Structural features of plant subtilases

Serine proteases of the subtilase family are present in Archaea, Bacteria and Eukarya. Many more subtilases are found in plants as compared to other organisms, implying adaptive significance for the expansion of the subtilase gene family in plants. Structural data, however, were hitherto available only for non-plant subtilases. We recently solved the first structure of a plant subtilase, SlSBT3 from tomato (Solanum lycopersicum). SlSBT3 is a multidomain enzyme displaying a subtilisin, a Protease-Associated (PA) and a fibronectin (Fn) III-like domain. Two prominent features set SlSBT3 apart from other structurally elucidated subtilases: (i) activation by PA domain-mediated homo-dimerization and (ii) calcium-independent activity and thermostability. To address the question whether these characteristics are unique features of SlSBT3, or else, general properties of plant subtilases, homology models were calculated for representative proteases from tomato and Arabidopsis using the SlSBT3 structure as template. We found the major structural elements required for the stabilization of the subtilisin domain to be conserved among all enzymes analyzed. PA domain-mediated dimerization as an auto-regulatory mechanism of enzyme activation, on the other hand, appears to be operating in only a subset of the analyzed subtilases.