The refined 2.0 8, X-ray crystal structure of the complex formed between bovine /3-trypsin and CMTI-I, a trypsin inhibitor from squash seeds (Cucurbita maxima) Received 7 November 1988The stoichiometric complex formed between bovine j?-trypsin and the Cucurbita maxima trypsin inhibitor I (CMTI-I) was crystallized and its X-ray crystal structure determined using Patterson search techniques. Its structure has been crystallographically refined to a final R value of 0.152 (6.0 -2.0 A). CMTI-I is of ellipsoidal shape; it lacks helices or B-sheets, but consists of turns and connecting short polypeptide stretches. The disulfide pairing is CYS-31-201, Cys-101-221 and Cys-161-281. According to the polypeptide fold and disulfide connectivity its structure resembles that of the carboxypep tidase A inhibitor from potatoes. Thirteen of the 29 inhibitor residues are in direct contact with trypsin; most of them are in the primary binding segment Val-21 (P4) -Glu-91 (P4') which contains the reactive site bond Arg-51 -Be-61 and is in a conformation observed also for other serine proteinase inhibitors.

Histone octamers were reconstituted from the following preparations: (a) natural histone H3‐H4 tetramer and histone H2A‐H2B dimer, either selectively extracted from chromatin with solutions of sodium chloride or prepared by dissociation of the natural octamer; (b) acid‐denatured core histones, either an unfractionated mixture or individually purified proteins. Complexes assembled from these histones elute from exclusion chromatography columns with octamer size as verified by cross‐linking with dimethylsuberimidate. The reconstituted octamers all crystallize in the same form of helical tubes as the natural octamer.

No abstract

A hybrid histone octamer was reconstituted from eythrocyte H2A and H2B, avian histone H3 and the sea-urchin sperm [73Cys]H4 variant. ['loCys-Des-thio]histone H3 was prepared by reaction of natural H3 with Raney nickel. The ability of the hybrid octamer to crystallise to the same form as the natural octamer demonstrated that the chemical modification of cysteine to alanine in H3 and the mutation from threonine to cysteine in sperm H4 do not alter histone-histone interactions in the octamer. Since the sulfhydryl groups of both H4 molecules are fully accessible to 5,5'-dithiobis(2-nitrobenzoate) these residues provide suitable sites for the introduction of a single cysteine-specific label per H4 molecule in the octamer.A detailed description of histone-histone and histone-DNA interactions at the level of natural core-particle structure is central to the understanding of structural changes in the core particle accompanying mitosis, replication and transcription. X-ray crystallographic studies of nucleosome structure will ultimately provide a sound structural basis for the interpretation of these interactions. The highest level of insight, however, has not yet been reached. This has primarily been due to the limited resolution attained during studies of the core particle [l] and the different ultrastructural descriptions of the core particle yielded from the studies of Richmond et al. [l] and Burlingame et al. [2].The reconstitution and physicochemical characterisation of histone octamers carrying reporter molecules in well-defined positions represent an alternative approach to the study of histone-histone and histone-DNA interactions. Such studies, undertaken in parallel with crystallographic investigations at the present and improved resolution, will increase the level of insight into the underlying mechanisms of biologically significant structural changes in the core particle.The histone octamer can be reconstituted from its natural products of dissociation or individually purified acid-denatured histones. These octamers crystallise to the same form as the natural one, namely the helical tube [3] previously reported by Klug et al. [4]. Crystalline tubes are also obtained from reconstituted hybrid octamers containing aurothiomalate-substituted H2A-H2B dimers [5]. We have now investigated the influence of the mutation from threonine to cysteine found in the natural sea-urchin MATERIALS AND METHODS Histone isolationChicken erythrocyte nuclei were prepared from washed cells as detailed previously [3]. Sperm from mature Parechinus angulosus males was collected after intracoelomic injection of 0.5 M KCl by inverting individuals on 100-ml beakers filled with sea water. The sea water was decanted and the sperm slurry filtered through a plastic mesh (pore size = 250 pm) before centrifugation at 200 x g for 5 min. The pelleted sperm was suspended in five volumes 0.14M NaCl, 0.01 M trisodium citrate and centrifuged as before. This washing procedure was repeated until the supernatant was clear.Total acid-extracted histones...