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Vitamin A transport and the transmembrane pore in the cell-surface receptor for plasma retinol binding protein.

Zhong M, Kawaguchi R, Ter-Stepanian M, Kassai M, Sun H - PLoS ONE (2013)

Bottom Line: We employ acute chemical modification to introduce chemical side chains to STRA6 in a site-specific manner.We found that modifications with specific chemicals at specific positions in or near the transmembrane domains of this receptor can almost completely suppress its vitamin A transport activity.These experiments provide the first evidence for the existence of a transmembrane pore, analogous to the pore of ion channels, for this new type of cell-surface receptor.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, Jules Stein Eye Institute, and Howard Hughes Medical Institute, David Geffen School of Medicine, University of California, Los Angeles, California, United States of America.

ABSTRACT
Vitamin A and its derivatives (retinoids) play diverse and crucial functions from embryogenesis to adulthood and are used as therapeutic agents in human medicine for eye and skin diseases, infections and cancer. Plasma retinol binding protein (RBP) is the principal and specific vitamin A carrier in the blood and binds vitamin A at 1:1 ratio. STRA6 is the high-affinity membrane receptor for RBP and mediates cellular vitamin A uptake. STRA6 mice have severely depleted vitamin A reserves for vision and consequently have vision loss, even under vitamin A sufficient conditions. STRA6 humans have a wide range of severe pathological phenotypes in many organs including the eye, brain, heart and lung. Known membrane transport mechanisms involve transmembrane pores that regulate the transport of the substrate (e.g., the gating of ion channels). STRA6 represents a new type of membrane receptor. How this receptor interacts with its transport substrate vitamin A and the functions of its nine transmembrane domains are still completely unknown. These questions are critical to understanding the molecular basis of STRA6's activities and its regulation. We employ acute chemical modification to introduce chemical side chains to STRA6 in a site-specific manner. We found that modifications with specific chemicals at specific positions in or near the transmembrane domains of this receptor can almost completely suppress its vitamin A transport activity. These experiments provide the first evidence for the existence of a transmembrane pore, analogous to the pore of ion channels, for this new type of cell-surface receptor.

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Related in: MedlinePlus

Scanning the seventh transmembrane helix of STRA6 for positions that enhance the sensitivity of STRA6 to MTSEA-biotin modification.Twenty eight residues in or near the seventh transmembrane domain of STRA6 were each changed to cysteine. Each mutant was tested for STRA6-catalyzed retinol release from holo-RBP. Grey traces, no modification. Red traces, MTSEA-biotin modification. All experiments were done in triplicate.
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pone-0073838-g005: Scanning the seventh transmembrane helix of STRA6 for positions that enhance the sensitivity of STRA6 to MTSEA-biotin modification.Twenty eight residues in or near the seventh transmembrane domain of STRA6 were each changed to cysteine. Each mutant was tested for STRA6-catalyzed retinol release from holo-RBP. Grey traces, no modification. Red traces, MTSEA-biotin modification. All experiments were done in triplicate.

Mentions: We further systematically scanned transmembrane helices VI and VII and adjacent regions by introducing cysteine to each residue. We chose to target these transmembrane helices in this study due to their immediate adjacency to the essential RBP binding domain of STRA6 [47]. We tested each STRA6 mutant for acute chemical modification (Figures 4 and 5). We found that most cysteine mutants behave similarly as the wild-type protein without cysteine modification (Figures 4 and 5). However, cysteine modification of a few of them highly suppresses STRA6′s retinol release activity. From the 60 mutants we created, we identified 20 positions whose modification can significantly suppress STRA6-mediated vitamin A transport (Figure 6). Interestingly, in helix wheel presentations, many key residues on the sixth or the seventh transmembrane helices are located on the same side of the helix (Figure 7A). As shown in the STRA6 transmembrane topology model (Figure 7B), many of the key residues are also located near the cytoplasmic side of STRA6 and are far away from the extracellular RBP binding domain [47].


