Membrane remodeling processes in eukaryotes, such as those involved in endocytosis and intracellular trafficking, are mediated by a large number of structural, accessory and regulatory proteins. These processes occur in all cell types, enabling the exchange of signals and/or nutrients with the external medium and with neighboring cells; likewise, they are required for the intracellular trafficking of various cargo molecules between organelles, as well as the recycling of these structures. Recent studies have demonstrated that some elements of the molecular machinery involved in regulating and mediating endocytosis in eukaryotic cells are also present in some bacteria, where they participate in processes such as cell division, sporulation and signal transduction. However, the mechanism whereby this prokaryotic machinery carries out such functions has barely begun to be elucidated. This review summarizes recent information about the cytoskeletal and membrane-organizing proteins for which bacterial homologs have been identified; given their known functions, they may be considered to be part of an ancestral membrane organization system that first emerged in prokaryotes and which further evolved into the more complex regulatory networks operating in eukaryotes. © 2017 IUBMB Life, 69(2):55-62, 2017.

Bacillus thuringiensis (Bt) Cry toxins are used to control Aedes aegypti, an important vector of dengue fever and yellow fever. Bt Cry toxin forms pores in the gut cells, provoking larvae death by osmotic shock. Little is known, however, about the endocytic and/or degradative cell processes that may counteract the toxin action at low doses. The purpose of this work is to describe the mechanisms of internalization and detoxification of Cry toxins, at low doses, into Mos20 cells from A. aegypti, following endocytotic and cytoskeletal markers or specific chemical inhibitors. Here, we show that both clathrin-dependent and clathrin-independent endocytosis are involved in the internalization into Mos20 cells of Cry11Aa, a toxin specific for Dipteran, and Cry1Ab, a toxin specific for Lepidoptera. Cry11Aa and Cry1Ab are not directed to secretory lysosomes. Instead, Mos20 cells use the Rab5 and Rab11 pathways as a common mechanism, most probably for the expulsion of Cry11Aa and Cry1Ab toxins. In conclusion, we propose that endocytosis is a mechanism induced by Cry toxins independently of specificity, probably as part of a basal immune response. We found, however, that actin is necessary for defense-specific response to Cry11Aa, because actin-silenced Mos20 cells become more sensitive to the toxic action of Cry11A toxin. Cry toxin internalization analysis in insect cell lines may contribute to a better understanding to Cry resistance in mosquitoes.

Spo0M has been previously reported as a regulator of sporulation in Bacillus subtilis; however, little is known about the mechanisms through which it participates in sporulation, and there is no information to date that relates this protein to other processes in the bacterium. In this work we present evidence from proteomic, protein-protein interaction, morphological, subcellular localization microscopy and bioinformatics studies which indicate that Spo0M function is not necessarily restricted to sporulation, and point towards its involvement in other stages of the vegetative life cycle. In the current study, we provide evidence that Spo0M interacts with cytoskeletal proteins involved in cell division, which suggest a function additional to that previously described in sporulation. Spo0M expression is not restricted to the transition phase or sporulation; rather, its expression begins during the early stages of growth and Spo0M localization in B. subtilis depends on the bacterial life cycle and could be related to an additional proposed function. This is supported by our discovery of homologs in a broad distribution of bacterial genera, even in non-sporulating species. Our work paves the way for re-evaluation of the role of Spo0M in bacterial cell.

In this mini-review, we present a perspective on the recent findings relating Spo0M structure and function that will stimulate and guide further studies in the characterization of this interesting protein. Cell division and sporulation constitute two of the best studied processes in the model organism Bacillus subtilis; however, there are many missing pieces in the giant regulatory puzzle that governs the independent and shared networks between them. Spo0M is a little studied protein that has been related to both, cell division and sporulation, but its biochemical function and its direct interactions have not been yet defined. Structural analysis of Spo0M revealed the presence of an arrestin-like domain and an FP domain (a dimerization domain present in proteasome elements), motifs more commonly found in eukaryotic proteins. The aim of this perspective is to present open questions regarding the functional and structural features of Spo0M that make this protein a good candidate for the ancestor of arrestins in bacteria and an important element in developmental and differentiation processes of Bacillus subtilis.