The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
AVT2 (systematic name YEL064C; UniProt P39981) encodes a predicted 480-amino-acid integral membrane protein in the budding yeast Saccharomyces cerevisiae (strain ATCC 204508 / S288c). The protein belongs to the amino acid/polyamine transporter 2 family (also referred to as the amino acid/auxin permease [AAAP] superfamily) and contains the AA_transpt_TM domain (IPR013057; PF01490) (russnak2001afamilyof pages 8-8, kawanokawada2018transportofamino pages 2-3). AVT2 is one of seven members (AVT1–AVT7) of the AVT family in S. cerevisiae, which are predicted to be membrane-spanning proteins related to the neuronal vesicular inhibitory amino acid transporter (VIAAT/VGAT) family, including the C. elegans UNC-47 protein (russnak2001afamilyof pages 1-1, russnak2001afamilyof pages 1-3).
The AVT family was identified and functionally characterized by Russnak, Konczal, and McIntire (2001) in a landmark study demonstrating that four of the seven family members mediate bidirectional amino acid transport across the vacuolar membrane (russnak2001afamilyof pages 1-1). The family subdivides phylogenetically into four branches: AVT1, AVT2, AVT3/AVT4, and AVT5/AVT6/AVT7 (russnak2001afamilyof pages 4-6). Functionally characterized members include:
The family members share conserved structural features with approximately 9–11 predicted transmembrane domains, and transport activity for characterized members is driven by the proton electrochemical gradient maintained by the vacuolar H⁺-ATPase (V-ATPase) (russnak2001afamilyof pages 4-6, kawanokawada2018transportofamino pages 2-3).
The following table summarizes the functional status of all AVT family members:
| Gene | Systematic name | Protein size (aa) | Known substrates | Transport direction | Functional status |
|---|---|---|---|---|---|
| AVT1 | YJR001W | 602 | Gln, Asn, Ile, Leu, Tyr, His | Import (cyt→vac) | Characterized (russnak2001afamilyof pages 8-8, kawanokawada2018transportofamino pages 2-3, bianchi2019regulationofamino pages 9-10) |
| AVT2 | YEL064C | 480 | Unknown | Unknown | Uncharacterized (russnak2001afamilyof pages 8-8, bianchi2019regulationofamino pages 9-10) |
| AVT3 | YKL146W | 692 | Neutral amino acids: Gln, Asn, Ile, Leu, Tyr | Export (vac→cyt) | Characterized (russnak2001afamilyof pages 8-8, russnak2001afamilyof pages 1-1, bianchi2019regulationofamino pages 9-10) |
| AVT4 | YNL101W | 713 | Neutral amino acids; also His, Lys, Arg | Export (vac→cyt) | Characterized (russnak2001afamilyof pages 8-8, kawanokawada2018transportofamino pages 2-3, bianchi2019regulationofamino pages 9-10) |
| AVT5 | YBL089W | 509 | Unknown | Unknown | Uncharacterized (russnak2001afamilyof pages 8-8, bianchi2019regulationofamino pages 9-10) |
| AVT6 | YER119C | 448 | Asp, Glu; recent evidence suggests additional neutral amino acids | Export (vac→cyt) | Characterized (russnak2001afamilyof pages 8-8, russnak2001afamilyof pages 1-1, nakajo2026significanceofamino pages 1-7) |
| AVT7 | YIL088C | 490 | Pro, Gln | Export / likely bidirectional depending on condition | Partially characterized (bianchi2019regulationofamino pages 9-10, nishida2016vacuolaraminoacid pages 6-7) |
Table: This table summarizes the seven S. cerevisiae AVT-family transporters, including systematic identifiers, protein lengths, known substrates, transport direction, and degree of functional characterization. It is useful for placing AVT2 in family context and highlighting that AVT2 remains one of the least characterized members.
