lin A and B Chains In the initial attempt to express recombinant human insulin in E. coli, insulin chain A and B had to 17855348 be fused to an E. coli bgalactosidase to provide the stable chain products separately. When the gene TSU 68 encoding insulin chain A was fused downstream to the gene of ThiS or Ub and cloned into prokaryotic expression vector pET28a, the fused insulin chain A protein was successfully expressed in E. coli BL21 pLysS, predominantly in inclusion bodies, by IPTG induction. The yield of ThiS fusion product was higher than Ub fusion in large scale expression. Anti-His-tag immunoblot of the proteins revealed the overexpressed bands as the target proteins. Trace amounts of soluble products were observed in Western blot, both for Ub fusion and ThiS fusion at similar level. Molecular weights of the expressed fusion proteins were as expected and confirmed by MALDI-TOF MS. Likewise, when insulin chain B was fused with ThiS or Ub, the fused proteins were also expressed predominantly in inclusion bodies, by IPTG induction. The yield of ThiS fusion product was also higher than Ub fusion in large scale production. The identities of the overexpressed proteins were confirmed by Anti-His-tag immunoblot and MALDI-TOF MS. Trace amounts of soluble products were observed in Western blot, at a higher level for Ub fusion than ThiS fusion. Ub and ThiS, although sharing same secondary structure of bgrasp domain, showed differential efficiency on enhancing the protein expression. This difference may not be attributed to the coden bias due to the prokaryotic origin of ThiS, since the coding gene of Ub used for fusion was synthesized according to the coden bias of E. coli. 2. Half-molecule of ThiS Fusion Enhanced the Expression Half-protein molecules of Ub were used as fusion tags. The splitted N- and C-terminal half-proteins are incapable of fast folding to a compact stable structure of the whole molecule of Ub. Fig. 2 showed the effect of fusion by the C-terminal 9751179 and Nterminal half-ThiS to insulin A and B chains. Insulin A fusion to the N-terminal half of Ub gave a protein yield of 31.5863.52 mg/ L, and the N-terminal and C-terminal half-ThiS fusions gave yields of 20.6263.09 and 13.6166.48 mg/L, respectively. The overexpressed proteins were confirmed by Anti-His-tag immunoblot and MALDITOF MS. The N-terminal and C-terminal half-ThiS fusions to insulin B chain had similar results to that of insulin A chain, while 2 ThiS Fusion for Heterologous Expression in E. coli the N-terminal half-Ub fusion gave a lower yield. MALDI-TOF MS indicated that the N-terminal and C-terminal halfThiS fusions of insulin B were expressed at molecular weights as expected. While the N-terminal half-Ub fusion product had a major peak about 1 kD smaller than expected. This may suggest a partial degradation of the target protein which was responsible for the lower expression level of the half-Ub fusion. 3. ThiS Fusion Expression of Murine Ribonuclease Inhibitor We suspected if ThiS fusion enhanced the target expression by improving its stability in vivo. Since murine Ribonuclease Inhibitor was shown as an unstable protein when expressed in E. coli, we tried to observe the effect of fusion of ThiS on the stability of mRI, and compared with that of Ub and SUMO. When mRI was coded as Ub and SUMO fusions in expression vector pVI, they were expressed predominantly as full length products in inclusion bodies, with degradation as fast migrating smaller fragments in Western blot. Like the

These results suggested that N-terminal residues 1140 of HIV-1 Vif are sufficient for CBFb binding

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