATα, leu2, trp1, ura3, prb−, pep4::LEU2 |
Deletion of vacuolar proteinase A and B |
[103] |
| S. cerevisiae |
JRY472 |
Δmpc1/2/3 |
Deletion of pyruvate carriers |
[104] |
| S. cerevisiae |
BY4741 gdt1Δpmr1Δ |
MATa his3Δ1 leu2Δ0 met15Δ0 ura3Δ0 gdt1::KanMX4 pmr1::KanMX4 |
Deletion of Ca2+ and Mg2+ transporters |
[105] |
2.2. Vector Choice
A yeast vector may exist as a self-replicating particle, independently of the yeast genome, or be stably integrated into the yeast genome. This last case should be considered to reduce expression fluctuations. Moreover, a chromosomally borne transgene confers the advantage of a uniform cell population containing the transgene. In contrast, plasmid numbers vary from cell-to-cell, with as many as 50% of cells in a culture under selection lacking the plasmid. Yeast vectors can be grouped into five general classes, based on their mode of replication in yeast: YIp, YRp, YCp, YEp, and YLp plasmids. Except for the YLp plasmids (yeast linear plasmids), all these plasmids can be maintained in
E. coli as well as in
S. cerevisiae and thus are referred to as shuttle vectors. Indeed, they contain two types of selectable genes: plasmid-encoded drug resistance genes for bacterial selection and a yeast gene which acts as dominant selectable marker only when the recipient yeast cell has a recessive mutation in the corresponding chromosomal copy of the cloned gene
[106]. Yeast integrating plasmids (YIp) contain selectable yeast genes but lack sequences that allow autonomous replication of the plasmid in yeast. The frequency of transformation of YIp plasmids is only 1 to 10 transformants/μg DNA, but transformation frequency can be increased 10- to 1000-fold by linearizing the plasmid within yeast sequences that are homologous to the intended site of integration on the yeast chromosome
[106]. Besides integrating vectors, there are three classes of circular yeast plasmids characterized by extrachromosomal autonomous replication sequences (ARS) which confer the ability to replicate autonomously: YRp (yeast replicating plasmids), YCp (yeast centromeric plasmids), and YEp (yeast episomal plasmids). YRp plasmids have high frequencies of transformation (10
3 to 10
4 transformants/μg DNA), but transformants are very unstable both mitotically and meiotically. YRp plasmids can be present in high copy number (up to 100 copies per plasmid-bearing cell, although the average copy number per cell is 1 to 10). Introducing DNA portion from yeast centromeres into YRp-generated YCp plasmids allows an increase in plasmid stability during mitosis and meiosis. These plasmids (present in 1 to 2 copies per cell), have a loss rate of approximately 1% per generation. The increase in plasmid stability is fundamental for allowing protein production. The YEp vectors contain sequences from a naturally occurring yeast plasmid called the 2 μ circle and confer high transformation frequencies (∼10
4 to 10
5 transformants/μg DNA). Since they are propagated quite stably through mitosis and meiosis in high copy number, these plasmids are commonly used for high-level gene expression in yeast
[106]. YLp (yeast linear plasmids) contain G-rich repeated sequences at their termini which act as telomeres and allow the plasmid to replicate as a linear molecule. They are highly unstable due to random segregation during mitosis. A crucial factor that drastically influences transcription and consequently translation rate is the promoter region. Commonly used promoters can be divided in two main classes: constitutive and inducible promoters (
Table 2). Constitutive promoters lead to stable expression levels across varying culture conditions, while ‘dynamic’ or inducible promoters drive huge changes in expression level in response to specific stimuli
[107] (
Table 2).
Table 2. Plasmids: promoters and regulation.
Normally, constitutive strong promoters are used for biotechnological application to produce high levels of non-toxic target. Sometimes, the constitutive expression of certain proteins can be detrimental to cell growth due to product toxicity and the imposed metabolic burden
[123]. To overcome this problem, several attempts at promoter engineering have been made aimed at tuning gene expression for biotechnological application
[124].
2.3. Selection Strategies
The original and most commonly used strategy for the selection of transformed yeast cells exploits the auxotrophic markers: TRP1
[125], HIS3
[126], LEU2, URA3
[127], MET15, and ADE2
[128]. Expression plasmids are paired with available
S. cerevisiae strains that are auxotrophic for tryptophan, histidine, leucine, uracil, methionine, and adenosine, respectively, by carrying full or functional knock-outs of these auxotrophic genes
[129]. Besides auxotrophic markers, antibiotics can also be used as an alternative selection tool in a rich growth medium. However, since antibiotics affect ribosome function, expression studies should be performed in liquid media lacking antibiotics
[129]. In the case of
P. pastoris, the transformant selection is mostly conducted by using the antibiotic Zeocin, a member of the bleomycin/phleomycin family isolated from
Streptomyces. Thus, clones deriving from multiple integration events can be selected increasing Zeocin concentration.