Protein self-assembly relies upon the formation of stabilizing noncovalent interactions across subunit interfaces. Identifying the determinants of self-assembly is crucial for understanding structure-function relationships in symmetric protein complexes and for engineering responsive nanoscale architectures for applications in medicine and biotechnology. Lumazine synthases (LS's) comprise a protein family that forms diverse quaternary structures, including pentamers and 60-subunit dodecahedral capsids. To improve our understanding of the basis for this difference in assembly, we attempted to convert the capsid-forming LS from Aquifex aeolicus (AaLS) into pentamers through a small number of rationally designed amino acid substitutions. Our mutations targeted side chains at ionic (R40), hydrogen bonding (H41), and hydrophobic (L121 and I125) interaction sites along the interfaces between pentamers. We found that substitutions at two or three of these positions could reliably generate pentameric variants of AaLS. Biophysical characterization indicates that this quaternary structure change is not accompanied by substantial changes in secondary or tertiary structure. Interestingly, previous homology-based studies of the assembly determinants in LS's had identified only one of these four positions. The ability to control assembly state in protein capsids such as AaLS could aid efforts in the development of new systems for drug delivery, biocatalysis, or materials synthesis.