RECOGNITION OF RNASE SA BY THE INHIBITOR BARSTAR - STRUCTURE OF THE COMPLEX AT 1.7 ANGSTROM RESOLUTION

The 1.7 Angstrom resolution structure of RNase Sa complexed with the p olypeptide inhibitor barstar is reported here. The crystals are in the hexagonal space group P6(5) with unit-cell dimensions a = b = 56.9, c = 135.8 Angstrom and the asymmetric unit contains one molecule of the complex. RNase Sa is an extracellular microbial ribonuclease produced by Streptomyces aureofaciens. Barstar is the natural inhibitor of bar nase. the ribonuclease of Bacillus amyloliquefaciens. It inhibits RNas e Sa and barnase in a similar manner by steric blocking of the active site. The structure of RNase Sa is very similar to that observed in cr ystals of the native enzyme and its complexes with nucleotides. Barsta r retains the structure found in its complex with barnase, The accessi ble surface area of protein buried in the complex is about 300 Angstro m(2) smaller and there are fewer hydrogen bonds in the enzyme-inhibito r interface in RNase Sa-barstar than in barnase-barstar, providing an explanation of the reduced binding affinity in the former. Previous st udies of barstar complexes have used mutants of the inhibitor and this is the first structure which includes wild-type barstar.