Molybdenum cofactor biosynthesis. The plant protein Cnx1 binds molybdopterin with high affinity

The molybdenum cofactor is an essential part of all eukaryotic molybdoenzymes. It is a molybdopterin (MPT) revealing the same core structure in all organisms. The plant protein Cnx1 from Arabidopsis thaliana is involved in the multi-step biosynthesis of molybdenum cofactor. Previous studies (Stallmeyer, B., Nerlich, A., Schiemann, J., Brinkmann, H., and Mendel, R. R. (1995) Plant J. 8, 751-762) suggested a function of Cnx1 in a late step of cofactor biosynthesis distal to the formation of MPT, i.e. conversion of MPT into molybdenum cofactor. Here we present the first biochemical evidences confirming this assumption. The protein Cnx1 consists of two domains (E and G) homologous to two distinct Escherichia coli proteins involved in cofactor synthesis. Binding studies with recombinantly expressed and purified Cnx1 and with its single domains revealed a high affinity of the G domain to MPT (kD = 0.1 microM) with equimolar binding. In contrast, the E domain of Cnx1 binds MPT with lower affinity (kD = 1.6 microM) and in a cooperative manner (nH = 1. 5). The entire Cnx1 showed a tight and cooperative MPT binding. Based on these data providing a common link between both domains that matches the previous characterization of plant and bacterial Cnx1 homologous mutants, we present a model for the function of Cnx1.