Extracytoplasmic disposition of lysine beta 165 of acetylcholine receptor

Lysine-691 of the anion exchanger from human erythrocytes is located on its cytoplasmic surface

A combination of vectorial modification and site-directed immunochemistry has been used to determine the disposition, with respect to the membrane, of Lys-691 of the anion exchanger from human erythrocytes. Intact erythrocytes and inside-out vesicles were vectorially modified in the same container with pyridoxal phosphate and sodium [3H]borohydride. The modified inside-out vesicles were separated from erythrocytes by differential centrifugation and the vesicles and erythrocyte membranes were treated with alkali and digested with trypsin and thermolysin to liberate the peptides IVSKPER and IVSK¿Nepsilon-[4'-(5'-phospho-[4-3H]pyridoxyl)]¿PER. These peptides, containing the unmodified and modified versions of Lys-691, were retrieved from the digests by site-directed immunochemistry and were identified by HPLC and liquid scintillation spectroscopy. Both the inside-out vesicles and the intact erythrocytes contained the peptide IVSKPER, however, the 3H-label from the phosphopyridoxylated peptide could be detected only in the inside-out vesicles. The incorporation of 3H into Lys-691 of the anion exchanger from inside-out vesicles was at least 30-fold greater than the incorporation into Lys-691 of the anion exchanger from intact erythrocytes. It follows that Lys-691 of the anion exchanger is located on the cytoplasmic surface of the plasma membrane.

Topological disposition of lysine 943 in native Na+/K(+)-transporting ATPase

Because of the conflicting conclusions that have been reached regarding the location of the two putative membrane-spanning segments from cysteine 911 through isoleucine 929 and from isoleucine 946 through cysteine 964 in the alpha subunit of native ovine Na+/K(+)-transporting ATPase, the disposition of lysine 943 with respect to the plane of the lipid bilayer was investigated. Sealed, right-side-out vesicles were modified with pyridoxal phosphate and Na[3H]BH4 in the presence and absence of saponin, a reagent that creates holes in the membranes. Modified alpha polypeptide was isolated, and digested with trypsin and chymotrypsin to release the desired peptides, QQGMK and QQGMK([3H]pyr)NK (where [3H]pyr designates the modification on lysine 943). These peptides, after cyclization of their amino-terminal glutamines, were isolated with an immunoadsorbent specific for the amino-terminal sequence pyroglutamyl-QGM-followed by high-pressure liquid chromatography on a C-18 reverse phase column. Comparisons were made of the extent of incorporation of radioactivity into lysine 943 between sealed vesicles and sealed vesicles pretreated with saponin. An increase in incorporation into lysine 943 of 5-fold to 18-fold was seen in vesicles pretreated with saponin prior to the modification with pyridoxal phosphate. This increase in incorporation is consistent with a cytoplasmic location for lysine 943. This conclusion places the residues on the carboxy-terminal side of the putative membrane-spanning segment from cysteine 911 through isoleucine 929 and the amino-terminal side of the putative membrane-spanning segment from isoleucine 946 through cysteine 964 in the ovine alpha subunit on the cytoplasmic side of the membrane.

A topological analysis of goldfish kainate receptors predicts three transmembrane segments

Glutamate receptors are the most abundant excitatory neurotransmitter receptors in vertebrate brain. We have previously cloned cDNAs encoding two homologous kainate receptors (GFKAR alpha, 45 kDa, and GFKAR beta, 41 kDa) from goldfish brain and proposed a topology with three transmembrane domains (Wo, Z. G., and Oswald, R. E. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 7154-7158). These studies have been extended using an in vitro translation/translocation system in conjunction with site-specific antibodies and point and deletion mutations. We report here that the entire region between the previously proposed third and fourth transmembrane segments is translocated and likely to be extracellular in mature receptors. This was based on the following results. 1) The entire segment was protected from Proteinase K and trypsin digestion and could be immunoprecipitated by a site-specific antibody. 2) Functional sites for N-glycosylation are present in the C-terminal half of the segment, and 3) a mutation, constructed with an additional consensus site for N-glycosylation in the N-terminal half of the segment, was found to be glycosylated at that site. Given the fact that the N terminus of the protein is likely to be extracellular, this would place an even number of transmembrane segments between the extracellular N terminus and the glycosylated segment. In addition, results of N-glycosylation and proteolysis protection assays of GFKAR alpha mutations indicated that the previously proposed second transmembrane segment is not a true transmembrane domain. These results provide further evidence in support of a topology with three transmembrane domains that has important implications for the relationship of structure to function in ionotropic glutamate receptors.