From pasta to pentamers

Cells are often said to be filled with cytosolic soup, a chunky broth where organelles and macromolecules carry out the mechanics of life. According to Michaela Jansen, the protein domains that inhabit this intracellular stew may be less like wet noodles than previously thought.

Jansen was the penultimate speaker in Sunday’s early-morning Symposium on Pentameric Ligand-Gated Ion Channels, chaired by University of Wisconsin Professor Cynthia Czajkowski. The field has experienced substantial progress in the past decade, with structures of ten distinct family members (or substantial domains thereof) determined by crystallography, NMR, or cryo-electron microscopy since 2005. However, no structure yet published has included a complete intracellular domain: variable in sequence and poorly behaved in isolation, this region is thought by some to be intrinsically disordered—floppy pasta tethered loosely to the better-characterized transmembrane and extracellular domains.

So it was stirring news that Jansen’s team, based at Texas Tech, has succeeded in expressing and purifying a soluble construct containing the intracellular domain of the serotonin-3A receptor. This channel is a target for drug developers as well as neurophysiologists, and contains one of the longest cytosolic loops of any family member. Surprisingly, the protein’s intracellular domain—fused to a soluble expression partner, the identity of which Jansen declined to share due to pending patents—forms pentamers on its own, mirroring the stoichiometry of the full-length channel. As Jansen described it, this “spaghetti conundrum” indicates the presence of previously-unidentified assembly determinants in the region: after all, “even well ordered pasta does not oligomerize.” Given that the corresponding extracellular and transmembrane domains often form noncanonical states in isolation, intrinsic intracellular domain assembly could help explain the selective oligomerization of many pentameric ligand-gated ion channels.

Jansen’s talk was one of several in her session to leverage comparative structures in this increasingly well-characterized family: preceding talks by Pei Tang (University of Pittsburgh) and Claudio Grosman (University of Illinois at Urbana-Champaign) took alternative chimera approaches to elucidate mechanisms of modulation and selectivity, among other things. Session chair Czajkowski closed the session with new spectroscopic and fluorescence data revealing precise gating motions in bacterial homologs. Czajkowski’s final remarks also provided some poignant perspective: when she began attending Biophysical Society meetings as a young scientist, she remembers finding only a handful of posters on pentameric ligand-gated ion channels; in contrast, this year’s meeting includes her own featured Symposium, a half-dozen additional subgroup and platform talks, and two dedicated poster sessions. Tantalizing data like Jansen’s further illustrate the potential of biophysics to open new lines of inquiry into such previously intractable proteins, including their most noodly domains.

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