2.A.90 The Vitamin A Receptor/Transporter (STRA6) Family

The bovine STRA6 (stimulated by retinoic acid-6) protein (668 aas) is a multispan integral membrane protein with 11-12 TMSs. Mammals contain multiple paralogues or partial paralogues showing regions of sequence similarity. These can be short (~200 aas) or long (1708 aas, EAW58841; the KIAA1528, isoform CRA_b of Homo sapiens; sequence similarity between residues 17-314 with residues 311-668 of STRA6). KIAA1528 has 5-6 putative N-terminal TMSs (residues 1-250) followed by an ~1500 residue long hydrophilic C-terminal region (Wolf 2007). These homologues do not show sequence similarity with any other proteins in TCDB. Vitamin A transporters play important roles in visual function and serve as membrane receptors for dietary Vitamin A uptake, storage, and transport to the eye (Martin Ask et al. 2021).

The bovine STRA6 homologue has been reported to be a receptor for retinol binding protein (RBP). It binds RBP with high affinity and apparently extracts the vitamin A from the RBP-vitamin A complex, transporting it into the cell (Kawaguchi et al., 2007). It is widely expressed in embryonic tissues and in adult organs. Homologues are found only in animals. STRA6 is essential for induction of vascular smooth muscle lineages in human embryonic cardiac outflow tract development (Zhou et al. 2023).

It has been suggested that STRA6 is a transmembrane pore which transports vitamin A bidirectionally between extra- and intracellular retinoid binding proteins and that vitamin A accumulation in cells is driven by coupling of transport with vitamin A esterification (Kelly and von Lintig 2015). Thus, it is not clear what the mechanisms of transport and energy coupling are. 

Vitamin A has diverse biological functions and has been used to treat human diseases including vision diseases, skin diseases, and cancer. Both insufficient and excessive vitamin A uptake are detrimental. STRA6 can mediate both cellular vitamin A influx and efflux. Zhong et al. 2020 purified and identified STRA6-associated proteins and found that the major STRA6-associated protein is calmodulin, consistent with the cryogenic electron microscopy (cryo-EM) study of zebrafish STRA6 associated with calmodulin. They showed that increased calcium/calmodulin promotes cellular vitamin A efflux and suppresses vitamin A influx through STRA6.  Also, calmodulin enhances the binding of apo-RBP to STRA6, and this enhancement is much more pronounced for apo-RBP than holo-RBP. Thus, calmodulin regulates STRA6's vitamin A influx vs efflux activity by modulating its preferential interaction with apo-RBP or holo-RBP (Zhong et al. 2020).

Loss-of-function studies in zebrafish and mouse models have unraveled the critical importance of STRA6 for vitamin A homeostasis of peripheral tissues. Impairment in vitamin A transport and uptake homeostasis are associated with diseases including type 2 diabetes and a microphthalmic syndrome known as Matthew Wood Syndrome (Kelly and von Lintig 2015).

Retinol-binding protein (RBP) is the sole carrier in the bloodstream for hydrophobic retinol, the main form in which vitamin A is transported. The integral membrane receptor, STRA6, mediates cellular uptake of vitamin A by recognizing RBP-retinol to trigger release and internalization of retinol. Chen et al. 2016 presented the structure of zebrafish STRA6, determined to 3.9-angstrom resolution by single-particle cryo-electron microscopy. STRA6 has one intramembrane and nine transmembrane helices in an intricate dimeric assembly. Calmodulin is bound tightly to STRA6 in a noncanonical arrangement. Residues involved with RBP binding map to an archlike structure that covers a deep lipophilic cleft. This cleft is open to the membrane, suggesting a possible mode for internalization of retinol through direct diffusion into the lipid bilayer.

The reaction catalyzed (reversibly?) by STRA6 is:

vitamin A (out) → vitamin A (in)


 

References:

and Sun H. (2012). Membrane receptors and transporters involved in the function and transport of vitamin A and its derivatives. Biochim Biophys Acta. 1821(1):99-112.

Alapatt, P., F. Guo, S.M. Komanetsky, S. Wang, J. Cai, A. Sargsyan, E. Rodríguez Díaz, B.T. Bacon, P. Aryal, and T.E. Graham. (2013). Liver retinol transporter and receptor for serum retinol-binding protein (RBP4). J. Biol. Chem. 288: 1250-1265.

Breen, C.J., D.S. Martin, H. Ma, K. McQuaid, R. O''Kennedy, and J.B. Findlay. (2015). Production of functional human vitamin A transporter/RBP receptor (STRA6) for structure determination. PLoS One 10: e0122293.

Chen, Y., O.B. Clarke, J. Kim, S. Stowe, Y.K. Kim, Z. Assur, M. Cavalier, R. Godoy-Ruiz, D.C. von Alpen, C. Manzini, W.S. Blaner, J. Frank, L. Quadro, D.J. Weber, L. Shapiro, W.A. Hendrickson, and F. Mancia. (2016). Structure of the STRA6 receptor for retinol uptake. Science 353:.

