Yet another job for the bacterial ribosome

Abstract

The ribosome is a sophisticated cellular machine, composed of RNA and protein, which translates the mRNA-encoded genetic information into protein and thus acts at the center of gene expression. Still, the ribosome not only decodes the genetic information, it also coordinates many ribosome-associated processes like protein folding and targeting. The ribosomal protein uL23 is crucial for this coordination and is located at the ribosomal tunnel exit where it serves as binding platform for targeting factors, chaperones and modifying enzymes. This includes the signal recognition particle (SRP), which facilitates co-translational protein targeting in pro- and eukaryotes, the chaperone Trigger Factor and methionine aminopeptidase, which removes the start methionine in many bacterial proteins. A recent report revealed the intricate interaction of uL23 with yet another essential player in bacteria, the ATPase SecA, which is best known for its role during post-translational secretion of proteins across the bacterial SecYEG translocon.

The Dynamic SecYEG Translocon

“…A co-translational targeting by SecA has been observed for the inner membrane protein RodZ, which contains a large cytosolic domain preceding its single transmembrane domain ( Rawat et al, 2015 ; Wang et al, 2017 ), and for the periplasmic maltose binding protein MBP ( Huber et al, 2017 ). This is in line with the ability of SecA to interact with translating and non-translating ribosomes ( Eisner et al, 2003 ; Karamyshev and Johnson, 2005 ; Huber et al, 2011 ; Knupffer et al, 2019 ; Origi et al, 2019 ; Wang S. et al, 2019 ). On the other hand, a post-translational interaction of SRP has been shown for the small bacterial membrane proteins YohP and YkgR ( Steinberg et al, 2020 ) and for the tail-anchored proteins DjlC, Flk, and SciP ( Pross et al, 2016 ; Peschke et al, 2018 ).…”

“…This is also the binding site for SRP and for many ribosome-associated chaperones and processing factors ( Kramer et al, 2002 , 2009 ; Denks et al, 2017 ; Knupffer et al, 2019 ). Importantly, it is the N-terminus of SecA that interacts with both the ribosome and with SecYEG or phospholipids ( Knupffer et al, 2019 ; Origi et al, 2019 ) and thus SecA binding to ribosomes or to SecYEG appears to be mutual exclusive. This suggests that co-translational targeting by SecA is followed by a post-translational translocation across the SecYEG translocon.…”

The largely unexplored biology of small proteins in pro‐ and eukaryotes

“…One possibility is the involvement of the ATPase SecA. SecA is the main targeting factor for secretory proteins in bacteria [219,220] and delivers them to the SecYEG translocon for transport SecA mainly acts posttranslationally [221,222], although it can also interact with RNCs [223][224][225]. So far, there are only few studies addressing the involvement of SecA in small protein targeting.…”