CRYSTAL-STRUCTURE OF ABRIN-A AT 2.14 ANGSTROM

The crystal structure of abrin-a, a type II ribosome-inactivating prot ein from the seeds of Abrus precatorius, has been determined from a no vel crystalline form by the molecular replacement method using the coo rdinates of ricin. The structure has been refined at 2.14 Angstrom to a R-factor of 18.9%. The root-mean-square deviations of bond lengths a nd angles from the standard values are 0.013 Angstrom and 1.82 degrees , respectively. The overall protein folding is similar to that of rici n, but there are differences in the secondary structure, mostly of the A-chain. Several parts of the molecular surface differ significantly; some of them are quite near the active site cleft, and probably influ ence ribosome recognition. The positions of invariant active site resi dues remain the same, except the position of Tyr74. Two water molecule s of hydrogen-bonded active site residues have been located in the act ive site cleft. Both of them may be responsible for hydrolyzing the N- C glycosidic bond. The current abrin-a structure is lactose free; this is probably essential for abrin-a crystallization. The B-chain is a g lycoprotein, and the positions of several sugar residues of two sugar chains linked to earlier predicted glycosylation sites were determined . One of the sugar chains is a bridge between two neighboring molecule s, since one of its mannose residues is connected to the galactose bin ding site of the neighboring molecule. Another sugar chain covers the surface of the B-chain.