CRYSTAL-STRUCTURES OF MS2 CAPSIDS WITH MUTATIONS IN THE SUBUNIT FG LOOP

The loop between the F and G beta strands (FG loop) of the bacteriopha ge MS2 coat protein subunit forms inter-subunit contacts around the 5- fold and 3-fold (quasi 6-fold) axes of the T = 3 protein shell. In cap sids, the loop is found in two very different conformations, one in B subunits, which form the 5-fold contact, and one in A and C subunits, which form the quasi 6-fold contact. One proline residue, Pro78, is st rictly conserved in the coat protein of all related bacteriophages, an d in the case of MS2 this proline residue is preceded by a cis peptide bond in the B subunit. In order to probe the role of the FG loop in c apsid assembly we have determined the crystal structures of two MS2 ca psids, formed by coat proteins with mutations at two positions in the FG loop, P78N or E76D. These mutants show conformational changes in th e FG loops that explain the reduced temperature stability of the capsi ds. The P78N mutant has a normal trans peptide bond at position 78. (C ) 1996 Academic Press Limited