Acting Like Actin

Abstract

Biochemistry In a dividing cell, DNA must be replicated and accurately partitioned between the two daughter cells. In eukaryotes, microtubules, which are linked to the centromeric DNA by kinetochores, separate the replicated chromosomes with the help of motor proteins. Prokaryotic plasmid-partitioning systems are similar in that they involve an adaptor protein that links DNA to a filament-forming protein; however, no motor proteins are involved, and nucleotidehydrolysis dependent filament dynamics are responsible for movement. To gain insight into the segregation of the Bacillus thuringiensis pBToxis plasmid, Aylett et al. determined the structure of two protofilaments of the tubulin/FtsZ-like protein, TubZ, which mediates plasmid segregation. A combination of crystallography and electron microscopy revealed that instead of forming tubulin-like straight protofilaments that organize into microtubules, TubZ forms twisted double filaments that can bundle, reminiscent of structures formed by ParM, an actin-like prokaryotic partitioning filament. That two disparate proteins have evolved to perform a similar function using similar superstructures suggests strong evolutionary constraints on partitioning systems. Proc. Natl. Acad. Sci. U.S.A. 107 , 19766 (2010).