Different Types Of Polymorphisms Within The F-actin And ParM Filaments

Abstract

Cytoskeletal proteins form dynamic networks in eukaryotic cells which are vital for chromosome segregation, movement of cellular organelles, motility and morphology. All three types of cytoskeletal elements found in eukaryotic cell - actins, tubulins, and intermediate filaments - are also present in bacteria. MreB and ParM are prokaryotic actin homologs that have little sequence identity with eukaryotic actin, but have a very similar fold. MreB maintains cell shape, while ParM is involved in plasmid segregation. Significant effort has been devoted to building an atomic model of the actin filament, but the structure of the ParM filament has been controversial. We used electron cryo-microscopy and the IHRSR helical reconstruction approach to compare the structures of actin and ParM filaments We show that F-actin, in addition to variable twist, possess a significant structural disorder and exists in multiple structural states. ParM filaments are even more heterogeneous. In addition to variability in twist, which is greater than that observed in F-actin, they have a variable axial rise. ParM filaments also have structural heterogeneity which arises from opening of the ATP-binding cleft. The subunit within the ParM filament, with a non-hydrolyzable ATP analog bound, can exist in both open and closed conformations. This variability in the opening of the cleft introduces a variability in the interface between the protomers in the ParM filament. Altogether, our results show that F-actin and ParM form very different filaments, and these filaments possess quite different types of structural heterogeneity. This is consistent with a lack of conservation between actin and ParM in the regions involved in the subunit-subunit interface within the filament. We conclude that both actin and ParM filaments are not uniform in their structure, and thus can not be described by a single atomic model.