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Chaperonin paralogues in cyanobacteria: Their non-classical nature

Hitoshi Nakamoto^a,*, Jittisak Senachak^b, Apiradee Hongsthong^b

 
ABSTRACT:     Chaperonin (GroEL, Hsp60) is a molecular chaperone involved in maintaining cellular protein homeostasis. It interacts with unfolded and misfolded proteins to assist in their folding. In this article, structures, functions, regulation, and significance of chaperonin in cyanobacteria will be reviewed. There are multiple kinds of groEL genes in cyanobacteria in contrast to the Escherichia coli chaperonin paradigm. The cyanobacterial groEL1 gene forms an operon with the single groES gene, similar to the E. coli groESL operon. In contrast, the cyanobacterial groEL2 gene is monocistronic. The regulation of expression and function of the groEL1 and groEL2 genes are mutually distinct. Transcription of the groESL1 operon and groEL2 gene is induced not only by heat but also by light. Unlike the E. coli groESL operon, the expression of the cyanobacterial groESL1 operon and groEL2 is not controlled by the transcription factor sigma32. Cis-regulatory elements such as K-box and CIRCE regulate it positively and/or negatively. Combinations of the elements in groESL1 and groEL2 are evolutionarily diversified. Functional studies suggested that GroEL1 is equivalent to E. coli GroEL, which is essential, whereas GroEL2 is nonessential but plays an important role under stress. The absence of GroEL2 affects proteome and phosphoproteome under stress conditions. Moreover, GroEL1 and GroEL2 are structurally different. We propose that the groEL2 gene is an outcome of neofunctionalization after groESL operon duplication. The groEL1 gene retains the original function essential for cellular activities under both normal and stress conditions, whereas groEL2 acquires a novel, beneficial function required under stress.

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* Corresponding author, E-mail: nakamoto@mail.saitama-u.ac.jp

Received 27 Oct 2022, Accepted 0 0000