Molecular analysis of Physarum haemagglutinin I: Lack of a signal sequence, sulphur amino acids and post-translational modifications

The cDNAs encoding haemagglutinin I from plasmodia of Physarum polycephalum have been cloned using PCR protocols. The composite haemagglutinin I cDNA sequence, derived from several overlapping clones from PCR fragments, spans 408 nt and the 315 bp ORF encodes a polypeptide of 104 aa without a typical signal sequence. The putative molecular mass deduced from the amino acid sequence (10760.76 Da) corresponds exactly to that determined by electrospray ionization MS (10759.86 ± 0.15 Da), suggesting that haemagglutinin I is not subject to post-translational modification. Haemagglutinin I lacks sulphur amino acids and has a β-sheet as the major secondary structure. Expression of the coding sequence in Escherichia coli yielded a product that exhibits the same sugar-binding specificity as natural haemagglutinin I. The deduced amino acid sequence shows little similarity to that of any known lectins and thus apparently represents a novel type of lectin.