Cyanobacteria possess CO2-concentrating mechanisms (CCM) which enables them to elevate the substrate concentrations around the primary CO2-fixing enzyme, D-ribulose-1,5-bisphosphate (RuBisCO). This allows the enzyme to operate at a more efficient rate. One of the key components of this mechanism is a bacterial micro-compartment known as the carboxysome, which encapsulates the RuBisCO enzyme and maintains a high CO2 environment from conversion of actively accumulated bicarbonate. In the β-carboxysome, the pre-assembly of the RuBisCO matrix is coordinated by binding to the CcmM protein, an interaction facilitated by RuBisCO-small-subunit-like domains present in CcmM. Although RuBisCO is known to bind to CcmM, the exact mechanism by which this occurs remains unknown.
To better understand the biogenesis of the β-carboxysome here we examine the interactions between RubisCO, RuBisCO chaperone proteins RbcX and Raf1 and the RuBisCO binding protein CcmM, from Thermosynechococcus elongatus BP-1. To date, the potential role/importance of RuBisCO chaperone proteins in carboxysomal biogenesis has yet to be studied, while the interaction between RuBisCO and CcmM remains poorly understood, in part, due to the insoluble nature of RuBisCO-CcmM complexes. Here we show that both RbcX and Raf1 significantly increase the amount of folded RuBisCO and RuBisCO activity both in the presence and absence of CcmM, indicating their importance in T. elongatus BP-1.