Limits...
Architecture and selectivity in aquaporins: 2.5 a X-ray structure of aquaporin Z.

Savage DF, Egea PF, Robles-Colmenares Y, O'Connell JD, Stroud RM - PLoS Biol. (2003)

Bottom Line: The 2.5 A resolution structure of AqpZ suggests aquaporin selectivity results both from a steric mechanism due to pore size and from specific amino acid substitutions that regulate the preference for a hydrophobic or hydrophilic substrate.This structure provides direct evidence on the molecular mechanisms of specificity between water and glycerol in this family of channels from a single species.It is to our knowledge the first atomic resolution structure of a recombinant aquaporin and so provides a platform for combined genetic, mutational, functional, and structural determinations of the mechanisms of aquaporins and, more generally, the assembly of multimeric membrane proteins.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Biophysics, University of California School of Medicine, San Francisco, California, USA.

ABSTRACT
Aquaporins are a family of water and small molecule channels found in organisms ranging from bacteria to animals. One of these channels, the E. coli protein aquaporin Z (AqpZ), has been shown to selectively conduct only water at high rates. We have expressed, purified, crystallized, and solved the X-ray structure of AqpZ. The 2.5 A resolution structure of AqpZ suggests aquaporin selectivity results both from a steric mechanism due to pore size and from specific amino acid substitutions that regulate the preference for a hydrophobic or hydrophilic substrate. This structure provides direct evidence on the molecular mechanisms of specificity between water and glycerol in this family of channels from a single species. It is to our knowledge the first atomic resolution structure of a recombinant aquaporin and so provides a platform for combined genetic, mutational, functional, and structural determinations of the mechanisms of aquaporins and, more generally, the assembly of multimeric membrane proteins.

Show MeSH
Structure of AqpZThree-dimensional fold of AqpZ with the quasi-2-fold related segments in yellow (residues 1–117) and blue (residues 188–231).(A) Cartoon representation of the AqpZ tetramer with OG detergent molecules represented as spheres; view is from the periplasmic side. Atoms are colored according to atom type (red, oxygen; gray, carbon; blue, nitrogen; yellow, sulfur).(B) Cartoon representation of the AqpZ monomer, with M2 and M6 removed for ease of viewing. Single-file water is shown hydrogen-bonding to carbonyls of main chain. Central water is shown accepting a hydrogen bond from the NH2 group of Asn63 and Asn186. Sidechains of the selectivity filter are also shown. Isopropanol molecules located in density are shown as sticks, just outside the channel.(C) A view from the membrane plane of the OG micelle interactions with the periplasmic segment of AqpZ. OG molecules pack against the aromatic sidechains, while making hydrogen bonds with the main chain carbonyls, E203, and each other. All figures were made with PyMOL (DeLano 2003).
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC300682&req=5

pbio.0000072-g001: Structure of AqpZThree-dimensional fold of AqpZ with the quasi-2-fold related segments in yellow (residues 1–117) and blue (residues 188–231).(A) Cartoon representation of the AqpZ tetramer with OG detergent molecules represented as spheres; view is from the periplasmic side. Atoms are colored according to atom type (red, oxygen; gray, carbon; blue, nitrogen; yellow, sulfur).(B) Cartoon representation of the AqpZ monomer, with M2 and M6 removed for ease of viewing. Single-file water is shown hydrogen-bonding to carbonyls of main chain. Central water is shown accepting a hydrogen bond from the NH2 group of Asn63 and Asn186. Sidechains of the selectivity filter are also shown. Isopropanol molecules located in density are shown as sticks, just outside the channel.(C) A view from the membrane plane of the OG micelle interactions with the periplasmic segment of AqpZ. OG molecules pack against the aromatic sidechains, while making hydrogen bonds with the main chain carbonyls, E203, and each other. All figures were made with PyMOL (DeLano 2003).

Mentions: The protomer structure of AqpZ displays the canonical aquaporin fold of six transmembrane helices and two half-membrane-spanning helices (M1–M8) in a right-handed helical bundle. As shown previously, the protomer oligomerizes to form a homotetramer (Figure 1A). The amino terminus begins on the cytoplasmic side. M1 crosses the membrane and loops to M2, which recrosses the membrane (Figure 1B). M2 is followed by a loop from residue 54 to residue 62 that contains the four carbonyls that project into the pore near the cytoplasmic side. Following this loop is helix M3, which contains the signature asparagine–proline–alanine (NPA) motif and is oriented in such a way as to point its positive dipole towards the central water position in the channel. The first domain of AqpZ ends with M4 ending on the periplasmic side. This is followed by a loop from residue 103 to residue 131 that descends into the periplasmic vestibule and leads into the carboxy-terminal segment. M5–M8 reiterates the amino-terminal topology, except now beginning on the periplasmic and ending on the cytoplasmic side. This pseudo-2-fold symmetry creates a general architecture in which the main chain carbonyls establish water-binding sites along the channel and in which sidechain variation determines channel size and chemistry (Figure 1B).


