Many mammalian viruses have acquired genes from their hosts during their evolution. The rationale for these acquisitions is usually quite clear: the captured genes are subverted to provide a selective advantage to the virus. Here we describe the opposite situation, where a viral gene has been sequestered to serve an important function in the physiology of a mammalian host. This gene, encoding a protein that we have called syncytin, is the envelope gene of a recently identified human endogenous defective retrovirus, HERV-W. We find that the major sites of syncytin expression are placental syncytiotrophoblasts, multinucleated cells that originate from fetal trophoblasts. We show that expression of recombinant syncytin in a wide variety of cell types induces the formation of giant syncytia, and that fusion of a human trophoblastic cell line expressing endogenous syncytin can be inhibited by an anti-syncytin antiserum. Our data indicate that syncytin may mediate placental cytotrophoblast fusion in vivo, and thus may be important in human placental morphogenesis.

We have developed a versatile Escherichia coli expression system based on the use of E. coli thioredoxin (trxA) as a gene fusion partner. The broad utility of the system is illustrated by the production of a variety of mammalian cytokines and growth factors as thioredoxin fusion proteins. Although many of these cytokines previously have been produced in E. coli as insoluble aggregates or "inclusion bodies", we show here that as thioredoxin fusions they can be made in soluble forms that are biologically active. In general we find that linkage to thioredoxin dramatically increases the solubility of heterologous proteins synthesized in the E. coli cytoplasm, and that thioredoxin fusion proteins usually accumulate to high levels. Two additional properties of E. coli thioredoxin, its ability to be specifically released from the E. coli cytoplasm by osmotic shock or freeze/thaw treatments and its intrinsic thermal stability, are retained by some fusions and provide convenient purification steps. We also find that the active-site loop of E. coli thioredoxin can be used as a general site for small peptide insertions, allowing for the high level production of soluble peptides in the E. coli cytoplasm.

This trial supports the hypothesis that systemic administration of myostatin inhibitors provides an adequate safety margin for clinical studies. Further evaluation of more potent myostatin inhibitors for stimulating muscle growth in muscular dystrophy should be considered.

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We have developed a system for probing protein/protein interactions which makes use of the bacterial flagellum to display random peptide libraries on the surface of E. coli. In developing the system the entire coding sequence of E. coli thioredoxin (trxA) was inserted into a dispensable region of the gene for flagellin (fliC), the major structural component of the E. coli flagellum. The resulting fusion protein (FLITRX) was efficiently exported and assembled into partially functional flagella on the bacterial cell surface. A diverse library of random dodecapeptides were displayed in FLITRX on the exterior of E. coli as conformationally constrained insertions into the thioredoxin active-site loop, a location known to be a highly permissive site for the insertion of exogenous peptide sequences into native thioredoxin. To demonstrate that members of this library could be bound and selected via specific protein/protein interactions to a target protein, a method was devised to enable efficient isolation of those bacteria displaying peptides with affinity to immobilized antibodies. We have unambiguously mapped three different antibody epitopes using this method. Peptides selected as FLITRX active-site fusions retain their binding specificity when made as native thioredoxin active-site loop fusions. This will facilitate future structural characterizations and broaden the general utility of the system for exploring other classes of protein-protein interactions.