Near-Neighbor Relationships of the Atypical Subunits that Form the Peripheral Stalk of the Mitochondrial ATP Synthase in Chlorophycean Algae

Abstract

Mitochondrial F1Fo-ATP synthase (complex V) is an oligomeric complex that exhibits a molecular mass around 550 kDa. The F1Fo-ATP synthase of the green chlorophycean alga Chlamydomonas reinhardtii and of its close colorless relative Polytomella sp. is isolated as a highly-stable dimer of 1600 kDa after solubilization of algal mitochondria with lauryl-maltoside. Each monomer of the enzyme is constituted by 17 polypeptides, eight of which are the conserved subunits alpha, beta, gamma, delta, epsilon, a (Atp6), c (Atp9), and OSCP, plus nine atypical polypeptides, named Asa1 to Asa9, that seem to be present only in the chlorophycean lineage. The Asa subunits (named after ATP Synthase Associated subunits) form the very robust peripheral stalk that has been observed in several electron-microscope studies of the algal enzyme. We have addressed the close-neighbor relationships of the ASA subunits and their interactions with the orthodox, conserved subunits. For this purpose, we have generated sub-complexes after partial dissociation of the dimeric ATP synthase with high detergent concentrations or with amphiphilic polymers; detected several subunit-subunit interactions based on cross-linking experiments; reconstituted various sub-complexes of the peripheral arm using recombinant subunits; and silenced the synthesis of certain subunits in vivo. Recently, an electron cryo-microscopy map of the enzyme was obtained. Putting together all the structural data available to date, we suggest a model of the complete topological disposition of the subunits that constitute the atypical, algal mitochondrial ATP synthase. The presence of such a stout peripheral stalk in the ATP synthase of chlorophycean alga, seems to have more than the necessary stiffness to counteract the rotation of the central stalk, but will probably make less twisting motions, and thus will store less transient elastic energy.