AVT2 remains one of the least characterized members of the AVT family. Despite extensive functional analysis using purified vacuolar membrane vesicles and radiolabeled amino acid transport assays, no vacuolar amino acid transport role has been assigned to AVT2 (russnak2001afamilyof pages 8-8). The substrates, direction of transport, and mechanism of action for AVT2 are all listed as "unknown" in the literature spanning from the original 2001 study through the most recent reviews (parzych2019vacuolarhydrolysisand pages 9-11, sekito2008novelfamiliesof pages 3-5, kawanokawada2018transportofamino pages 2-3, russnak2001afamilyof pages 8-8).
Russnak et al. (2001) explicitly noted that they were unable to detect any amino acid transport function for AVT2 under their standard assay conditions, which tested a panel of neutral, acidic, and basic amino acids (russnak2001afamilyof pages 8-8). The authors suggested that AVT2 may transport other organic compounds or may require additional cofactors not included in standard experimental conditions (russnak2001afamilyof pages 8-8). A comprehensive review by Bianchi et al. (2019) confirmed that "no activity has been observed for two of the predicted family members, Avt2 and Avt5" (bianchi2019regulationofamino pages 9-10).
Similarly, when Nishida et al. (2016) examined the effects of AVT gene deletions on proline distribution, single deletion of AVT2 did not clearly alter the subcellular localization of proline (nishida2016vacuolaraminoacid pages 6-7). This contrasts with the clear effects of deleting AVT1 (reduced vacuolar proline) or AVT3 (increased vacuolar proline) (nishida2016vacuolaraminoacid pages 6-7).
The key finding from the original characterization is summarized below:
“In the original functional analysis of the S. cerevisiae AVT family, AVT2 (YEL064C) emerged as an outlier: sequence comparisons placed it closer to the mammalian system N transporter SN1/NAT1 than to the better-characterized yeast AVT uptake/export branches, yet no vacuolar amino acid transport activity could be assigned to AVT2 under the assay conditions used. AVT2-HA did not show the vacuolar staining pattern seen for AVT1, AVT4, or AVT6; instead, it produced punctate staining reminiscent of endoplasmic reticulum-resident proteins. The authors therefore concluded that AVT2 might function in a different cellular region, while also cautioning that the tagged protein could have been mislocalized; alternatively, AVT2 may transport other organic solutes or require additional cofactors absent from the standard transport assays.” (russnak2001afamilyof pages 8-8)
Blockquote: This blockquote condenses the main conclusion about AVT2 from Russnak et al. 2001: despite family membership and homology to mammalian transporters, AVT2 remained functionally unassigned and showed uncertain, possibly ER-like localization.
The subcellular localization of AVT2 is uncertain and may not be exclusively vacuolar. In indirect immunofluorescence experiments using HA-tagged AVT2, the staining pattern was "punctate staining reminiscent of proteins that reside in the endoplasmic reticulum," which was distinctly different from the vacuolar membrane staining observed for AVT1-HA, AVT4-HA, and AVT6-HA (all of which showed patterns indistinguishable from the vacuolar ATPase subunit VMA2) (russnak2001afamilyof pages 8-8). The authors cautioned that this localization pattern could potentially represent misfolded derivatized (HA-tagged) molecules retained in the ER, but the result raises the possibility that AVT2 may function at the ER or at other intracellular membranes rather than exclusively at the vacuole (russnak2001afamilyof pages 8-8).
Notably, at the amino acid sequence level, Avt1 and Avt2 do not cluster with any of the other Avt members in phylogenetic analysis, suggesting a distinct evolutionary trajectory within the family (bianchi2019regulationofamino pages 9-10).
AVT2 is more closely related to the mammalian system N transporter SN1/NAT1 than to the other AVT family members (russnak2001afamilyof pages 8-8). SN1/NAT1 is a plasma membrane glutamine/histidine transporter that resides on mammalian cell surfaces and requires Na⁺ cotransport in addition to a pH gradient for function (russnak2001afamilyof pages 8-8). This homology to SN1/NAT1 provides a potential clue to AVT2's substrate specificity: it may preferentially transport glutamine and/or histidine, though this has not been experimentally confirmed.
