Aug 2026· Annual Review of Microbiology· 0 citations
Medicine
TL;DR
Current knowledge on T9SS architecture and function is summarized, the embedded gliding machinery is described, and conceptual advances and open questions regarding the mechanisms, dynamics, and ecological implications of this unique system are highlighted.
Abstract
The type IX secretion system (T9SS) is a Bacteroidota-specific multiprotein machine that supports a wide range of biological processes, from nutrient acquisition and surface modification to host interaction and gliding motility. T9SS effectors represent a structurally diverse repertoire of enzymes, adhesins, and surface proteins that all possess a C-terminal domain that addresses them to their final destination. Recent structural and mechanistic information has revealed the modular organization of the T9SS and molecular details governing effector selection, transport, processing, and sorting. In motile Bacteroidota, the T9SS has been co-opted, evolved, and specialized for gliding motility. In this review, we summarize current knowledge on T9SS architecture and function, describe the embedded gliding machinery, and highlight conceptual advances and open questions regarding the mechanisms, dynamics, and ecological implications of this unique system.
SUMMARY One of the first gram-negative protein secretion systems to be discovered was the type II secretion system (T2SS), in which proteins first cross the inner membrane mainly via the Sec translocon, and then are recognized and carried across the outer membrane by the multiprotein T2SS apparatus. With its erstwhile appellation as the main terminal branch of the general secretory pathway, the T2SS has sometimes been described as (only) secreting a relatively small number of degradative enzymes for the purpose of nutrient acquisition. However, as comprehensively reviewed here, the T2SS is a great deal more than this, e.g., it (i) can secrete and expose on the cell surface over 100 proteins, (ii) is a major mediator of virulence against humans, animals, and plants, secreting diverse substrates that target essentially every step in the pathogenic process, (iii) promotes biofilm formation and environmental reactions such as metal reduction, (iv) has functional interactions with other secretion systems, and (v) has a homolog in mitochondria. Moreover, current assessment of the literature reveals an underappreciated role for the T2SS in both interbacterial antagonism and bacterial competition with fungi, protozoa, and perhaps algae. We also assembled data indicating that the T2SS mediates bacterial resistance to antibiotics. Finally, the T2SS is a fascinatingly complex “machine” whose contributing parts can vary and whose ability to recognize substrates is primarily conformational, involving spatially separated elements that interact with the T2SS apparatus through transient, multivalent interactions rather than a single dedicated binding motif.
N. Cianciotto, Joshua Mayoral, James A Garnett· Microbiology and Molecular B...· 0 citations
The extent to which chaperones are universally required for effector secretion is evaluated, through analysis of the conservation of chaperone sequence and structure, to discuss how these proteins interact with and support the secretion of diverse substrates.
Kyra Roepke, Alexia J Galsworthy, Adam Agbamu et al.· Microbiology· 0 citations
The type VI secretion system (T6SS) is a widespread contractile nanomachine in pathogenic bacteria that delivers diverse effectors directly into prokaryotic or eukaryotic target cells. These effectors confer competitive fitness advantages to attacker cells through multifaceted target manipulation. Current classification categorizes T6SS effectors as "specialized" or "cargo" types based on delivery mechanisms. This review systematically examines secretory pathways mediating effector translocation via covalent/non-covalent interactions with the Hcp-VgrG-PAAR puncturing complex. Subsequently, it characterizes the distinctive features of Hcp-, VgrG-, and PAAR-dependent effectors. In addition, this review highlights the indispensable role of adaptor proteins in effector-core component recognition. By synthesizing the secretion modalities of diverse effectors via the T6SS puncturing structure, this review provides a comprehensive framework for understanding the loading and delivery mechanisms of T6SS effectors.
Tiange Ma, Yan Hu, Jian Xu et al.· The FEBS Journal· 0 citations
ABSTRACT The type VI secretion system (T6SS) is a nanomachine utilized by Gram-negative bacteria to secrete toxic effectors. The T6SS is employed against both prokaryotic and eukaryotic targets, allowing for competitive advantage and virulence of the attackers in a wide variety of environments. Recent work has revealed the great diversity of T6SS systems. Beyond the canonical, conserved architectural genes of the T6SS, diverse T6SS-associated genes (TAGs) are employed to optimize T6SS activity, compensate for variations in architecture, and dictate T6SS use. Although various TAGs have been discovered in T6SS gene clusters, they display unique activities in modifying their cognate T6SS. In this review, we discuss several of the better-studied TAGs, as well as highlight under-researched TAGs that are likely of great significance. As a means to better understand TAG roles, identify common principles and unifying themes, we present the following six functional categories: Firing Coordinators, Target Recognition Factors, Structural Factors, Transcriptional Regulators, Secretion Regulators, and Unknown TAGs.
C. Latario, Benjamin D. Ross· Journal of Bacteriology· 0 citations
Key determinants of secretion specificity and endopilus stability are identified, revealing how minor sequence variations in conserved nanomachines drive functional adaptation to diverse environments.
ABSTRACT The type VII secretion system (T7SS) is an ATPase-powered molecular machine encoded by Actinomycetota (T7SSa) and Bacillota (T7SSb) that exports small alpha-helical effectors with functions in interbacterial competition, host colonization, and virulence. While T7SSb has been well characterized in Staphylococcus aureus and Bacillus subtilis, its presence and function within the order Lactobacillales have only been thoroughly investigated within the past decade. Experimental evidence for T7SSb function has now been established in select Streptococcus and Enterococcus species. Here, we review the current understanding of T7SSb in these genera, covering core machinery and secreted substrates, including LXG toxins and WXG100 proteins. We discuss the roles of T7SSb during interbacterial antagonism and host-pathogen interactions, including contributions to cytotoxicity and virulence, as well as mucosal colonization and persistence. We further review known regulators of the T7SSb and evaluate the ecological significance of T7SSb-mediated competition for microbial community stability. Finally, we highlight outstanding questions in the field, including mechanisms of effector delivery, structural determination of the apparatus, impact of effectors on host immunity, and potential therapeutic targeting of T7SSb components.
Caitlin S. Wiafe-Kwakye, Abigail E. Glenn, Brady L. Spencer· Journal of Bacteriology· 0 citations
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