Light-dependent assembly of ribulose-1,5-bisphosphate carboxylase

Abstract

Ribulose-1,5-bisphosphate carboxylase [RuP(2)Case; 3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39] is composed of eight small subunits (M(r), 14,000) and eight large subunits (M(r), 55,000). Newly synthesized large subunits are associated with two complexes having sedimentation coefficients of 7 and 29 S. Assembly of RuP(2)Case occurs in isolated intact chloroplasts in the light but not in the dark. When extracts of chloroplasts are treated with ATP or GTP, RuP(2)Case assembly is accelerated while the 29S large subunit complex is maintained. In the presence of Mg(2+), ATP brings about almost complete dissociation of the 29S complex, whereas GTP and a nonhydrolyzable analog of ATP are without effect. These results indicate the existence of a complex set of reactions involving nucleotides, Mg(2+), and several putative intermediates in RuP(2)Case assembly. It is postulated that these reactions at least partly account for the light dependence of RuP(2)Case assembly. In particular, ATP and GTP promote the assembly of large subunits into RuP(2)Case.