Molecular cloning of a protein associated with soybean seed oil bodies that is similar to thiol proteases of the papain family

A 34,000-Da protein (P34) is one of the four major soybean oil body proteins observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of isolated organic solvent-extracted oil bodies from mature seeds. P34 is processed during seedling growth to a 32,000-Da polypeptide (P32) by the removal of an amino-terminal decapeptide (Herman, E. M., Melroy, D. L., and Buckhout, T. J. (1990) Plant Physiol, in press). A soybean λ ZAP II cDNA library constructed from RNA isolated from midmaturation seeds was screened with monoclonal antibodies directed against two different epitopes of P34. The isolated cDNA clone encoding P34 contains 1,350 base pairs terminating in a poly(A)⁺ tail and an open reading frame 1,137 base pairs in length. The open reading frame includes a deduced amino acid sequence which matches 23 of 25 amino-terminal amino acids determined by automated Edman degradation of P34 and P32. The cDNA predicts a mature protein of 257 amino acids and of 28,641 Da. The open reading frame extends 5′ from the known amino terminus of P34 encoding a possible precursor and signal sequence segments with a combined additional 122 amino acids. Prepro-P34 is deduced to be a polypeptide of 42,714 Da, indicating that the cDNA clone apparently encodes a polypeptide of 379 amino acids. A comparison of the nucleotide and deduced amino acid sequences in the GenBank™ Data Bank with the sequence of P34 has shown considerable sequence similarity to the thiol proteases of the papain family. Southern blot analysis of genomic DNA indicated that the P34 gene has a low copy number.