Recruitment of heterologous substrates by bacterial secretion systems for transkingdom translocation

Abstract

Bacterial secretion systems mediate the selective exchange of macromolecules between bacteria and their environment, playing a pivotal role in processes such as horizontal gene transfer or virulence. Among the different families of secretion systems, Type III, IV and VI (T3SS, T4SS and T6SS) share the ability to inject their substrates into human cells, opening up the possibility of using them as customized injectors. For this to happen, it is necessary to understand how substrates are recruited and to be able to engineer secretion signals, so that the transmembrane machineries can recognize and translocate the desired substrates in place of their own. Other factors, such as recruiting proteins, chaperones, and the degree of unfolding required to cross through the secretion channel, may also affect transport. Advances in the knowledge of the secretion mechanism have allowed heterologous substrate engineering to accomplish translocation by T3SS, and to a lesser extent, T4SS and T6SS into human cells. In the case of T4SS, transport of nucleoprotein complexes adds a bonus to its biotechnological potential. Here, we review the current knowledge on substrate recognition by these secretion systems, the many examples of heterologous substrate translocation by engineering of secretion signals, and the current and future biotechnological and biomedical applications derived from this approach.

Xanthomonas immunity proteins protect against the cis-toxic effects of their cognate T4SS effectors

“…1B ). This is a relevant hypothesis considering that cytoplasmic substrates of the T4SS are known to interact with various T4SS subcomplexes along the secretion pathway (Cascales and Christie, 2004a ; Atmakuri et al, 2004 ; Cascales and Christie, 2004b ; Guzmán-Herrador et al, 2023 ). To address this question, we deleted the genes coding for several X-T4SS subunits that are associated with the bacterial inner membrane (VirB4, VirB6, VirB8, and VirD4) (Macé et al, 2022 ; Llosa et al, 2003 ), the outer membrane (VirB7, VirB9, and VirB10) (Fronzes et al, 2009 ; Chandran et al, 2009 ; Souza et al, 2011 ; Oliveira et al, 2016 ; Sgro et al, 2018 ) and the VirB5 subunit believed to form part of the extracellular pilus (Alvarez-Martinez and Christie, 2009 ; Christie et al, 2014 ; Sheedlo et al, 2022 ).…”

Secretion of functional interferon by the type 3 secretion system of enteropathogenic Escherichia coli

“…This translocation is achieved by passing through the T3SS apparatus, a syringe-like structure, anchored within the bacterial membranes, with a long extracellular needle that extends toward the host cell membrane. It was previously demonstrated that EPEC, like other T3SS-containing bacteria, can deliver heterologous proteins into diverse cell types [18][19][20][21][22] or secrete them to the culture medium [23][24][25][26][27]. Most of these proteins were reporter proteins, facilitating the study of bacterial translocation mechanisms.…”

Gonococcal Genetic Island in the Global Neisseria gonorrhoeae Population: A Model of Genetic Diversity and Association with Resistance to Antimicrobials

“…To release DNA into the external environment, the gonococcus uses a type IV secretion system (T4SS) encoded by the genes of the gonococcal genetic island (GGI). In general, T4SSs are widespread in both Gram-positive and Gram-negative microorganisms and are subdivided into three families according to their biological functions [ 7 ]: (1) DNA conjugation, (2) protein transfer to a recipient cell (direct contact with the recipient cell is required), and (3) DNA secretion into the external environment (no contact with the cell is required). The gonococcal T4SS belongs to the latter family and releases single-stranded DNA into the medium in a contact-independent manner [ 8 ].…”