Type 1 secretion necessitates a tight interplay between all domains of the ABC transporter

Type I secretion systems (T1SS) facilitate the secretion of substrates in one step across both membranes of Gram-negative bacteria. A prime example is the hemolysin T1SS which secretes the toxin HlyA. Secretion is energized by the ABC transporter HlyB, which forms a complex together with the membrane fusion protein HlyD and the outer membrane protein TolC. HlyB features three domains: an N-terminal C39 peptidase-like domain (CLD), a transmembrane domain (TMD) and a C-terminal nucleotide binding domain (NBD). Here, we created chimeric transporters by swapping one or more domains of HlyB with the respective domain(s) of RtxB, a HlyB homolog from Kingella kingae. We tested all chimeric transporters for their ability to secrete pro-HlyA when co-expressed with HlyD. The CLD proved to be most critical, as a substitution abolished secretion. Swapping only the TMD or NBD reduced the secretion efficiency, while a simultaneous exchange abolished secretion. These results indicate that the CLD is the most critical secretion determinant, while TMD and NBD might possess additional recognition or interaction sites. This mode of recognition represents a hierarchical and extreme unusual case of substrate recognition for ABC transporters and optimal secretion requires a tight interplay between all domains.

Redefining the bacterial Type I protein secretion system

Type I secretion systems (T1SS) are versatile molecular machines for protein transport across the Gram-negative cell envelope. The archetypal Type I system mediates secretion of the Escherichia coli hemolysin, HlyA. This system has remained the pre-eminent model of T1SS research since its discovery. The classic description of a T1SS is composed of three proteins: an inner membrane ABC transporter, a periplasmic adaptor protein and an outer membrane factor. According to this model, these components assemble to form a continuous channel across the cell envelope, an unfolded substrate molecule is then transported in a one-step mechanism, directly from the cytosol to the extracellular milieu. However, this model does not encapsulate the diversity of T1SS that have been characterized to date. In this review, we provide an updated definition of a T1SS, and propose the subdivision of this system into five subgroups. These subgroups are categorized as T1SSa for RTX proteins, T1SSb for non-RTX Ca²⁺-binding proteins, T1SSc for non-RTX proteins, T1SSd for class II microcins, and T1SSe for lipoprotein secretion. Although often overlooked in the literature, these alternative mechanisms of Type I protein secretion offer many avenues for biotechnological discovery and application.

Secretory production of a camelid single-domain antibody (VHH, nanobody) by the Serratia marcescens Lip system in Escherichia coli

Escherichia coli is one of the most popularly used hosts to produce recombinant proteins. Most recombinant proteins are produced in the cytoplasm and periplasm, requiring multiple steps to extract and purify recombinant proteins. The Serratia marcescens Lip system (LipB-LipC-LipD) is a type 1 secretion system that selectively secretes LipA from the intracellular to extracellular space in a single step. This study aimed to establish a secretory production system for nanobodies, camelid-derived small molecule antibody fragments, using the S. marcescens Lip system. Surprisingly, E. coli harboring only LipC, a membrane fusion protein of the Lip system, could secrete an anti-green fluorescent protein (GFP)-Nb, a nanobody against GFP, without the addition of a long amino acid sequence. The LipC-based secretion system recognized the Val-Thr-Val sequence at the C-terminus of the nanobody. Finally, Strep-tagged anti-GFP-Nb was purified from culture supernatants of E. coli harboring LipC by Strep-affinity chromatography at a final yield of >5 mg per liter of culture supernatant. These results potently supported that the S. marcescens LipC-based secretion system has the potential to establish an efficient secretory production system for nanobodies.

High-quality complete genome sequence of Microbacterium sp. SUBG005, a plant pathogen

Microbacterium sp. SUBG005 is a Gram positive bacterium, isolated from infected leaf of Mangifera indica L. in Rajkot (22.30°N, 70.78°E), Gujarat, India. The genome sequencing of Microbacterium sp. SUBG005 is having type I secretion system genes of pathogenicity as well as heavy metal resistance unique genes. The genome size is 7.01 Mb with G + C content of 64.80% and contains rRNA sequences. Genome sequencing analysis provides information about the microbe role in host–pathogen interaction. The whole genome sequencing has been deposited in DDBJ/EMBL/GenBank under the accession number JNNT00000000.

Biotechnological applications of bacterial protein secretion: from therapeutics to biofuel production

Recent years have witnessed significant progresses in engineering of recombinant protein secretion. The relatively simple secretion mechanisms, Type I and Type V (autotransporters), are increasingly used for secretion of recombinant proteins. The secretion level of target proteins varied from milligrams to grams per liter. The range of proteins was significantly expanded beyond medical application. Notable additions include biofuel productions from renewable feedstock. Despite the progress, almost all successes in the engineering efforts come with significant trials and errors, highlighting the need for a better understanding of secretion systems and rational based methods.