The broader AVT family belongs to the AAAP (amino acid/auxin permease) superfamily, which is phylogenetically related to the mammalian SLC32 family (VGAT/VIAAT vesicular GABA/glycine transporters), the SLC36 family (proton-coupled amino acid transporters, including LYAAT-1), and plant amino acid/auxin permeases (kawanokawada2018transportofamino pages 2-3, sekito2008novelfamiliesof pages 2-3, russnak2001afamilyof pages 1-3). The AVT proteins share varying degrees of sequence identity with each other: AVT5 and AVT6 share 52% identity, AVT3 and AVT4 share 38% identity, and AVT7 shares 35% identity with AVT5, while UNC-47 and VGAT share 35% identity (russnak2001afamilyof pages 1-3).
Although AVT2 function is unknown, its transcription is regulated in a manner consistent with a role in amino acid homeostasis. Nishida et al. (2016) identified all seven AVT genes (including AVT2) as novel proline-responsive genes: mRNA levels of all AVT genes were significantly elevated 1–5 hours after the addition of exogenous proline, with increases ranging from 1.5-fold to 5-fold depending on the gene (nishida2016vacuolaraminoacid pages 3-6). The transcriptional changes of the AVT genes were significantly greater than those of all other genes tested (p < 10⁻¹⁵) (nishida2016vacuolaraminoacid pages 3-6). This co-regulation with the other AVT family members supports the hypothesis that AVT2 participates in vacuolar amino acid homeostasis, even though its specific transport function remains undetectable under standard assay conditions.
Notably, the AVT gene promoters do not contain typical Put3-recognition sites (the proline-dependent transcriptional activator), and the cis and trans elements responsible for proline-dependent induction of AVT genes remain to be identified (nishida2016vacuolaraminoacid pages 3-6). Additionally, AVT2 has been suggested as a candidate for transport of S-adenosylmethionine (AdoMet) across the vacuolar membrane, based on its potential for vacuolar amino acid-related transport (chan2003regulationofsadenosylmethionine pages 5-6), though this has not been experimentally validated.
The vacuolar amino acid transport system in which AVT2 participates (by family membership) plays a critical role in autophagy-mediated amino acid recycling. During nitrogen starvation, autophagy delivers bulk cytoplasmic proteins to the vacuole for degradation, and the resulting amino acids must be exported back to the cytosol for reuse in protein synthesis and cell survival (yang2006atg22recyclesamino pages 1-2, yang2006atg22recyclesamino pages 6-7). The related transporter Atg22, together with Avt3 and Avt4, functions as partially redundant vacuolar amino acid effluxers that are essential for recycling leucine, isoleucine, and tyrosine from the vacuole during starvation (yang2006atg22recyclesamino pages 1-2, yang2006atg22recyclesamino pages 5-6).
However, AVT2 is conspicuously absent from the functionally important transporters in this context. In the most recent study by Nakajo et al. (2026), which comprehensively examined vacuolar amino acid recycling using an avt3Δ avt4Δ avt6Δ avt7Δ quadruple mutant, AVT2 was not included among the vacuolar amino acid exporters (nakajo2026significanceofamino pages 1-7). This quadruple mutant showed reduced protein synthesis under nitrogen starvation and impaired sporulation, phenocopying autophagy-defective mutants (nakajo2026significanceofamino pages 1-7). The exclusion of AVT2 from this analysis underscores the current consensus that AVT2 does not have a detectable role in vacuolar amino acid efflux.
