A Potassium Channel-MiRP Complex Controls Neurosensory Function in Caenorhabditis elegans

Abstract

MinK-related peptides (MiRPs) are single transmembrane proteins that associate with mammalian voltagegated K ؉ subunits. Here we report the cloning and functional characterization of a MiRP ␤-subunit, MPS-1, and of a voltage-gated pore-forming potassium subunit, KVS-1, from the nematode Caenorhabditis elegans. mps-1 is expressed in chemosensory and mechanosensory neurons and co-localizes with kvs-1 in a subset of these. Inactivation of either mps-1 or kvs-1 by RNA interference (RNAi) causes partially overlapping neuronal defects and results in broad-spectrum neuronal dysfunction, including defective chemotaxis, disrupted mechanotransduction, and impaired locomotion. Inactivation of one subunit by RNAi dramatically suppresses the expression of the partner subunit only in cells where the two proteins co-localize. Co-expression of MPS-1 and KVS-1 in mammalian cells gives rise to a potassium current distinct from the KVS-1 current. Taken together these data indicate that potassium currents constitute a basic determinant for C. elegans neuronal function and unravel a unifying principle of evolutionary significance: that potassium channels in various organisms use MiRPs to generate uniqueness of function with rich variation in the details.MinK-related peptides (MiRPs or KCNEs) 1 are single-transmembrane proteins that associate with pore-forming ion-channel subunits to form stable complexes with channel properties markedly distinct from those of the isolated pore-forming subunits (1, 2). MiRPs were identified recently in an attempt to find a ␤-subunit for the cardiac potassium channel HERG, which in heterologous expression systems behaves differently than in native cardiomyocytes. Using in silico approaches three MiRPs (MiRP1, MiRP2, and MiRP3) were recognized by their homology with MinK, a putative ␤-subunit of HERG and KCNQ1 (2-5). The last member of the family, MiRP4 was identified later (6). Although MiRPs were initially identified as cardiac proteins, they were soon found to be expressed and function in other tissues and to cause acquired and congenital disease (2, 7-11). For instance, MiRP2 is expressed in skeletal muscle where it associates with Kv3.4 and, when defective, can cause periodic paralysis (8). Mutations in MinK and MiRP1 genes can lead to Long QT syndrome, a specific form of polymorphic ventricular tachycardia characterized by impaired ventricular repolarization (2, 10 -12). A well-established characteristic of MiRPs is the capacity to associate with multiple pore-forming subunits in heterologous systems. For instance, MiRP1 can associate with HERG, KCNQ1, HCN, and Kv4.2 subunits (2, 13-15). Indeed, MiRP "promiscuity" has considerable biomedical implications, because, if a single MiRP coassembles with multiple pore-forming subunits, genetic mutations would be predicted to lead to disruption of multiple currents simultaneously.To date MiRPs have been reported only in vertebrates, suggesting that MiRPs might be relatively young in the evolutionary scale. We speculated that they might underlie a more ge...

An Arrhythmia Susceptibility Gene in Caenorhabditis elegans

“…To strengthen the notion that the two proteins assemble in the pharynx we assessed the effect that epigenetic inactivation of mps-4 by RNAi had on the expression of EXP-2 tagged by GFP. Generally, KCNE proteins are essential components of their respective complexes that become unstable and/or cannot traffic to the plasma membrane without them (22,31). As expected, mps-4 RNAi selectively attenuated GFP signals in isthmus and terminal bulb (Fig.…”

“…This disease predisposes to polymorphic tachyarrhythmias, and life threatening ventricular fibrillation (19). An important feature of kcne genes is that they are conserved across phyla (21)(22)(23). There are four kcne-related genes in Caenorhabditis elegans termed mps genes that play a prominent role in the nervous system of the animal.…”

“…There are four kcne-related genes in Caenorhabditis elegans termed mps genes that play a prominent role in the nervous system of the animal. The founding member of the family, MPS-1, assembles with the pore-forming subunit KVS-1 in a subset of sensory neurons to form a complex that contributes to both their maintenance and sensitivity (22). Two other members, MPS-2 and MPS-3, appear to form a ternary complex with KVS-1 that regulates taste sensitivity to sodium (23).…”

Cumulative Activation of Voltage-Dependent KVS-1 Potassium Channels

“…One of these transmembrane domain motif (S4) contains the positive charges responsible for sensing voltage. KVS-1 channels were cloned previously and expressed in a heterologous expression system and shown to have activation and inactivation kinetics resembling potassium channels of the Shal (Kv4) family (Bianchi et al 2003). However, a National Center for Biotechnology Information Blast comparison of the amino acid sequence of the KVS-1 ␣ subunit with the ␣ subunits of all 10 families of voltagedependent potassium channels showed that the KVS-1 channel was most similar in sequence to the Shab (Kv2) family.…”

“…We now report the reverse phenomenon, which we term "cumulative activation" (CA). In the KVS-1 channel, cloned from Caenorhabditis elegans (Bianchi et al, 2003), we observe a progressive frequency-dependent increase in outward current amplitude in response to repetitive depolarization. Thus, when two depolarizing pulses are repeated with an interpulse interval as long as 20 s, the peak outward current evoked by the second pulse is greater than the peak current of the preceding pulse.…”