AMINO-TERMINAL TRUNCATIONS OF THE RIBULOSE-BISPHOSPHATE CARBOXYLASE SMALL-SUBUNIT INFLUENCE CATALYSIS AND SUBUNIT INTERACTIONS

The first 20 residues at the amino terminus of the small subunit of sp inach ribulose-1,5-bisphosphate carboxylase form an irregular arm that makes extensive contacts with the large subunit and also with another small subunit (S. Knight, I. Andersson, and C.-I. Branden [1990] J Mo l Biol 215: 113-160). The influence of these contacts on subunit bindi ng and, indirectly, on catalysis was investigated by constructing trun cations from the amino terminus of the small subunit of the highly hom ologous enzyme from Synechococcus PCC 6301 expressed in Escherichia co li. Removal of the first six residues (and thus the region of contact with a neighboring small subunit) affected neither the affinity with w hich the small subunits bound to the large subunits nor the catalytic properties of the assembled holoenzyme. Extending the truncation to in clude the first 12 residues (which encroaches into a highly conserved region that interacts with the large subunit) also did not weaken inte rsubunit binding appreciably, but it reduced the catalytic activity of the holoenzyme nearly 5-fold. Removal of an additional single residue (i.e. removal of a total of 13 residues) weakened intersubunit bindin g approximately 80-fold. Paradoxically, this partially restored cataly tic activity to approximately 40% of that of the wild-type holoenzyme. None of these truncations materially affected the K(m) values for rib ulose-1,5-bisphosphate or CO2. Removal of all 20 residues of the irreg ular arm (thereby deleting the conserved region of contact with large subunits) totally abolished the small subunit's ability to bind to lar ge subunits to form a stable holoenzyme. However, this truncated small subunit was still synthesized by the E. coli cells. These data are in terpreted in terms of the role of the amino-terminal arm of the small subunit in maintaining the structure of the holoenzyme.