CcmK1-4 and CcmL proteins found in Synechocystis sp. strain PCC 6803 make up the beta carboxysome shell. These CcmK proteins have been shown to form hexameric units, while the CcmL proteins have been shown to form pentameric units. Together these proteins further assemble into the flat facets of the polyhedral carboxysome shell. The structures suggest that the central pores and the gaps between hexamers limit the transport of metabolites into and out of the the carboxysome.