I. Janzik et al., LeSBT1, a subtilase from tomato plants - Overexpression in insect cells, purification, and characterization, J BIOL CHEM, 275(7), 2000, pp. 5193-5199
The cDNA of a tomato subtilase designated LeSBT1 was cloned from a tomato f
lower cDNA library. The deduced amino acid sequence indicated for LeSBT1 th
e structure of a prepro-protein targeted to the secretory pathway by virtue
of an amino-terminal signal peptide. LeSBT1 was expressed in the baculovir
us/insect cell system and a processed 73-kDa form of LeSBT1, lacking both s
ignal peptide and prodomain, was purified to homogeneity from culture super
natants, This 73-kDa LeSBT1, however, lacked proteolytic activity. Zymogen
activation to yield 68-kDa LeSBT1 required the additional processing of an
amino-terminal autoinhibitory peptide in a strictly pH-dependent manner. Ma
ture 68-kDa LeSBT1 showed highest activity at acidic pH consistent with its
presumed localization in the apoplast of the plant cell. In comparison to
other plant subtilases, LeSBT1 exhibited a narrower substrate specificity i
n that it cleaves only polypeptide substrates preferentially but not exclus
ively carboxyl-terminal of glutamine residues. The possible involvement of
LeSBT1 in selective proprotein processing is discussed with reference to th
e related mammalian proprotein convertases.