The structure of a haemopexin-fold protein from cow pea (Vigna unguiculata) suggests functional diversity of haemopexins in plants

The haemopexin fold is present in almost all life forms and is utilized for carrying out diverse physiological functions. The structure of CP4, a haemopexin-fold protein from cow pea (Vigna unguiculata), was determined at 2.1 angstrom resolution. The protein exists as a monomer both in solution and in the crystal. The structure revealed a typical four-bladed beta-propeller topology. The protein exhibits 42% sequence similarity to LS-24 from Lathyrus sativus, with substantial differences in the surface-charge distribution and in the oligomeric state. A structure-based sequence analysis of haemopexin-fold proteins of plant and mammalian origin established a sequence signature associated with the haemopexin motif. This signature sequence enabled the identification of other proteins with possible haemopexin-like topology of both plant and animal origin. Although CP4 shares a structural fold with LS-24 and other haemopexins, biochemical studies indicated possible functional differences between CP4 and LS-24. While both of these proteins exhibit spermine-binding potential, CP4 does not bind to haem, unlike LS-24.