Morulin Pm: A Modified Polypeptide Containing TOPA and 6-Bromotryptophan from the Morula Cells of the Ascidian,Phallusia mammillata

Abstract

A novel polypeptide containing the unusual posttranslationally modified amino acidsl-3,4,5-trihydroxyphenylalanine (TOPA) andl-6-bromotryptophan (6-BrW) has been isolated from the morula cells of the vanadium-accumulating ascidian,Phallusia mammillata.The polypeptide, designated Morulin Pm, has a molecular weight of 3825 ± 0.6 and has a simple amino acid composition consisting mainly of TOPA and 6-BrW as well as Ser, Leu, Phe, and Ala. To our knowledge, this is the first reported example of multiple sites of brominated tryptophan in a polypeptide of this size. Edman degradation revealed the N-terminal sequence to be BrW-Leu-Phe-BrW before sequencing was blocked. While the N-terminal tripeptide could be isolated from chymotrypsin digests of Morulin Pm, the rest of the polypeptide resisted further cleavage by the proteases, a feature common among this class of peptides. However, unlike other ascidian blood cell peptides examined to date, microheterogeneity was minimal. For the first time a detailed NMR investigation could be undertaken on a member of this class of polypeptides. In addition to signals assignable to the constituent amino acids by extensive 2D experiments, resonances were present both in the13C and1H spectra not typical of a simple linear peptide. Two proton resonances were identified with a cross peak in the correlation spectrum strongly indicative of a C-terminal decarboxy-Δ2,3-unsaturated TOPA residue as observed in certain tunichromes and clionamide. Chemical degradation experiments were undertaken in an effort to produce identifiable fragments to which these signals could be assigned, including full and partial acid hydrolysis and tryptophan-targeted BNPS-skatole treatment. However, the nature of the modification remains unknown. Possible structures for the modification, which may represent the source of the difficulties encountered in the structural elucidation of this and related peptides, are assessed. Conjecture is made as to the biological relevance of Morulin Pm, based on its localization and chemical characteristics.