THE 2 MONOFUNCTIONAL DOMAINS OF OCTAMERIC FORMIMINOTRANSFERASE-CYCLODEAMINASE EXIST AS DIMERS

Formiminotransferase - cyclodeaminase is a bifunctional enzyme arrange d as a circular tetramer of dimers that exhibits the ability to effici ently channel polyglutamylated folate between catalytic sites. Through deletion mutagenesis we demonstrate that each subunit consists of an N-terminal transferase active domain and a C-terminal deaminase active domain separated by a linker sequence of minimally eight residues. Th e full-length enzyme and both isolated domains have been expressed as C-terminally histidine-tagged proteins. Both domains self-dimerize, pr oviding direct evidence for the existence of two types of subunit inte rfaces. The results suggest that both the transferase and the deaminas e activities are dependent on the formation of specific subunit interf aces. Because channeling is not observed between isolated domains, onl y the octamer appears able to directly transfer pentaglutamylated inte rmediate between active sites.