Expression of the complete ORF2 of human astrovirus serotype 1 (HAstV-1) in the baculovirus system led to the formation of virus-like particles (VLPs) of around 38 nm. The same kind of VLPs were also obtained either with the expression of a truncated form of ORF2 lacking the first 70 amino acids (aa), or with the same truncated form in which those 70 aa were replaced by the green fluorescent protein. All three kinds of VLPs were equally recognized by an anti-HAstV-1 polyclonal antibody and by two monoclonal antibodies (MAbs; 8E7 and 5B7), indicating a nonessential role of those amino acids neither in the capsid assembly nor in the antigen structure. A second type of structure consisting of 16-nm ring-like units was observed in all of the cases, mostly after disassembling the 38-nm VLPs through the addition of EDTA. The removal of the EDTA and the addition of Mg 2؉ ions promoted the reassembly of the 38-nm VLPs. The nature of these 16-nm ring-like structures, capsomers or T ؍ 1 VLPs, still remains unclear. Biochemical analysis revealed no differences between the 38-nm VLPs and the 16-nm structures, whereas antigenically, they shared the 8E7 MAb epitope but differed in the 5B7 MAb epitope, with the latter structures being more readily recognized.Human astroviruses (HAstV) are a frequent causal agent of gastroenteritis in children worldwide (5,10,12,14,24,27,28,35,38), although they have also been associated with the elderly (4, 29, 34). Eight serotypes have been described, with serotype 1 (HAstV-1) being the most globally prevalent (12,14,18,27,28,33,36). Astroviruses are nonenveloped viruses whose capsid is around 28 to 41 nm in diameter and contains a plus-sense single-stranded RNA of around 6.9 kb organized in three open reading frames (ORFs) (20,32,37). ORF1a and ORF1b encode the nonstructural proteins (19,21,22), whereas ORF2 encodes the structural proteins through a subgenomic RNA (25). ORF2 of HAstV-1 encodes a polyprotein of 787 amino acids (aa) in length, with a molecular mass of around 87 kDa (26), which is the precursor of the smaller 24-to 26-kDa, 29-to 31-kDa, and 32-to 34-kDa structural proteins (1,3,26). The proteolytical processing from the precursor to the mature structural proteins is still very controversial, and to date, three different models have been proposed (1,13,23). In the first model, Bass and Qiu (1) proposed an intracellular processing of the precursor polyprotein at the amino terminus between residues 70(R) and 71(K) before capsid assembly, with this capsid being further processed extracellularly by the action of trypsin and giving the above-mentioned mature proteins. In subsequent studies, the intracellular processing of this model was refused (13), with the complete precursor being the assembly unit. Later on, Méndez and colleagues (23) proposed a third model in which the structural precursor of HAstV-8 is intracellularly processed at the carboxy terminus prior to its assembly into the capsid, although the cleavage site has not yet been identified.The expression of the genomes en...