AVT2 (YEL064C) encodes a predicted membrane-spanning amino acid transporter of the AAAP superfamily in S. cerevisiae. Despite its classification as a "vacuolar amino acid transporter" based on family membership and domain architecture, AVT2 remains functionally uncharacterized after more than two decades of research on the AVT family. Key unresolved questions include:
The closest mammalian homolog, SN1/NAT1 (a system N glutamine/histidine transporter), provides an evolutionary clue suggesting possible glutamine or histidine transport activity. AVT2 transcription is co-regulated with other AVT family members in response to proline, supporting a role in amino acid homeostasis. However, the protein may require specific cofactors, operate under specific physiological conditions, or transport non-amino acid organic solutes that have not been tested. Future studies employing reconstituted proteoliposome assays, broader substrate screening, or high-resolution localization approaches may be needed to finally assign a function to this enigmatic transporter.
References
(russnak2001afamilyof pages 8-8): Roland Russnak, David Konczal, and Steven L. McIntire. A family of yeast proteins mediating bidirectional vacuolar amino acid transport*. The Journal of Biological Chemistry, 276:23849-23857, Jun 2001. URL: https://doi.org/10.1074/jbc.m008028200, doi:10.1074/jbc.m008028200. This article has 223 citations.
(kawanokawada2018transportofamino pages 2-3): Miyuki Kawano-Kawada, Yoshimi Kakinuma, and Takayuki Sekito. Transport of amino acids across the vacuolar membrane of yeast: its mechanism and physiological role. Biological & pharmaceutical bulletin, 41 10:1496-1501, Oct 2018. URL: https://doi.org/10.1248/bpb.b18-00165, doi:10.1248/bpb.b18-00165. This article has 42 citations and is from a peer-reviewed journal.
(russnak2001afamilyof pages 1-1): Roland Russnak, David Konczal, and Steven L. McIntire. A family of yeast proteins mediating bidirectional vacuolar amino acid transport*. The Journal of Biological Chemistry, 276:23849-23857, Jun 2001. URL: https://doi.org/10.1074/jbc.m008028200, doi:10.1074/jbc.m008028200. This article has 223 citations.
(russnak2001afamilyof pages 1-3): Roland Russnak, David Konczal, and Steven L. McIntire. A family of yeast proteins mediating bidirectional vacuolar amino acid transport*. The Journal of Biological Chemistry, 276:23849-23857, Jun 2001. URL: https://doi.org/10.1074/jbc.m008028200, doi:10.1074/jbc.m008028200. This article has 223 citations.
(russnak2001afamilyof pages 4-6): Roland Russnak, David Konczal, and Steven L. McIntire. A family of yeast proteins mediating bidirectional vacuolar amino acid transport*. The Journal of Biological Chemistry, 276:23849-23857, Jun 2001. URL: https://doi.org/10.1074/jbc.m008028200, doi:10.1074/jbc.m008028200. This article has 223 citations.
(nakajo2026significanceofamino pages 1-7): Haruto Nakajo, Takayuki Sekito, Ryogo Okamura, Shiori Nakagawa, Nodoka Hamada, Yusuke Yamamoto, Wakaba Yagi, Natsumi Ozaka, Takumi Kimura, Soshi Abe, Naoko Sugimoto, Koichi Akiyama, and Miyuki Kawano-Kawada. Significance of amino acid recycling from vacuoles for viability and sporulation of yeast cells under starvation conditions. Scientific Reports, Mar 2026. URL: https://doi.org/10.1038/s41598-026-42129-3, doi:10.1038/s41598-026-42129-3. This article has 0 citations and is from a peer-reviewed journal.
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(yang2006atg22recyclesamino pages 5-6): Zhifen Yang, Ju Huang, Jiefei Geng, Usha Nair, and Daniel J. Klionsky. Atg22 recycles amino acids to link the degradative and recycling functions of autophagy. Molecular biology of the cell, 17 12:5094-104, Dec 2006. URL: https://doi.org/10.1091/mbc.e06-06-0479, doi:10.1091/mbc.e06-06-0479. This article has 403 citations and is from a domain leading peer-reviewed journal.