Self-catalyzed growth of S layers via an amorphous-to-crystalline transition limited by folding kinetics

• Published in: Proceedings of the National Academy of Sciences - PNAS 2010-09, Vol.107 (38), p.16536-16541
• Main Authors: Chung, Sungwook, Shin, Seong-Ho, Bertozzi, Carolyn R., De Yoreo, James J.
• Format: Article
• Language: English
• Subjects: Bacillaceae - chemistry; Bacterial Proteins - chemistry; Biological Sciences; Biophysical Phenomena; Catalysis; Cell walls; Condensation; Crystal lattices; Crystallization; Crystals; Enzyme kinetics; Kinetics; Lipids; Membrane Glycoproteins - chemistry; Microscopy; Microscopy, Atomic Force; Models, Molecular; Molecules; Monomers; Nucleation; Order disorder transformations; Phase Transition; Protein Folding; Protein Multimerization; Proteins
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1008280107

The importance of nonclassical, multistage crystallization pathways is increasingly evident from theoretical studies on colloidal systems and experimental investigations of proteins and biomineral phases. Although theoretical predictions suggest that proteins follow these pathways as a result of fluctuations that create unstable dense-liquid states, microscopic studies indicate these states are long-lived. Using in situ atomic force microscopy to follow 2D assembly of S-layer proteins on supported lipid bilayers, we have obtained a molecular-scale picture of multistage protein crystallization that reveals the importance of conformational transformations in directing the pathway of assembly. We find that monomers with an extended conformation first form a mobile adsorbed phase, from which they condense into amorphous clusters. These clusters undergo a phase transition through S-layer folding into crystalline clusters composed of compact tetramers. Growth then proceeds by formation of new tetramers exclusively at cluster edges, implying tetramer formation is autocatalytic. Analysis of the growth kinetics leads to a quantitative model in which tetramer creation is rate limiting. However, the estimated barrier is much smaller than expected for folding of isolated S-layer proteins, suggesting an energetic rationale for this multistage pathway.