Ana Saurí

Extracellular production of recombinant proteins using bacterial autotransporters.

Escherichia coli is still a very popular host for the production of recombinant proteins at an analytical or industrial scale. Secretion of the proteins into the culture medium or display at the cell surface would be preferred in many applications but is hampered by the complex two-layered cell envelope. The autotransporter pathway is used by E. coli to secrete virulence factors via a relatively simple but efficient and specific mechanism. Here we discuss recent progress in the structural and mechanistic analysis of this pathway and the implications for future development of a versatile platform for secretion and display of heterologous proteins.

Insertion and Topology of a Plant Viral Movement Protein in the Endoplasmic Reticulum Membrane

Virus-encoded movement proteins (MPs) mediate cell-to-cell spread of viral RNA through plant membranous intercellular connections, the plasmodesmata. The molecular pathway by which MPs interact with viral genomes and target plasmodesmata channels is largely unknown. The 9-kDa MP from carnation mottle carmovirus (CarMV) contains two potential transmembrane domains. To explore the possibility that this protein is in fact an intrinsic membrane protein, we have investigated its insertion into the endoplasmic reticulum membrane. By using in vitro translation in the presence of dog pancreas microsomes, we demonstrate that CarMV p9 inserts into the endoplasmic reticulum without the aid of any additional viral or plant host components. We further show that the membrane topology of CarMV p9 is Ncyt-Ccyt (N and C termini of the protein facing the cytoplasm) by in vitro translation of a series of truncated and full-length constructs with engineered glycosylation sites. Based on these results, we propose a topological model in which CarMV p9 is anchored in the membrane with its N- and C-terminal tail segments interacting with its soluble, RNA-bound partner CarMV p7, to accomplish the viral cell-to-cell movement function.

A Conserved Aromatic Residue in the Autochaperone Domain of the Autotransporter Hbp Is Critical for Initiation of Outer Membrane Translocation

Autotransporters are bacterial virulence factors that share a common mechanism by which they are transported to the cell surface. They consist of an N-terminal passenger domain and a C-terminal β-barrel, which has been implicated in translocation of the passenger across the outer membrane (OM). The mechanism of passenger translocation and folding is still unclear but involves a conserved region at the C terminus of the passenger domain, the so-called autochaperone domain. This domain functions in the stepwise translocation process and in the folding of the passenger domain after translocation. In the autotransporter hemoglobin protease (Hbp), the autochaperone domain consists of the last rung of the β-helix and a capping domain. To examine the role of this region, we have mutated several conserved aromatic residues that are oriented toward the core of the β-helix. We found that non-conservative mutations affected secretion with Trp1015 in the cap region as the most critical residue. Substitution at this position yielded a DegP-sensitive intermediate that is located at the periplasmic side of the OM. Further analysis revealed that Trp1015 is most likely required for initiation of processive folding of the β-helix at the cell surface, which drives sequential translocation of the Hbp passenger across the OM.

Autotransporter β-Domains Have a Specific Function in Protein Secretion beyond Outer-Membrane Targeting

Autotransporters (ATs) of Gram-negative bacteria contain an N-proximal passenger domain that is transported to the extracellular milieu and a C-terminal β-domain that inserts into the outer membrane (OM) in a β-barrel conformation. This β-domain facilitates translocation of the passenger domain across the OM and has long been considered to be the translocation pore. However, available crystal structures of β-domains show that the β-barrel pore is too narrow for the observed transport of folded elements within the passenger domains. ATs have recently been shown to interact with the β-barrel assembly machinery. These findings questioned a direct involvement of the β-domain in passenger translocation and suggested that it may only target the passenger to the β-barrel assembly machinery pore. To address the function of the β-domain in more detail, we have replaced the β-domain of the Escherichia coli AT hemoglobin protease by β-domains originating from other OM proteins. Furthermore, we have modified the diameter of the β-domain pore. The mutant proteins were analyzed for their capacity to insert into the OM and for surface display of the passenger. Our results show that efficient passenger secretion requires a specific β-domain that not only functions as a targeting device but also is directly involved in the translocation of the passenger to the cell surface.

Estimating the size of the active translocation pore of an autotransporter.

Autotransporters (ATs) are large virulence factors secreted by Gram-negative bacteria. The passenger domain, carrying the virulence functions, is transported across the bacterial outer membrane in a step that is facilitated by a C-terminal β-domain. This domain folds into a β-barrel with a central aqueous pore of ∼1 nm inner diameter according to crystal structures. However, these static dimensions are not compatible with the observed secretion of passengers that may contain natural short-spaced disulfide bonds or artificially fused folded elements. Here, we have systematically analyzed the dimensions of the active AT passenger translocator by inserting peptides of different length and structural complexity in the passenger of the AT hemoglobin protease. The peptides were introduced in a short loop protruding from the main structure and flanked by two single cysteines. Our results show that the attained secondary structure may be more critical for secretion than the length of peptide inserted. Furthermore, the data suggest that, during passenger translocation, at least four extended polypeptides or an extended polypeptide and an α-helix are accommodated in the translocator, indicating that the diameter of the active translocation pore is up to 1.7 nm. If the β-domain functions as the translocator, it must be forced into an expanded conformation during passenger translocation.

Membrane protein integration into the endoplasmic reticulum

Most integral membrane proteins are targeted, inserted and assembled in the endoplasmic reticulum membrane. The sequential and potentially overlapping events necessary for membrane protein integration take place at sites termed translocons, which comprise a specific set of membrane proteins acting in concert with ribosomes and, probably, molecular chaperones to ensure the success of the whole process. In this minireview, we summarize our current understanding of helical membrane protein integration at the endoplasmic reticulum, and highlight specific characteristics that affect the biogenesis of multispanning membrane proteins.

Viral membrane protein topology is dictated by multiple determinants in its sequence.

The targeting, insertion, and topology of membrane proteins have been extensively studied in both prokaryotes and eukaryotes. However, the mechanisms used by viral membrane proteins to generate the correct topology within cellular membranes are less well understood. Here, the effect of flanking charges and the hydrophobicity of the N-terminal hydrophobic segment on viral membrane protein topogenesis are examined systematically. Experimental data reveal that the classical topological determinants have only a minor effect on the overall topology of p9, a plant viral movement protein. Since only a few individual sequence alterations cause an inversion of p9 topology, its topological stability is robust. This result further indicates that the protein has multiple, and perhaps redundant, structural features that ensure that it always adopts the same topology. These critical topogenic sequences appear to be recognized and acted upon from the initial stages of protein biosynthesis, even before the ribosome ends protein translation.

Transient structural ordering of the RNA-binding domain of carnation mottle virus p7 movement protein modulates nucleic acid binding.

Plant viral movement proteins bind to RNA and participate in the intra‐ and intercellular movement of the RNAs from plant viruses. However, the role and magnitude of the conformational changes associated with the formation of RNA–protein complexes are not yet defined. Here we describe studies on the relevance of a preexisting nascent α‐helix at the C terminus of the RNA‐binding domain of p7, a movement protein from carnation mottle virus, to RNA binding. Synthetic peptide analogues and single amino acid mutation at the RNA‐binding domain of recombinant p7 protein were used to correlate the transient structural order in aqueous solution with RNA‐binding potential.