During the preparation of this manuscript, another group reported the structure of TipαN34, which is a truncated form of Tipα lacking the N-terminal 13
Adrenorphin [26]. They reported two different dimeric forms occurring at pH 4 and pH 8.5, with the latter being the more relevant in solution, and suggested that two cysteine residues (C5 and C7) are important for maintaining the dimeric formation under acidic condition [26]. Notably, at pH 8.5 both the monomeric and dimeric structures of TipαN34 are essentially the same as that of del-Tipα, though the crystallization conditions differ: whereas del-Tipα dimer shows crystallographic symmetry, TipαN34 dimer shows non-crystallographic symmetry. That two groups independently observed a common dimer formation is indicative of the likely importance of the oligomerization of del-Tipα, and is probably common to Tipα. Furthermore, we found that at least one of the cysteine residues is required for potent induction of biological activity at neutral pH. This suggests formation of a stable Tipα dimer with a disulfide bond is required for full activity [15]. Although removal of the two cysteine residues from Tipα caused a reduction in activity, del-Tipα binds to the same molecule on gastric epithelial
cells as Tipα, but with less affinity [15]. Notably, we recently identified a specific Tipα binding protein [28] and confirmed that this protein also binds del-Tipα with low affinity, which strongly supports the idea that del-Tipα assumes a unique dimeric structure similar to that of Tipα, despite the absence of disulfide bonds. We anticipate that the insight gained from our analysis of del-Tipα will facilitate future clarification of the importance of the N-terminal structure of Tipα to its interactions with other proteins and the carcinogenesis related to H. pylori infection.
