A novel plant ferritin subunit from soybean that is related to a mechanismin iron release

Ferritin is a multimeric iron storage protein composed of 24 subunits. Ferr itin purified from dried soybean seed resolves into two peptides of 26.5 an d 28 kDa. To date, the 26.5-kDa subunit has been supposed to be generated f rom the 28-kDa subunit by cleavage of the N-terminal region. We performed a mino acid sequence analysis of the 28-kDa subunit and found that it had a d ifferent sequence from the 26.5-kDa subunit, thus rendering it novel among known soybean ferritins, We cloned a cDNA encoding this novel subunit from 10-day-old seedlings, each of which contained developed bifoliates, an epic otyl and a terminal bud. The 26.5-kDa subunit was found to be identical to that identified previously lacking the C-terminal 16 residues that correspo nd to the E helix of mammalian ferritin. However, the corresponding region in the 28-kDa soybean ferritin subunit identified in this study was not sus ceptible to cleavage. We present evidence that the two different ferritin s ubunits in soybean dry seeds show differential sensitivity to protease dige stions and that the novel, uncleaved 28-kDa ferritin subunit appears to sta bilize the ferritin shell by co-existing with the cleaved 26.5-kDa subunit. These data demonstrate that soybean ferritin is composed of at least two d ifferent subunits, which have cooperative functional roles in soybean seeds .