Factors deciding the assembly and thermostability of the DmrB cage.

Dihydromethanopterin reductase (DmrB), is a naturally occurring cage protein found in various archaeal and a few bacterial species. It exists as 24mer with cubic geometry where 8 trimeric subunits are present at the corners of each cube. Each trimer is made up of three monomeric units and six FMN, where two molecules of FMN are present at the interface of each monomer. DmrB is involved in the conversion of dihydromethanopterin to tetrahydromethanopterin using FMN as a redox equivalent. In the present study, we have used spectroscopic and biochemical techniques along with complementary bio-informatic work to understand the assembly principles of the DmrB. Our results show a concentration dependant self-assembly of DmrB which is mediated by ionic interactions. The co-factor FMN stabilizes and preserves the secondary and quaternary structure of DmrB against thermal insult, indicating that the higher order assembly of DmrB is very thermostable. Our work provides an interesting piece of information regarding the role of the co-factors in the thermostability of these classes of cage proteins. The understanding of the assembly and disassembly of this thermostable cage would enable the downstream usage of this system in various nano-biotechnological applications.