Cloning of the cDNA of alpha-momorcharin: a ribosome inactivating protein.

Using a lambda gt11 cDNA library constructed from the seeds of the bitter melon (Momordica charantia), we have obtained a full length cDNA containing the entire sequence of alpha-momorcharin by immunoscreening. The length of this cDNA is 1044 basepairs long and it consists of an open reading frame coding for a polypeptide of 286 amino acids. The first 23 residues of this polypeptide probably code for a signal sequence. The N-terminal sequence of the deduced protein is exactly identical to that determined by peptide sequencing. The sequence identity between alpha-momorcharin and other ribosome inactivating proteins, such as trichosanthin and ricin A chain, is high, i.e., 34-63%. Examination of the predicted secondary structure of alpha-momorcharin and trichosanthin indicates that these proteins have regions of high structural similarities and this may account for the common biological activities that they share, viz., abortificant, immunosuppressive, antitumor and inhibition of HIV-1.