Unraveling membrane remodeling by Bacterial Dynamin-like Protein (BDLP) from Nostoc punctiforme

Abstract

Membranes are fundamental to biological systems as they serve as physical barriers that enable compartmentalization and functional specialization of the cytoplasm. But membranes must undergo constant and regulated mass and shape changes by fission, fusion and remodelling during organelle formation and maintenance. The large GTPase dynamins are well characterized for membrane remodelling in eukaryotes. Interestingly, dynamins are widely present in extant prokaryotes but their functions remain less characterized. The first bacterial dynamin to be characterized was the Bacterial Dynamin-like Protein (BDLP) from the cyanobacterium Nostoc punctiforme. In this study, we characterize lipid binding, enzymatic and membrane remodelling activities of BDLP. Our findings reveal that BDLP preferentially binds to negatively charged membranes composed of phosphatidylglycerol (PG). Interestingly, unlike classical fission dynamins such as dynamin-1, which exhibit a 100-fold increase in GTPase activity upon membrane binding and has been shown to utilize this energy to severe tubular neck like structure, BDLP shows little stimulation in GTPase activity on PG-containing membranes, suggesting a distinct membrane-associated function. This enzymatic feature is similar to that seen among the fusion dynamins like mitofusins to which BDLP shows structural similarities. However, unlike mitofusins, we find that BDLP is incapable of membrane fusion in bulk liposome-based assays. To further understand BDLP functions, we turned to microscopic analysis of BDLP on membranes, which reveal that the GTP-bound BDLP forms a rigid coat or scaffold while the GDP-bound state displays a loose organization on membranes. Remarkably, both nucleotide-bound states were found to tether PG-containing liposomes in a heterotypic manner. On account of its ability to form scaffolds, the GTP-bound state serves to locally confine tethered liposomes while the GDP-bound state does not do so. Interestingly, at high protein to lipid ratios, we observed that GDP-bound BDLP induces extensive membrane remodeling, resembling the effects of GppNHp-bound BDLP at lower protein densities. This suggests that GDP-bound BDLP, at high concentrations, may remodel membranes through enhanced protein packing, increased protein-membrane interactions and deeper membrane insertion. Together, our results provide unique insights into BDLP functions and point to a physiological role in managing both membrane tethering and membrane remodelling.