Kawaguchi, R., J. Yu, J. Honda, J. Hu, J. Whitelegge, P. Ping, P. Wiita, D. Bok, and H. Sun. (2007). A membrane receptor for retinol binding protein mediates cellular uptake of vitamin A. Science 315: 820-825.

Kawaguchi, R., J. Yu, P. Wiita, M. Ter-Stepanian, and H. Sun. (2008). Mapping the membrane topology and extracellular ligand binding domains of the retinol binding protein receptor. Biochemistry 47: 5387-5395.

Kawaguchi, R., M. Zhong, M. Kassai, M. Ter-Stepanian, and H. Sun. (2015). Vitamin A Transport Mechanism of the Multitransmembrane Cell-Surface Receptor STRA6. Membranes (Basel) 5: 425-453.

Kelly, M. and J. von Lintig. (2015). STRA6: role in cellular retinol uptake and efflux. Hepatobiliary Surg Nutr 4: 229-242.

Martin Ask, N., M. Leung, R. Radhakrishnan, and G.P. Lobo. (2021). Vitamin A Transporters in Visual Function: A Mini Review on Membrane Receptors for Dietary Vitamin A Uptake, Storage, and Transport to the Eye. Nutrients 13:.

Radhakrishnan, R., M. Leung, A.K. Solanki, and G.P. Lobo. (2023). Mapping of the extracellular RBP4 ligand binding domain on the RBPR2 receptor for Vitamin A transport. Front Cell Dev Biol 11: 1105657.

Wolf, G. (2007). Identification of a membrane receptor for retinol-binding protein functioning in the cellular uptake of retinol. Nutr Rev 65: 385-388.

Xiao, Q., L. Wang, S. Supekar, T. Shen, H. Liu, F. Ye, J. Huang, H. Fan, Z. Wei, and C. Zhang. (2020). Structure of human steroid 5α-reductase 2 with the anti-androgen drug finasteride. Nat Commun 11: 5430.

Zhong, M., R. Kawaguchi, B. Costabile, Y. Tang, J. Hu, G. Cheng, M. Kassai, B. Ribalet, F. Mancia, D. Bok, and H. Sun. (2020). Regulatory mechanism for the transmembrane receptor that mediates bidirectional vitamin A transport. Proc. Natl. Acad. Sci. USA 117: 9857-9864.

Zhong, M., R. Kawaguchi, M. Ter-Stepanian, M. Kassai, and H. Sun. (2013). Vitamin a transport and the transmembrane pore in the cell-surface receptor for plasma retinol binding protein. PLoS One 8: e73838.

Zhou, C., T. Häneke, E. Rohner, J. Sohlmér, P. Kameneva, A. Artemov, I. Adameyko, and M. Sahara. (2023). STRA6 is essential for induction of vascular smooth muscle lineages in human embryonic cardiac outflow tract development. Cardiovasc Res. [Epub: Ahead of Print]

Examples:

TC#NameOrganismal TypeExample
2.A.90.1.1

The STRA6 vitamin A transporter/RBP receptor mediates cellular uptake of Vitamin A (Sun, 2011). It has 668 aas and probably 9 TMSs (Kawaguchi et al. 2015).  Transport is reversible, and is probably mediated by a carrier or group translocation mechanism (Kelly and von Lintig 2015).  Mechanistic features have been discussed (Kawaguchi et al. 2015), and it was concluded that energy-independent facilitated diffusion, from external retinol binding protein (RBP) to internal CRBP-1 or the corresponding CRABP (for retinoic acid) after release of the substrate into the iipid environment of the membrane or an enzyme that forms retinol esters (LRAT) provides the most probable mechanism. These vitamin A transporters play roles in visual functions. They serve as membrane receptors for dietary vitamin A uptake, storage, and transport to the eye (Martin Ask et al. 2021).

Animals

STRA6 of Bos taurus (ABG81428)

 
2.A.90.1.2

Stimulated by retinoic acid gene 6, STRA6 (670 aas; 8-10 TMSs predicted with N- and C-termini outside) (Kawaguchi et al., 2008).  Mechanistic features have been discussed (Kawaguchi et al. 2015), and it was concluded that energy-independent facilitated diffusion, from external holo-retinol binding protein (RBP) to internal CRBP-1 or the corresponding CRABP (for retinoic acid) after release of the substrate into the iipid environment of the membrane or an enzyme that forms retinol esters (LRAT) provides the most probable mechanism.

