Structural basis for Ca<superscript>2+</superscript>-independence and activation by homodimerization of tomato subtilase 3.

Subtilases are serine proteases found in Archae, Bacteria, yeasts, and higher eukaryotes. Plants possess many more of these subtilisin-like endopeptidases than animals, e.g., 56 identified genes in Arabidopsis compared with only 9 in humans, indicating important roles for subtilases in plant biology. We report the first structure of a plant subtilase, SBT3 from tomato, in the active apo form and complexed with a chloromethylketone (cmk) inhibitor. The domain architecture comprises an N-terminal protease domain displaying a 132 aa protease-associated (PA) domain insertion and a C-terminal sevenstranded jelly-roll fibronectin (Fn) Ill-like domain. We present the first structural evidence for an explicit function of PA domains in proteases revealing a vital role in the homo-dimerization of SBT3 and in enzyme activation. Although Ca 2+ -binding sites are conserved and critical for stability in other subtilases, SBT3 was found to be Ca 2+ -free and its thermo stability is Ca 2+ -indepenc1ent. [ABSTRACT FROM AUTHOR]

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