. The interfacial activation of Pseudomonas lipases involves conformational rearrangements of surface loops and appears to conform to models of activation deduced from the structures of fungal and mammalian lipases. Factors controlling the conformational rearrangement are not understood, but a comparison of crystallization conditions and observed conformation suggests that the conformation of the protein is determined by the solution conditions, perhaps by the dielectric constant.

No abstract

# 1999 International Union of Crystallography Printed in Denmark ± all rights reserved 2u -Globulin (A2U) is the major urinary protein excreted by adult male rats. The structure of a monoclinic crystal form of A2U was reported in 1992 [Bo È cskei et al. (1992). Nature (London), 360, 186±188]. The structures of an orthorhombic crystal form of A2U at 2.5 A Ê resolution (re®ned to an R factor of 0.248; R free = 0.264) and of a complex between A2U and d-limonene 1,2-epoxide (DLO) at 2.9 A Ê resolution (R factor = 0.248; R free = 0.260) are presented here. DLO is one of a diverse group of chemicals which cause a male rat-speci®c renal carcinogenesis called hyaline-droplet nephropathy. The rate-determining step in the development of this disorder is the binding of the toxin to A2U. Comparison of the cavities in A2U and in the corresponding mouse urinary protein (MUP) reveal that the former is tailor-made for small oval hydrophobic ligands such as DLO. The cavity in MUP is more shallow and elongated and cannot easily accommodate such ligands.

No abstract

Variants designed using PROTEUS have been produced in an attempt to engineer stabilizing salt bridges into subtilisin BPN'. All the mutants constructed by site-directed mutagenesis were secreted by Bacillus subtilis, except L75K. Q19E, expressed as a single variant and also in a double variant, Q19E/Q271E, appears to form a stabilizing salt bridge based on X-ray crystal structure determination and differential scanning calorimeter measurements. Although the double mutant was found to be less thermodynamically stable than the wild-type, it did exhibit an autolytic stability about two-fold greater under hydrophobic conditions. Four variants, A98K, S89E, V26R and L235R, were found to be nearly identical to wild-type in thermal stability, indicative of stable structures without evidence of salt bridge formation. Variants Q271E, V51K and T164R led to structures that resulted in varying degrees of thermodynamic and autolytic instability. A computer-modeling analysis of the PROTEUS predictions reveals that the low percentage of salt bridge formation is probably due to an overly simplistic electrostatic model, which does not account for the geometry of the pairwise interactions.