Structural and functional studies of 1) Actin interacting protein 5, a novel actin assembly regulator in Saccharomyces cerevisiae 2) CbbX, a red-type Rubisco activase in Cyanidioschyzon merolae

Abstract

Polarisome, a fuzzy multiprotein complex, regulates polarized cell growth in both budding yeast and filamentous fungi through actin polymerization. Here we reported a previously uncharacterized gene YFR016C that encodes a novel fourth type of actin nucleation factor, named Aip5. Through X-ray crystallography, we have unveiled the structure of Aip5 C-terminal domain (Aip5-C), which is the functional component for actin nucleation. Structural analysis of both the wild type and mutant Aip5-C revealed that Aip5-C directly interacts with G-actin via a loop region to nucleate actin filaments.

Rubisco is the key enzyme for carbon fixation during photosynthesis. Owing to its sluggish enzymatic activity, Rubisco activase is required to modulate Rubisco’s activity. In the unicellular red algae Cyanidioschyzon merolae, there are two isoforms (plastid and nuclear) of the CbbX protein that complex to form a functional Rubisco activase. Through X-ray crystallography, we have determined the structure of CbbX plastid isoform. RuBP and ATP binding sites appeared to be conserved. However, more effort is required to improve the crystal diffraction of CbbX nuclear isoform and CbbX complex to elucidate its regulatory mechanism.