Motivated by the octomer structure of RuBisCO, we constructed model cargo particles, each containing eight pseudoatoms positioned at the corners of a cube, with center-to-center spacing of . Each pseudoatom interacts with other particles in the system as described for cargo particles in appendix 1. Parameters are (A) , , , and (B) , , , (the magnitude of the shell-cargo and cargo-cargo interaction strengths is reduced because multiple cargo pseudoatoms participate in each interaction). To keep the cargo volume fraction equal to those in other simulations in this manuscript, we set the number of octomer cargo particles to 132 (each has a volume of ). Other parameters are as described in the Model section.