Unconventional translation initiation of human trypsinogen4 at a CUG codon with an N-terminal leucine

• Published in: The FEBS journal 2007-03, Vol.274 (6), p.1610-1620
• Main Authors: Nemeth, Attila L, Medveczky, Peter, Toth, Julia, Siklodi, Erika, Schlett, Katalin, Patthy, Andras, Palkovits, Miklos, Ovadi, Judit, Tokesi, Natalia, Nemeth, Peter, Szilagyi, Laszlo, Graf, Laszlo
• Format: Article
• Language: English
• ISSN: 1742-464X; 1742-4658
• DOI: 10.1111/j.1742-4658.2007.05708.x

SummaryChromosomal rearrangements apparently account for the presence of a primate-specific gene (protease serine3) in chromosome9. This gene encodes, as the result of alternative splicing, both mesotrypsinogen and trypsinogen4. Whereas mesotrypsinogen is known to be a pancreatic protease, neither the chemical nature nor biological function of trypsinogen4 has been explored previously. The trypsinogen4 sequence contains two predicted translation initiation sites: an AUG site that codes for a 72-residue leader peptide on IsoformA, and a CUG site that codes for a 28-residue leader peptide on IsoformB. We report studies that provide evidence for the N-terminal amino acid sequence of trypsinogen4 and the possible mechanism of expression of this protein in human brain and transiently transfected cells. We raised mAbs against a 28-amino acid synthetic peptide representing the leader sequence of IsoformB and against recombinant trypsin4. By using these antibodies, we isolated and chemically identified trypsinogen4 from extracts of both post mortem human brain and transiently transfected HeLa cells. Our results show that IsoformB, with a leucine Nterminus, is the predominant (if not exclusive) form of the enzyme in post mortem human brain, but that both isoforms are expressed in transiently transfected cells. On the basis of our studies on the expression of a series of trypsinogen4 constructs in two different cell lines, we propose that unconventional translation initiation at a CUG with a leucine, rather than a methionine, Nterminus may serve as a means to regulate protein expression.