Primary structure of the hydrophobic plant protein crambin

Crambin, a hydrophobic plant seed protein, consists of a single chain of 46 amino acids with a calculated molecular weight of 4720. The primary structure was determined by using solid-phase sequencing techniques and was confirmed through X-ray crystallographic analysis of the protein at 1.5-A resolution [Hendrickson, W. A., & Teeter, M. M. (1981) Nature (London) 290, 107-112]. High-performance liquid chromatographic separation of the proteolytic fragments from crambin led to the identification of two sites of microheterogeneity. The three disulfide bonds were located at positions 3-40, 4-32, and 16-26 from the crystallographic data. Comparison of the primary structure with known sequences revealed that crambin is homologous with the plant toxins purothionin and viscotoxin. Methods to estimate protein secondary structure were applied and found to predict all of crambin's structure except its amphiphilic helix.