Lipidic Cubic Phase-Induced Membrane Protein Crystallization: Interplay between Lipid Molecular Structure, Mesophase Structure and Properties, and Crystallogenesis

Obtaining well-diffracting crystals of membrane proteins, which is crucial to the molecular-level understanding of their intrinsic three-dimensional structure, dynamics, and function, represents a fundamental bottleneck in the field of structural biology. One of the major advances in the field of membrane protein structural determination was the realization that the nanostructured lipidic cubic phase (LCP) environment constitutes a membrane mimetic matrix that promotes solubilization, stabilization, and crystallization of specific membrane proteins. Despite two decades passing since the introduction of LCP-based membrane protein crystallization, the research community's understanding of the processes that drive protein nucleation and macromolecular assembly in the LCP generally remains limited. In the current study, we present a detailed, systematic investigation into the relationship between the chemical structure of the lipid, the physical properties of the ensuing mesophase, the translational diffusion of the encapsulated membrane protein, and the resulting protein crystallization. Importantly, we show for the first time that cubic phase transport properties directly modulate the size and morphology of fully grown protein crystals without affecting their crystallographic space group. These findings provide a deeper understanding of the LCP-based crystal growth process, setting the stage for the engineering of membrane protein crystals at the molecular level via intentional design of the host cubic phase, and, ultimately, accelerating progress in the field of membrane protein structural biology.