Mapping the Kidney Potassium Channel ROMK1

Abstract

ROMK1, also known as Kir 1.1, is an inwardly rectifying K ؉ channel and is the prototypical member of the large Kir gene family. The accepted model of Kir topology predicts intracellular NH 2 and COOH termini, and two membrane-spanning segments, M1 and M2, connected by an intramembranous pore-forming segment, H5. The sequence of H5 is similar in voltage-dependent K ؉ channels and features a strictly conserved GY/FG in its mid-region, which has been proposed as the selectivity filter of the pore. We have been using N-glycosylation substitution mutants to map the extracellular topology of ROMK1 biochemically and have described several loci in H5 that were glycosylated. We now report glycosylation at loci Tyr 144 and Phe 146 , which indicates that the signature GYG sequence (143-145) rather than being intramembranous is extracellular. The COOH terminus was predicted to begin at position 178, but contrary to the model, we observed that position 257 was glycosylated and surrounding positions at 199, 222, and 298 were unglycosylated. N-Glycosylation sequon substitution at the latter three positions abolished K ؉ /Na ؉ selectivity. Our results suggest a major revision of the topology of ROMK1 with H5 and the pore signature sequence now completely extracellular. The COOH terminus appears to form two additional membrane-spanning segments and to contribute to the ion conduction pathway.ROMK1 is a weak, inwardly rectifying ATP-regulated K ϩ channel that was cloned from rat kidney outer medulla (1). There are three isoforms, which differ in their NH 2 terminus as a result of alternative splicing (2, 3). The isoforms are differentially expressed along the loop of Henle and distal nephron (4). In the thick ascending loop of Henle, ROMK provides K ϩ to the Na-K-2Cl cotransporter and thereby enhances Na ϩ uptake (5, 6). This physiological role is supported by mutations in ROMK that have been linked to the antenatal form of Bartter's syndrome, characterized by hypokalemic metabolic alkalosis, hypotension, renal salt wasting, and hypercalciuria (7, 8). Furthermore, characterization of some of these Bartter's ROMK1 mutations revealed disruption of function by phosphorylation, proteolytic processing, and protein transport. The topological model of ROMK1 and related members of the Kir gene family was predicted from hydropathy plots and, in the case of H5, from sequence similarity to Kv channels 2 (Fig. 1A). The NH 2 and COOH termini were cytoplasmic, based on the absence of a signal sequence in the open reading frame, and the ␣-helical transmembrane segments M1 and M2 were separated by a linker containing a 17-residue stretch called H5 (9 -13). In Kv channels, H5 is thought to form an intramembranous hairpin structure with the selectivity filter at the highly conserved GYG, which is at loci 143-145 of ROMK1. Recently, we showed that several H5 loci of ROMK1 could be glycosylated and that one of them, Q139N, in both its glycosylated and unglycosylated forms was nonselective between K ϩ and Na ϩ (14). That H5 is a hotspot for residue...

Site-directed Glycosylation Tagging of Functional Kir2.1 Reveals That the Putative Pore-forming Segment Is Extracellular

“…3). K ϩ selectivity was also retained when the two corresponding residues were mutated in Kir1.1 (16). The double mutants also had N-linked oligosaccharide at positions 147 and 146 of Kir2.1 and Kir1.1, respectively.…”

“…Experimental Design-In our topological model of Kir1.1, the putative H5 segment is extracellular (15,16). To assess whether the extracellularity of this segment is novel to Kir1.1 or is a general structural feature of Kirs, we have studied a strong inwardly rectifying K ϩ channel, Kir2.1, using N-glycosylation tagging.…”

“…Beads were resuspended in 125-150 l of reducing SDS-PAGE sample buffer (2ϫ) and either heated at 90°C for 4 min or incubated overnight at room temperature and then diluted appropriately with SDS-PAGE sample buffer (1ϫ) prior to separation on 12% SDS-polyacrylamide gels. The electrophoresed proteins were transferred to Immobilon P membranes and probed with the M2 antibody (16). The gels were run until the 46-kDa rainbow marker was 0.25-2.5 cm from the bottom of the gel.…”

“…Our findings for Kir1.1 were inconsistent with the accepted Kir topological model, because positions 117, 128, 133, 139, 144, 146, and 153 between M1 and M2, including the signature sequence, could be glycosylated. The results indicate that H5 is part of the extracellular loop between M1 and M2 and does not penetrate the membrane (15)(16)(17). To assess whether this biochemically derived topological structure is an accurate description of other Kirs, we examined a different member of the Kir class, Kir2.1 (see Fig.…”

“…We have mapped the topological structure of Kir1.1, a weak inwardly rectifying K ϩ channel, using biochemical methods to identify which novel N-glycosylation sites of Kirs are utilized (15)(16)(17). The initial steps of N-glycosylation occur on the luminal side of the endoplasmic reticulum, and therefore only extracytoplasmic segments of membrane proteins can be glycosylated (18).…”