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 S-domain 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 = 194.6Å, c = 272.1Å 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 poly-peptide 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
