Determination of the structure of the recombinant T=1 capsid of Sesbania mosaic virus

Abstract

The recombinant coat protein (CP) of Sesbania mosaic virus lacking segments of different lengths from the N-terminus expressed in E. coli was shown to selfassemble into a variety of distinct capsids encapsidating 23S rRNA from the host and CP mRNA in vivo. Particles with 60 copies (T=1) of protein subunits were observed when protein lacking 65 amino acids from the N-terminus was expressed. This recombinant protein possesses the sequence corresponding to the Sdomain of the native, T=3 icosahedral particles but lacks the β-annulus, the βA strand (residues 67-70) and the arginine-rich ARM motif (residues 28-36). Purified T=1 particles crystallized in the monoclinic space group P2₁ with cell parameters of a=188.4 Å, b=194.6 Å, c=272.1 Å and β=92.6°. The structure of the T=1 particles was determined by X-ray diffraction at 3.0 Å resolution. As expected, the polypeptide fold of the subunit closely resembles that of the S-domain of the native virus. The recombinant particles bind calcium ions in a manner indistinguishable from that of the native capsids. The structure reveals the major differences in the quaternary organization responsible for the formation of T=1 against T=3 particles.