Characterization of the SecB Protein, a Chaperone that Facilitates Protein Secretion in Escherichia coli

• Main Author: Altman, Elliot Charles
• Format: Others
• Language: en
• Published: 1991
• Online Access: https://thesis.library.caltech.edu/2612/1/Altman_e_1991.pdf; Altman, Elliot Charles (1991) Characterization of the SecB Protein, a Chaperone that Facilitates Protein Secretion in Escherichia coli. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/tq2t-3k47. https://resolver.caltech.edu/CaltechETD:etd-06152007-080238 <https://resolver.caltech.edu/CaltechETD:etd-06152007-080238>

<p>It has become increasingly clear that in Escherichia coli, most exported proteins are translocated either posttranslationally or late in their synthesis, and that a component of the export apparatus, SecB, facilitates the export of a subset of the secreted proteins by maintaining them in an export-competent, unfolded form. In an effort to understand how SecB functions as an antifolding factor, we mapped and characterized the sites of SecB interaction in the outer membrane protein LamB. We found that the interaction of SecB with LamB was dependent on the LamB signal sequence as well as on a region in the mature LamB protein. The simplest interpretation of these findings is that SecB binds to both the LamB signal sequence and a mature region in LamB, and that this interaction promotes the antifolding activity of SecB.</p> <p>Given the fact that several heat-shock proteins have also been shown to function as antifolding factors, we wanted to investigate whether heat-shock proteins might act in a manner analogous to SecB in facilitating the export process. We found that induction of the heat-shock response could substitute for SecB function (SecB is not a heat-shock protein), and that a basal level of heat-shock proteins was necessary for the cell to survive in the absence of SecB protein. These results suggested that heat-shock proteins might indeed be involved in the secretory process and function in a manner similar to that of SecB.</p> <p>In an attempt to identify these proteins, suppressors of a secB null mutation were isolated and characterized. Not unexpectedly, most of these suppressors mapped to the rpoH locus. Since rpoH encodes σ³², the heat-shock transcription factor, it is likely that these suppressors affect the synthesis levels of heat-shock proteins, which can substitute for SecB function. The remaining suppressors did not map to any known heat-shock or export genes, and potentially represent unidentified heat-shock proteins or export factors that act in a manner similar to SecB in facilitating the export process in E. coli.</p>