A Proposed Chaperone of the Bacterial Type VI Secretion System Functions To Constrain a Self-Identity Protein

Abstract

The bacterium can communicate identity through the secretion of the self-identity protein IdsD via the type VI secretion (T6S) system. IdsD secretion is essential for self-versus-nonself recognition behaviors in these populations. Here we provide an answer to the unresolved question of how the activity of a T6S substrate, such as IdsD, is regulated before secretion. We demonstrate that IdsD is found in clusters that form independently of the T6S machinery and activity. We show that the IdsC protein, which is a member of the proposed DUF4123 chaperone family, is essential for the maintenance of these clusters and of the IdsD protein itself. We provide evidence that amino acid disruptions in IdsC are sufficient to disrupt IdsD secretion but not IdsD localization into subcellular clusters, strongly supporting the notion that IdsC functions in at least two different ways: maintaining IdsD levels and secreting IdsD. We propose that IdsC, and likely other DUF4123-containing proteins, functions to regulate T6S substrates in the donor cell both by maintaining protein levels and by mediating secretion at the T6S machinery. Understanding the subcellular dynamics of self-identity proteins is crucial for developing models of self-versus-nonself recognition. We directly addressed how a bacterium restricts self-identity information before cell-cell exchange. We resolved two conflicting models for type VI secretion (T6S) substrate regulation by focusing on the self-identity protein IdsD. One model is that a cognate immunity protein binds the substrate, inhibiting activity before transport. Another model proposes that DUF4123 proteins act as chaperones in the donor cell, but no detailed molecular mechanism was previously known. We resolve this discrepancy and propose a model wherein a chaperone couples IdsD sequestration with its localization. Such a molecular mechanism restricts the communication of identity, and possibly other T6S substrates, in producing cells.

Accumulation of dead cells from contact killing facilitates coexistence in bacterial biofilms

“…92 Such cell debris may eventually prevent competing cells from contacting. Similar 93 observations have been made previously when studying T6SS-mediated 94 interactions [50][51][52].…”

Peer pressure from a Proteus mirabilis self-recognition system controls participation in cooperative swarm motility

“…Briefly, two proteins, IdsD and IdsE, govern self identity. IdsD is transferred between cells in a Type VI secretion system (T6SS)-dependent fashion; disruption of the T6SS prevents all Ids signal transfer [17, 21]. A cell in a swarm is considered to be self if it produces an IdsE protein that can bind IdsD proteins sent from neighboring cells.…”

“…Disruption of these IdsD-IdsE interactions, either through deletion of idsE or through production of non-self IdsD or IdsE variants, result in strains that display extreme attenuation in swarm expansion without loss of viability [16, 17]. Fig 1A shows cartoon representations of Ids-mediated self recognition, where endogenous IdsD and IdsE are represented by capital letters “D” and “E”, and a non-self variant of IdsD is represented by lowercase “d.” The IdsD protein must be incoming: endogenous IdsD and IdsE proteins produced within a single cell do not impact self-recognition behaviors [21]. We describe conditions that lead to non-self recognition as “Ids mismatch.”…”

“…Excluded cells grow and divide at a comparable rate to non-excluded cells when isolated from swarms [17]. While Ids has been described as a toxin-antitoxin system [22, 23], this characterization is inconsistent with experimental data and is likely due to the reliance on the T6SS for transport of IdsD [17, 21]. The T6SS is often characterized as a lethal toxin delivery mechanism [24].…”

A family of contact-dependent nuclease effectors contain an exchangeable, species-identifying domain

“…Strains lacking a functional idrD gene are unable to merge colonies with their wild-type parent 15 or dominate in mixed migrating populations (10) and possibly in the human host (11). IdrD is exported by a Type VI secretion system, allowing for transport through direct physical contact with adjacent cells (10,12). The C-terminal end of IdrD encodes a potential cell-contact dependent effector protein, IdrD-CT.…”