Size of human lens beta-crystallin aggregates are distinguished by N-terminal truncation of betaB1.

The aggregates formed by the interactions of the human lens beta-crystallins have been particularly difficult to characterize because the beta-crystallins comprise several proteins of similar structure and molecular weight and because their sequences were not known until recently. Previously, it could not be ascertained whether the species of various acidities were different proteins or modifications of the same proteins. The recent determination of the sequences permits calculation of molecular weights and unambiguous identification of the various beta-crystallins and their modified forms by mass spectrometry. In this investigation, the components of the three sizes of beta-crystallin aggregates, beta1 (approximately 150,000), beta2 (approximately 92,000), and beta3 (approximately 46,000), were determined. The principal differences among the different beta-crystallin aggregates was the presence of betaA4 in beta1 and beta2, but not beta3, and the length of the N-terminal extension of betaB1. The size of the beta-crystallin aggregate correlated with the length of the N-terminal extension of betaB1, indicating that the flexible N terminus of betaB1 is critical to the formation of higher molecular weight aggregates of beta-crystallins. Separation of the components by ion exchange under non-denaturing conditions showed that betaB2 occurs as homo-dimers and homo-tetramers as well as contributing to hetero-oligomers. Other beta-crystallins were present only as hetero-oligomers.