Proteins with RBM(ring-building motif)-like domain involved in bactaria sporulation
- Authors
- Publication Date
- Nov 09, 2020
- Source
- HAL-Descartes
- Keywords
- Language
- English
- License
- Unknown
- External links
Abstract
Specialized secretion systems found in Gram-negative bacteria allow the transport of molecules across their double-membrane cell envelope. Components of these nanomachines include ring-forming proteins from the PrgK and PrgH families, which are part of the inner membrane platform in Type-III secretion systems, or the InvG and GspD secretins from Type-III and Type-II secretion systems, respectively. Homo-oligomerization of these proteins involves a domain called RBM for "Ring-Building Motif". Despite low sequence identity, RBM domains display a conserved wedge-shaped fold composed of a three-stranded β-sheet packed against two α-helices.Because the cell envelope of Gram-positive bacteria possess a single membrane, double-membrane spanning machineries are not necessary for secretion. During spore formation in Gram-positive bacteria however, the mother cell engulfs the developing spore, encasing it with a double membrane. Communication between the two cells involves a large multi-protein complex called the SpoIIIA-SpoIIQ complex. The global architecture and function of this new machinery remains mysterious but its components display structural similarities with essential constituents of specialized secretion systems. In particular, some of the SpoIIIA-SpoIIQ proteins possess RBM-like domains and one of them, called SpoIIIAG, forms large oligomeric rings that display remarkable similarities and differences with PrgK and PrgH rings from Type-III secretion systems. Ring formation by SpoIIIAG provides evidence that the SpoIIIA-SpoIIQ complex might serve as a secretion machinery between the mother cell and forespore but assembly of a trans-envelope channel requires oligomerization of other SpoIIIA-SpoIIQ proteins.To get further insights into the capacity of RBM-containing SpoIIIA-SpoIIQ proteins to form rings, I produced, purified and characterized full-length membrane and truncated soluble forms of these proteins. This part of my work showed that the RBM domain alone in SpoIIIAG is not sufficient to promote oligomerization in vitro, and that additional secondary structures observed in non-canonical RBM domain is not what prevents them from forming rings in vitro.Intriguingly, RBM domains were also found in proteins that are not related to the SpoIIIA-SpoIIQ complex and raised the hypothesis that other putative secretion systems might form during sporulation. In order to investigate this, I studied the structure and oligomerization ability of one of these proteins, which is called YhcN and is likely involved in spore germination. The crystallographic structure of YhcN revealed the presence of a non-canonical RBM domain and the protein did not show any oligomerization ability.Altogether, my work questions the ring-forming function associated with RBM domains and suggests that some of these domains might have evolved to fulfill different roles.
