What Is Motion? Recent Advances in the Study of Molecular Movement Patterns of the Peptidoglycan Synthesis Machines

How proteins move through space and time is a fundamental question in biology. While great strides have been made toward a mechanistic understanding of protein movement, many questions remain. We discuss the biological implications of motion in the context of the peptidoglycan (PG) synthesis machines. We reviewed systems in several bacteria, including Escherichia coli, Bacillus subtilis, and Streptococcus pneumoniae, and present a comprehensive view of our current knowledge regarding movement dynamics. Discrepancies are also addressed because "one size does not fit all". For bacteria to divide, new PG is synthesized and incorporated into the growing cell wall by complex multiprotein nanomachines consisting of PG synthases (transglycosylases [TG] and/or transpeptidases [TP]) as well as a variety of regulators and cytoskeletal factors. Advances in imaging capabilities and labeling methods have revealed that these machines are not static but rather circumferentially transit the cell via directed motion perpendicular to the long axis of model rod-shaped bacteria such as E. coli and B. subtilis. The enzymatic activity of the TG:TPs drives motion in some species while motion is mediated by FtsZ treadmilling in others. In addition, both directed and diffusive motion of the PG synthases have been observed using single-particle tracking technology. Here, we examined the biological role of diffusion regarding transit. Lastly, findings regarding the monofunctional transglycosylases (RodA and FtsW) as well as the Class A PG synthases are discussed. This minireview serves to showcase recent advances, broach mechanistic unknowns, and stimulate future areas of study.