Characterization of the DnaK Chaperone Network in the Cyanobacterium Synechocystis sp. PCC 6803

Molecular chaperones of the Hsp70 family are involved in diverse cellular processes, such as in ensuring proper folding of newly synthesized polypeptide chains, in protein transport and translocation and in preventing protein misfolding and aggregation. The activity of DnaK is regulated by the Hsp40 (DnaJ) and the nucleotide exchange factor GrpE co-chaperones. The mesophilic cyanobacterium Synechocystis sp. PCC 6803 encodes three DnaK proteins, which share a high degree of sequence identity both in their N-terminal ATPase domain and in the adjacent peptide binding domain. However, the C-terminal DnaK domain is highly variable. Interestingly, while the canonical dnaK gene can be deleted in Escherichia coli, in Synechocystis only the dnaK1 gene can be deleted, whereas dnak2 and dnak3 are essential. In contrast to the cytosolic proteins DnaK1 and DnaK2, the Dnak3 protein is attached to thylakoid membranes via its prolonged C-terminus (by a thus far uncharacterised mechanism). Analysis of the promoter activity and the relative protein amounts clearly indicated physiological differences of the three proteins in vivo, and DnaK2 appears to have a key function in Synechocystis. For all three DnaK proteins we could already show specific binding of the GrpE co-chaperone, and currently aim to resolve specific interactions of the three DnaK proteins with the nine DnaJ co-chaperones of Synechocystis.