Recombinant production, purification, crystallization, and structure analysis of human transforming growth factor ß2 in a new conformation.

Transforming growth factor ß is a disulfide-linked dimeric cytokine that occurs in three highly related isoforms (TGFß1-TGFß3) engaged in signaling functions through binding of cognate TGFß receptors. To regulate this pathway, the cytokines are biosynthesized as inactive pro-TGFßs with an N-terminal latency-associated protein preceding the mature moieties. Due to their pleiotropic implications in physiology and pathology, TGFßs are privileged objects of in vitro studies. However, such studies have long been limited by the lack of efficient human recombinant expression systems of native, glycosylated, and homogenous proteins. Here, we developed pro-TGFß2 production systems based on human Expi293F cells, which yielded >2 mg of pure histidine- or Strep-tagged protein per liter of cell culture. We assayed this material biophysically and in crystallization assays and obtained a different crystal form of mature TGFß2, which adopted a conformation deviating from previous structures, with a distinct dimeric conformation that would require significant rearrangement for binding of TGFß receptors. This new conformation may be reversibly adopted by a certain fraction of the mature TGß2 population and represent a hitherto undescribed additional level of activity regulation of the mature growth factor once the latency-associated protein has been separated.

An improved recombinant mammalian cell expression system for human transforming growth factor-beta2 and -beta3 preparations.

Transforming growth factor-beta2 and -beta3 (TGF-beta2 and -beta3) are important members of TGF-beta family which play important roles in the growth, maintenance, and repair processes of developing embryos, neonates, and adults. Preparation of large quantities of these two cytokines, which is necessary for structural studies and other applications, has proven to be extremely difficult. We have developed a novel Chinese hamster ovary cell-based expression system for high-level expression and high recovery of recombinant human TGF-beta2 and -beta3. In this system, we used a mammalian expression vector which contains a glutamine synthetase coding region for amplification, together with a modified TGF-beta2 or -beta3 open reading frame for expression. The leader peptide of TGF-beta2 or -beta3 was replaced by that from the V-J2-C region of a mouse immunoglobulin kappa-chain, and a poly-histidine tag was inserted immediately after the leader sequence to facilitate protein purification without changing the mature TGF-beta2 or -beta3 amino acid sequence. In addition, the extreme N-terminal cysteine residue of TGF-beta2 or -beta3 was replaced by a serine residue. The resulting expression constructs produced two stable cell clones expressing 10 mg of TGF-beta2 and 8 mg of TGF-beta3 per liter of spent medium. The purification scheme involved the use of two simple chromatographic steps with a typical yield of 5 mg of TGF-beta2 and 4 mg of TGF-beta3. This method represents a significant improvement over previously published methods and may be applicable to other TGF-beta superfamily members. We further confirmed that latent TGF-beta2 and -beta3 can be activated by proteolysis and glycolysis, which have not been reported before.