Abstract. The ot2-macroglobulin (ot2M) receptor/lowdensity lipoprotein receptor-related protein (LRP) is important for the clearance of proteases, proteaseinhibitor complexes, and various ligands associated with lipid metabolism. While the regulation of receptor function is poorly understood, the addition of high concentrations of the 39-kD receptor-associate d protein (RAP) to cells inhibits the binding and/or uptake of many of these ligands. Previously, we (Kounnas, M. Z., J. Henkin, W. S. Argraves, D. K. Strickland. 1992. J. Biol. Chem. 267:12420-12423) showed that Pseudomohas exotoxin (PE) could bind immobilized LRP. Also, the addition of RAP blocked toxin-mediated cell killing. These findings suggested that PE might use LRP to gain entry into toxin-sensitive cells. Here we report on a strategy to select PE-resistant lines of Chinese hamster ovary cells that express altered amounts of LRP. An important part of this strategy is to screen PE-resistant clones for those that retain sensitivity to both diphtheria toxin and to a fusion protein composed of lethal factor (from anthrax toxin) fused to the adenosine diphosphate-ribosylating domain of PE. Two lines, with obvious changes in their expression of LRP, were characterized in detail. The 14-2-1 line had significant amounts of LRP, but in contrast to wild-type cells, little or no receptor was displayed on the cell surface. Instead, receptor protein was found primarily within cells, much of it apparently in an unprocessed state. The 14-2-1 line showed no uptake of chymotrypsin-aEM and was 10-fold resistant to PE compared with wild-type cells. A second line, 13-5-1, had no detectable LRP mRNA or protein, did not internalize OtEM-chymotrypsin, and exhibited a 100-fold resistance to PE. Resistance to PE appeared to be due to receptor-specific defects, since these mutant lines showed no resistance to a PE chimeric toxin that was internalized via the transferrin receptor. The results of this investigation confirm that LRP mediates the internalization of PE.

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Many clostridial proteins are poorly produced in Escherichia coli. It has been suggested that this phenomena is due to the fact that several types of codons common in clostridial coding sequences are rarely used in E. coli and the quantities of the corresponding tRNAs in E. coli are not sufficient to ensure efficient translation of the corresponding clostridial sequences. To address this issue, we amplified three E. coli genes, ileX, argU, and leuW, in E. coli; these genes encode tRNAs that are rarely used in E. coli (the tRNAs for the ATA, AGA, and CTA codons, respectively). Our data demonstrate that amplification of ileX dramatically increased the level of production of most of the clostridial proteins tested, while amplification of argU had a moderate effect and amplification of leuW had no effect. Thus, amplification of certain tRNA genes for rare codons in E. coli improves the expression of clostridial genes in E. coli, while amplification of other tRNAs for rare codons might not be needed for improved expression. We also show that amplification of a particular tRNA gene might have different effects on the level of protein production depending on the prevalence and relative positions of the corresponding codons in the coding sequence. Finally, we describe a novel approach for improving expression of recombinant clostridial proteins that are usually expressed at a very low level in E. coli.Clostridial proteins, such as tetanus toxin and seven serologically distinct botulinum neurotoxins (botulinum neurotoxin serotype A [BoNT/A], BoNT/B, BoNT/C, BoNT/D, BoNT/E, BoNT/F, and BoNT/G) that are produced by Clostridium tetani, Clostridium botulinum, Clostridium argentiensis, and Clostridium baratti, are powerful tools for studying the mechanisms of synaptic vesicle exocytosis (3,(20)(21)(22)(23). These toxins have been also used for therapeutic purposes, such as the treatment of strabismus, blepharospasms (24,25), and many other neurological conditions, as well as in clinical dermatology (4).Currently, BoNT/A and other clostridial neurotoxins and their fragments are purified from native Clostridium strains by using traditional purification protocols. Because these microorganisms are anaerobes, they pose technical problems. In addition, gene manipulation methods have not been developed for these microorganisms. Therefore, it has been difficult to construct Clostridium strains that produce derivatives of neurotoxins and other proteins. Genes for all eight clostridial neurotoxins have been cloned, and their sequences have been identified (2,6,9,18,30,31). Many attempts to express fragments of clostridial neurotoxins in Escherichia coli have failed because of the unusually high AT content of clostridial DNA. Makoff et al. successfully expressed a tetanus toxin fragment in E. coli (12) by optimizing sequences for codon usage in E. coli by complete synthesis of these sequences de novo. This approach, however, is very laborious and expensive.Recently, several groups of workers have demonstrated that rarely used codons can...