Architecture and selectivity in aquaporins: 2.5 a X-ray structure of aquaporin Z.

Savage DF, Egea PF, Robles-Colmenares Y, O'Connell JD, Stroud RM - PLoS Biol. (2003)

Structure of AqpZThree-dimensional fold of AqpZ with the quasi-2-fold related segments in yellow (residues 1–117) and blue (residues 188–231).(A) Cartoon representation of the AqpZ tetramer with OG detergent molecules represented as spheres; view is from the periplasmic side. Atoms are colored according to atom type (red, oxygen; gray, carbon; blue, nitrogen; yellow, sulfur).(B) Cartoon representation of the AqpZ monomer, with M2 and M6 removed for ease of viewing. Single-file water is shown hydrogen-bonding to carbonyls of main chain. Central water is shown accepting a hydrogen bond from the NH2 group of Asn63 and Asn186. Sidechains of the selectivity filter are also shown. Isopropanol molecules located in density are shown as sticks, just outside the channel.(C) A view from the membrane plane of the OG micelle interactions with the periplasmic segment of AqpZ. OG molecules pack against the aromatic sidechains, while making hydrogen bonds with the main chain carbonyls, E203, and each other. All figures were made with PyMOL (DeLano 2003).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC300682&req=5

pbio.0000072-g001: Structure of AqpZThree-dimensional fold of AqpZ with the quasi-2-fold related segments in yellow (residues 1–117) and blue (residues 188–231).(A) Cartoon representation of the AqpZ tetramer with OG detergent molecules represented as spheres; view is from the periplasmic side. Atoms are colored according to atom type (red, oxygen; gray, carbon; blue, nitrogen; yellow, sulfur).(B) Cartoon representation of the AqpZ monomer, with M2 and M6 removed for ease of viewing. Single-file water is shown hydrogen-bonding to carbonyls of main chain. Central water is shown accepting a hydrogen bond from the NH2 group of Asn63 and Asn186. Sidechains of the selectivity filter are also shown. Isopropanol molecules located in density are shown as sticks, just outside the channel.(C) A view from the membrane plane of the OG micelle interactions with the periplasmic segment of AqpZ. OG molecules pack against the aromatic sidechains, while making hydrogen bonds with the main chain carbonyls, E203, and each other. All figures were made with PyMOL (DeLano 2003).
Mentions: The protomer structure of AqpZ displays the canonical aquaporin fold of six transmembrane helices and two half-membrane-spanning helices (M1–M8) in a right-handed helical bundle. As shown previously, the protomer oligomerizes to form a homotetramer (Figure 1A). The amino terminus begins on the cytoplasmic side. M1 crosses the membrane and loops to M2, which recrosses the membrane (Figure 1B). M2 is followed by a loop from residue 54 to residue 62 that contains the four carbonyls that project into the pore near the cytoplasmic side. Following this loop is helix M3, which contains the signature asparagine–proline–alanine (NPA) motif and is oriented in such a way as to point its positive dipole towards the central water position in the channel. The first domain of AqpZ ends with M4 ending on the periplasmic side. This is followed by a loop from residue 103 to residue 131 that descends into the periplasmic vestibule and leads into the carboxy-terminal segment. M5–M8 reiterates the amino-terminal topology, except now beginning on the periplasmic and ending on the cytoplasmic side. This pseudo-2-fold symmetry creates a general architecture in which the main chain carbonyls establish water-binding sites along the channel and in which sidechain variation determines channel size and chemistry (Figure 1B).

Bottom Line: The 2.5 A resolution structure of AqpZ suggests aquaporin selectivity results both from a steric mechanism due to pore size and from specific amino acid substitutions that regulate the preference for a hydrophobic or hydrophilic substrate.This structure provides direct evidence on the molecular mechanisms of specificity between water and glycerol in this family of channels from a single species.It is to our knowledge the first atomic resolution structure of a recombinant aquaporin and so provides a platform for combined genetic, mutational, functional, and structural determinations of the mechanisms of aquaporins and, more generally, the assembly of multimeric membrane proteins.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Biophysics, University of California School of Medicine, San Francisco, California, USA.

ABSTRACT
Aquaporins are a family of water and small molecule channels found in organisms ranging from bacteria to animals. One of these channels, the E. coli protein aquaporin Z (AqpZ), has been shown to selectively conduct only water at high rates. We have expressed, purified, crystallized, and solved the X-ray structure of AqpZ. The 2.5 A resolution structure of AqpZ suggests aquaporin selectivity results both from a steric mechanism due to pore size and from specific amino acid substitutions that regulate the preference for a hydrophobic or hydrophilic substrate. This structure provides direct evidence on the molecular mechanisms of specificity between water and glycerol in this family of channels from a single species. It is to our knowledge the first atomic resolution structure of a recombinant aquaporin and so provides a platform for combined genetic, mutational, functional, and structural determinations of the mechanisms of aquaporins and, more generally, the assembly of multimeric membrane proteins.

Show MeSH