By "protection" the authors mean that in the presence of NusA, some proteins are less prone to aggregation after heat shock. "Protection", in general, could also refer to stabilization i.e. reduction in the degradation rates of the protein.
To detect the influence of NusA on heat-induced aggregation, we used
lactate dehydrogenase (LDH). LDH was incubated at 48°C in the absence
or presence of NusA. As shown in Fig. 2b, LDH slowly aggregated at
48°C without the protection of NusA. However, when this experiment was
performed in the presence of NusA, aggregation was visibly reduced.
These results suggest that NusA also protects unfolded proteins from
irreversible aggregation during thermal stress.
In E. coli, RpoA and RpoB, the major components of RNAP, are prone to
aggregation under heat stress in vivo. They were reported to interact
with GroEL and DnaK, mainly for the protection from aggregation or
refolding into a nonnative state48,49. However, based on the strong
interaction between NusA and RNAP, we propose that the chaperone
activity of NusA could provide more direct and concentrated protection
for RNAP under stress, i.e., NusA could simultaneously act as a latent
protector of RNAP during transcription elongation and termination.