Electron Microscopic Study on the Influence of Deimination on Casein Micelle Formation

Abstract

The role of arginine residues in the formation of casein micelles was examined by means of an electron microscope. The positively charged arginyl residues were enzymically modified to noncharged citrullyl residues by protein arginine deiminase (EC 3.5.3.15). When the casein micelles were reconstituted from unmodified κ-casein (CN) and deiminated αs1-CN in the presence of 25mM Ca2+, particles that were similar to submicelles and that had a diameter similar to that of the unmodified type predominated, but those particles were not integrated into the casein micelles. Deimination of the reconstructed casein micelles from unmodified αs1- and κ-CN resulted in an increased diameter of the casein micelles in which the arginyl residues of αs1-CN were partially modified. The electron microscopy revealed that this phenomenon was not due to any enlargement of the casein micelle size itself; instead, the increased diameter was due to an increased number of submicelles on the periphery of the casein micelles, which were possibly linked loosely with one another.