Hepatitis E virus (HEV) is enterically transmitted and endemic to tropical areas of the world. The major capsid protein of HEV is pORF2 (similar to74 kDa), encoded by open reading frame 2 (ORF2). When expressed in insect cells, it is processed into a similar to55 kDa form (n-pORF2). We also generated a mutant, m-pORF2, lacking a C-terminal hydrophobic region shown earlier to be required for its homo-oligomerization. Circular dichroism, was used to measure the secondary structure and stability of these proteins as a function of pH and temperature. With decreasing pH both proteins acquired increasing alpha-helicity and thermal stability in terms of midpoint of denaturation and the Gibbs energy change. (C) 2003 Elsevier Inc. All rights reserved.