EMBL tomograms of frozen bacteria link protein production to membrane transport

"Both projects are essentially proofs of concept for this kind of analysis," said Joe Dobbs, lead author of one of the two papers and a former Ph.D. fellow in Julia Mahamid's group at EMBL Heidelberg ^[2][12]. He also described the analysis. "Rather than looking at a single type of molecule in isolation, both papers examine interactions between different cellular systems with a common thread of understanding how different processes are coordinated," he said ^[3].

According to the Phys.org report, past research described the cell's individual components, but scientists lacked the tools to probe how those components interact ^[14]. Cryo-ET can get at interactions because it images flash-frozen cells in 3D, so each molecular machine is caught where it was working ^[4]. Dobbs's team then made the images countable. They sorted ribosomes, the cell's protein-making machines, by functional state across hundreds of *Mycoplasma pneumoniae* cells ^[5].

The first result is structural evidence for long-hypothesized "supercomplexes", new complexes that tie transcription, the copying of DNA into mRNA, directly to translation ^[6]. The thing this doesn't tell you is how common they are. If most ribosomes in a cell sit in such complexes, coupling is routine for the bacterium. If only a small share do, it is one arrangement among several.

The second result concerns ribosomes at rest. Parts of the ribosome stayed attached to the cell membrane even when they were not making protein, and the authors suggest the subunits let go only when conditions allow a new round of synthesis to begin ^[7]. A tomogram fixes where each particle sat at the moment of freezing. The order of events, parked first and released later, is therefore inferred from many still positions. Mammalian cells were seen doing something similar decades ago, and the team suggests the behavior may be conserved from bacteria to humans ^[8]. The observation itself was made in a bacterium ^[5].

Rasmus Jensen, who led the second project in the same group, worked in reverse. "Joe started with a biological question he wanted to answer," Jensen said. "I started from the opposite direction." ^[13] "We'd seen something interesting in the cell and worked backwards to figure out what it was, eventually discovering that it's a new configuration of a molecular machine that helps proteins either be exported out of the cell or inserted into its membrane," he said ^[9].

Working backwards took three methods: cryo-ET, proteomics and computational protein structure prediction ^[10]. The tomogram supplies a shape. The protein inventory and the predicted structures help put names on it. Together they showed the machine contains the Sec-translocon, the well-known membrane channel that moves newly made proteins out of the cell, plus three previously unknown proteins that, according to the report, help transported proteins fold into their working shapes ^[10].

The same work produced the most detailed images so far of the conserved part of the bacterial protein transport machinery ^[11]. Those images show how proteins translated by ribosomes begin moving through the cell membrane ^[11].