Vitamin A transport and the transmembrane pore in the cell-surface receptor for plasma retinol binding protein.

Zhong M, Kawaguchi R, Ter-Stepanian M, Kassai M, Sun H - PLoS ONE (2013)

Scanning the seventh transmembrane helix of STRA6 for positions that enhance the sensitivity of STRA6 to MTSEA-biotin modification.Twenty eight residues in or near the seventh transmembrane domain of STRA6 were each changed to cysteine. Each mutant was tested for STRA6-catalyzed retinol release from holo-RBP. Grey traces, no modification. Red traces, MTSEA-biotin modification. All experiments were done in triplicate.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3815300&req=5

pone-0073838-g005: Scanning the seventh transmembrane helix of STRA6 for positions that enhance the sensitivity of STRA6 to MTSEA-biotin modification.Twenty eight residues in or near the seventh transmembrane domain of STRA6 were each changed to cysteine. Each mutant was tested for STRA6-catalyzed retinol release from holo-RBP. Grey traces, no modification. Red traces, MTSEA-biotin modification. All experiments were done in triplicate.
Mentions: We further systematically scanned transmembrane helices VI and VII and adjacent regions by introducing cysteine to each residue. We chose to target these transmembrane helices in this study due to their immediate adjacency to the essential RBP binding domain of STRA6 [47]. We tested each STRA6 mutant for acute chemical modification (Figures 4 and 5). We found that most cysteine mutants behave similarly as the wild-type protein without cysteine modification (Figures 4 and 5). However, cysteine modification of a few of them highly suppresses STRA6′s retinol release activity. From the 60 mutants we created, we identified 20 positions whose modification can significantly suppress STRA6-mediated vitamin A transport (Figure 6). Interestingly, in helix wheel presentations, many key residues on the sixth or the seventh transmembrane helices are located on the same side of the helix (Figure 7A). As shown in the STRA6 transmembrane topology model (Figure 7B), many of the key residues are also located near the cytoplasmic side of STRA6 and are far away from the extracellular RBP binding domain [47].

Bottom Line: We employ acute chemical modification to introduce chemical side chains to STRA6 in a site-specific manner.We found that modifications with specific chemicals at specific positions in or near the transmembrane domains of this receptor can almost completely suppress its vitamin A transport activity.These experiments provide the first evidence for the existence of a transmembrane pore, analogous to the pore of ion channels, for this new type of cell-surface receptor.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, Jules Stein Eye Institute, and Howard Hughes Medical Institute, David Geffen School of Medicine, University of California, Los Angeles, California, United States of America.

ABSTRACT
Vitamin A and its derivatives (retinoids) play diverse and crucial functions from embryogenesis to adulthood and are used as therapeutic agents in human medicine for eye and skin diseases, infections and cancer. Plasma retinol binding protein (RBP) is the principal and specific vitamin A carrier in the blood and binds vitamin A at 1:1 ratio. STRA6 is the high-affinity membrane receptor for RBP and mediates cellular vitamin A uptake. STRA6 mice have severely depleted vitamin A reserves for vision and consequently have vision loss, even under vitamin A sufficient conditions. STRA6 humans have a wide range of severe pathological phenotypes in many organs including the eye, brain, heart and lung. Known membrane transport mechanisms involve transmembrane pores that regulate the transport of the substrate (e.g., the gating of ion channels). STRA6 represents a new type of membrane receptor. How this receptor interacts with its transport substrate vitamin A and the functions of its nine transmembrane domains are still completely unknown. These questions are critical to understanding the molecular basis of STRA6's activities and its regulation. We employ acute chemical modification to introduce chemical side chains to STRA6 in a site-specific manner. We found that modifications with specific chemicals at specific positions in or near the transmembrane domains of this receptor can almost completely suppress its vitamin A transport activity. These experiments provide the first evidence for the existence of a transmembrane pore, analogous to the pore of ion channels, for this new type of cell-surface receptor.

Show MeSH
Related in: MedlinePlus