Macromolecular crowding decelerates aggregation of a β-rich protein, bovine carbonic anhydrase: a case study

The majority of in vitro investigations concerning protein aggregation have been performed in dilute systems, which poorly reflect the crowded in vivo scenario. Cell interior is highly crowded with soluble and insoluble macromolecules that alter macromolecular properties. Macromolecular crowding is known to enhance the rate and/or extent of protein aggregation. However, most of the understandings were derived from studies with alpha-rich or predominantly alpha-proteins. Indeed, alpha-proteins fold faster than beta-proteins and conversion of alpha-helices to cross beta-sheets are responsible for aggregate/amyloid formation. Therefore, it is important to investigate how macromolecular crowding affects the aggregation propensity of beta-rich proteins. In this study, we investigated the effect of synthetic macromolecular crowders on bovine carbonic anhydrase (BCA, a beta-rich protein) aggregation. In contrast to the effect of macromolecular crowding on alpha-rich proteins, BCA aggregation was observed to be reduced due to decrease in the population of aggregation-prone intermediates as a consequence of increased native state stability. In addition, the extent of aggregation was found to depend on the nature of the crowder under consideration. Combining the published data on alpha-proteins and this study, we conclude that macromolecular crowding can have opposite consequences on protein aggregation process depending on the fold type of the protein.