Animals

STRA6 of Mus musculus (O70491)

 
2.A.90.1.3

STRA6 of 667 aas and 9 - 12 putative TMSs (Zhong et al. 2013).  Mechanistic features have been discussed (Kawaguchi et al. 2015), and it was concluded that energy-independent facilitated diffusion, from external holo-retinol binding protein (RBP) to internal CRBP-1 or the corresponding CRABP (for retinoic acid) after release of the substrate into the iipid environment of the membrane or an enzyme that forms retinol esters (LRAT) provides the most probable mechanism of transport.  STRA6 binds RBP with a 1:1 stoichiometry (Breen et al. 2015). Zhong et al. 2020 purified and identified STRA6-associated proteins and found that the major STRA6-associated protein is calmodulin, consistent with the cryo-EM study of zebrafish STRA6 associated with calmodulin. They showed that increased calcium/calmodulin promotes cellular vitamin A efflux and suppresses vitamin A influx through STRA6.  Also, calmodulin enhances the binding of apo-RBP to STRA6, and this enhancement is much more pronounced for apo-RBP than holo-RBP. Thus, calmodulin seems to regulate STRA6's vitamin A influx vs efflux activities by modulating its preferential interaction with apo-RBP or holo-RBP (Zhong et al. 2020).

Animals

STRA6 of Homo sapiens

 
2.A.90.1.4

STRA6 of 670 aas, mediates cellular uptake of vitamin A by recognizing RBP-retinol to trigger release and internalization of retinol. Chen et al. 2016 presented the structure of zebrafish STRA6, determined to 3.9-angstrom resolution by single-particle cryo-electron microscopy. STRA6 has one intramembrane and nine transmembrane helices in an intricate dimeric assembly. Calmodulin is bound tightly to STRA6 in a noncanonical arrangement. Residues involved with RBP binding map to an arch-like structure that covers a deep lipophilic cleft. This cleft is open to the membrane, suggesting a possible mode for internalization of retinol through direct diffusion into the lipid bilayer.

STRA6 of Danio rerio (Zebrafish) (Brachydanio rerio)

 
Examples:

TC#NameOrganismal TypeExample
2.A.90.2.1

Sea urchin protein of unknown function

Animals

Uncharacterized protein of Strongylocentrotus purpuratus

 
2.A.90.2.2

Rotifer Stra6 protein of 655 aas

Animals

Stra6 homologue of Adineta (Callidina) vaga

 
2.A.90.2.3

Stra6 homologue of 716 aas

Ichthyosporea (protists; amoebae)

Stra6 homologue of Capsaspora owczarzaki

 
2.A.90.2.4

Stra6 homologue of 1327 aas

Choanoflagellida

Stra6 homologue of Monosiga brevicollis

 
2.A.90.2.5

Stimulated by retinoic acid gene 6 protein-like, Str61, RBPR2, of 621 aas and 11 or 12 TMSs. It acts as a high-affinity cell-surface receptor for retinol-binding protein RBP4 and mediates RBP4-dependent retinol uptake in the liver (Alapatt et al. 2013). The extracellular RBP4 ligand binding domain on the RBPR2 receptor for Vitamin A transporthas been mapped (Radhakrishnan et al. 2023).

RBPR2 of Mus musculus (Mouse) 

 
Examples:

TC#NameOrganismal TypeExample
2.A.90.3.1

Harmful bloom algal protein of unknown function, of 962 aas and 16 putative TMSs.  TMSs 1-5 or 6 correspond to synaptic glycoprotein SC2, possibly a sterol reductase (TC@2.A.90.3.2 and3.3), while the last 5 TMSs are homologous to TMSs 7 - 11 in the 11 TMS STRA6 protein (2.A.90.1.1).  Thus, this protein may be a fusion protein.   

Algae

Uncharacterized protein of Aureococcus anophagefferens

 
2.A.90.3.2

Synaptic glycoprotein SC2 of 299 aas and 6 TMSs.

SC2 of Culex quinquefasciatus (Southern house mosquito) (Culex pungens)

 
2.A.90.3.3

Putative steroid reductase required for elongation of the very long chain fatty acids, of 285 aas and 6 TMSs.

Reductase of Ixodes ricinus (Common tick)

 
2.A.90.3.4

Steroid 5-alpha reductase, putative, of 252 aas and 6 TMSs.

Reductase of Entamoeba histolytica

 
2.A.90.3.5

Uncharacterized protein of 287 aas and 5 or 6 TMSs.

UP of Entamoeba histolytica

 
2.A.90.3.6

The human steroid 5alpha-reductase 2 of 254 aas and 7 TMSs.  The x-ray structure has been determined to 2.8 Å with the anti-androgen drug finasteride bound (Xiao et al. 2020). It catalyzes the reduction of testosterone to dihydrotestosterone, and mutations in the SRD5A2 gene have been linked to 5alpha-reductase deficiency and prostate cancer. Finasteride and dutasteride, as SRD5A2 inhibitors, are widely used antiandrogen drugs for benign prostate hyperplasia. Xiao et al. 2020 showed a unique 7-TMS topology and an intermediate adduct of finasteride and NADPH as NADP-dihydrofinasteride in a largely enclosed binding cavity inside the transmembrane domain. Structural analysis together with computational and mutagenesis studies reveal the molecular mechanisms of the catalyzed reaction and of finasteride inhibition involving residues E57 and Y91. Molecular dynamics simulation results indicated  conformational dynamics of the cytosolic region that regulate NADPH/NADP(+) exchange (Xiao et al. 2020).

SRD5A2 of Homo sapiens