Cyanobacteria and some chemoautotrophs sequester Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in close proximity to CO2 using large proteinaceous microcompartments called carboxysomes, to improve carbon fixation efficiency. Carboxysomes are built from tens of thousands of protein subunits with a closed icosahedral shell that is selectively permeable for certain substrates and metabolites. This shell encases functional enzymes, including Rubisco, within. CcmM is an interior β-carboxysome protein that has been implicated in organizing the carboxysome interior. The C-terminal domain of CcmM is built of three to five subunits that closely resemble the small subunit of Rubisco, and have been implicated in organizing Rubisco into a core that the carboxysomal shell forms around. This thesis reports the first known X-ray crystallographic structure of the first C-terminal domain in the chain “SS1” from T. elongatus BP-1. Förster Resonance Energy Transfer, Surface Plasmon Resonance and Native mass spectrometry experiments show CcmM binds Rubisco with micromolar affinities. Six SS1 domains can bind to Rubisco with no indication of RbcS release, suggesting that binding does not fully displace RbcS. A novel molecular binding model for the CcmM-Rubisco complex is proposed.