Identical precursors for serum transferrin and egg white conalbumin

The NH2-terminal sequences of egg white conalbumin and chicken serum transferrin were examined and found to be identical. Conalbumin, when synthesized in a rabbit reticulocyte cell-free translation system, was found to contain an NH2-terminal extension of 19 amino acid residues. Sequential Edman degradation of this precursor (pre-conalbumin) labeled with radioactive amino acids revealed the following sequence: formula see text: The vertical line indicates the site at which pre-conalbumin is cleaved to yield authentic conalbumin. The sequence represents the primary translation product since the NH2-terminal methionine was shown to be derived from initiator Met-tRNAfMet. A partial NH2-terminal sequence of transferrin synthesized in vitro was also determined (underlined residues) and it is identical with that of pre-conalbumin.