Janus chaperones: Assistance of both RNA- and protein-folding by ribosomal proteins

Ribosomal proteins assist the assembly and increase the stability of ribosomal RNA, without requiring ATP for their action. Some ribosomal proteins are also known to have essential functions outside the ribosome, i.e. promiscuity of functions that appears to correlate with their structural disorder. Here we addressed if certain ribosomal proteins with RNA chaperone activity and with a significant level of disorder also have protein-chaperone activity in vitro. Four proteins of the large subunit of Escherichia coli ribosome, L15, L16, L18 and L19 have been tested in three chaperone assays, in which all of them exhibited potent chaperone activity, commensurable with that of heat shock protein 90 kDa. These observations highlight possible novel aspects of the promiscuous functions of ribosomal proteins outside of the ribosome.