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Tuberous sclerosis protein complex (pTSC) nucleates a proteinaceous signaling hub that integrates information about the internal and external energy status of the cell in the regulation of growth and energy consumption. Biochemical and cryo-electron microscopy studies of recombinant pTSC have revealed its structure and stoichiometry and hinted at the possibility that the complex may form large oligomers. Here, we have partially purified endogenous pTSC from fasted mammalian brains of rat and pig by leveraging a recombinant antigen binding fragment (Fab) specific for the TSC2 subunit of pTSC. We demonstrate Fab-dependent purification of pTSC from membrane-solubilized fractions of the brain homogenates. Negative stain electron microscopy of the samples purified from pig brain demonstrates rod-shaped protein particles with a width of 10 nm, a variable length as small as 40 nm, and a high degree of conformational flexibility. Larger filaments are evident with a similar 10 nm width and a ≤1 μm length in linear and weblike organizations prepared from pig brain. Immunogold labeling experiments demonstrate linear aggregates of pTSC purified from mammalian brains. These observations suggest polymerization of endogenous pTSC into filamentous superstructures.

Purpose of review Current research on the human band 3 glycoprotein, the red cell chloride/bicarbonate anion exchanger (AE1), is highlighted and placed within a structural context. Recent Findings The determination of the crystal structure of the membrane domain of human band 3, the founding member of the solute carrier 4 (SLC4) family of bicarbonate transporters, is a major breakthrough toward understanding the mechanism of action of this membrane transport protein, its interaction with partner proteins, and how mutations linked to disease affect its ability to fold and function. Summary Band 3 contains 14 transmembrane segments arranged in a 7+7 transmembrane inverted repeat topology common to all members of the SLC4 family and the unrelated SLC26 anion transporter family. A functional feature of this fold is the presence of a core and a gate domain: The core domain contains two short transmembrane helices (TM3 and 10) that face each other in the middle of the membrane with the positive N-terminal helix dipoles creating the anion-binding site, whereas the gate domain forms the dimer interface. During transport, the movement of these two domains relative to each other provides the intracellular and extracellular compartments with alternating access to the central anion-binding site.

The Interferon-Induced Proteins with Tetratricopeptide Repeats (IFITs) are a group of potently expressed Interferon Stimulated Genes that mediate antiviral innate immunity. Previous studies have revealed that most IFITs partake in higher order structures, potentially as part of an ‘IFIT interactome’ that results in viral inhibition. Recent crystal structures of a mutated, monomeric form of IFIT1 revealed the molecular basis of how it recognizes non-self, capped viral mRNAs to selectively inhibit their translation. However, wild-type IFIT1 forms dimers in solution and the role of dimerization was not examined in detail. Here we present a structural and biochemical analysis of wild-type IFIT1 in complex with capped and uncapped RNA. Wild-type IFIT1 forms an antiparallel, elongated dimer that is in stark contrast to the domain-swapped, parallel dimer found in IFIT2. Dimerization takes place through a small, C-terminal interface that is evolutionarily conserved in IFIT1 and IFIT1B proteins. The interface is modular and can be grafted onto IFIT5, which is natively monomeric, to induce dimerization. Mutational analysis of this interface showed that homo-dimerization is not required for full RNA binding or translational inhibition by IFIT1. Sedimentation velocity analytical ultracentrifugation measurements demonstrated a reversible monomer-dimer equilibrium, suggesting that dimerization is of low affinity and could play a role under physiological concentrations, possibly in regulating IFIT interactome assembly. Finally, conformational changes in IFIT1 that occur upon RNA binding provide insight into how RNA enters its binding site in solution.