# Specialized shuttle proteins recognize Type IX secretion signals and target effectors to their final destinations in Flavobacterium johnsoniae

## TL;DR

This paper explores how Flavobacterium johnsoniae uses specialized shuttle proteins to direct proteins with Type B CTDs to their final destinations via the Type IX secretion system.

## Contribution

The study identifies specific shuttle proteins for Type B CTDs and reveals distinct secretion and targeting signals within these domains.

## Key findings

- Two Type B substrates require their specific PorP/SprF shuttle proteins for secretion.
- Five conserved motifs in Type B CTDs are necessary but not sufficient for shuttle recognition.
- CTDs contain both a secretion signal and a targeting signal for final localization.

## Abstract

Members of the phylum Bacteroidota utilize the type IX secretion system (T9SS) to transport diverse substrates into the environment or onto their surface. T9SS substrates feature a Sec-dependent signal peptide for export to the periplasm and a conserved C-terminal domain (CTD), recognized by the T9SS, for translocation across the outer membrane. Following translocation, substrates engage with a shuttle protein, which ensures their final localization. Most CTDs are classified into two major families. Type A CTDs are all recognized by the PorV shuttle. Recognition and transport of Type B CTDs remain less explored. Flavobacterium johnsoniae encodes 12 Type B substrates, often genetically linked to genes encoding PorP/SprF-like shuttle proteins. We demonstrate that two Type B substrates indeed rely on their cognate PorP/SprF specialized shuttle proteins for secretion and identify the shuttle responsible for the secretion of three orphan Type B CTDs. Our findings also reveal that five conserved motifs within Type B CTDs are necessary for secretion but not sufficient for their specific recognition by cognate shuttle proteins. Our results further suggest that CTDs contain a secretion signal, sufficient for secretion of substrates by the T9SS, and a targeting signal, which directs substrates to their final localization.

Bacteroidota use the Type IX secretion system to secrete proteins with a conserved C-terminal domain (CTD) secretion signal domain. Type B CTDs require specific shuttle proteins for secretion and target substrates to their final localization.