RBMP2

RBMP2 (Rubisco-binding membrane protein 2; locus Cre09.g416850) is a large (1690 residues, about 165 kDa), nuclear-encoded membrane protein of the chloroplast pyrenoid in the green alga Chlamydomonas reinhardtii. It has an N-terminal region predicted to contact membranes, a central rhodanese-like domain flanked on both sides by predicted transmembrane helices, and a long disordered C-terminal region with six Rubisco-binding motifs, the last at the exact C-terminus. The rhodanese-like domain is non-catalytic: it contains no cysteine, and an aspartate occupies the position of the catalytic cysteine of sulfurtransferases and CDC25 phosphatases. RBMP2 resides in the pyrenoid tubules, the thylakoid-derived membranes that cross the Rubisco matrix, and is concentrated in their central reticulated region. It builds that region. In rbmp2 mutants the cylindrical tubules still enter the matrix but often end bluntly before the centre, the narrow reticulated network is absent, the lumenal carbonic anhydrase CAH3 is mislocalized to the matrix periphery, and growth at very low CO2 is impaired. Overexpression enlarges the reticulated region. Domain deletions separate two steps: the rhodanese-like domain is needed for tubules to extend to the centre, and the membrane-contact and transmembrane regions are needed to convert them into narrow reticulated tubules. The Rubisco-binding motifs bind Rubisco as peptides but are dispensable for forming the reticulated region. RBMP2 was first proposed to tether the Rubisco matrix to the tubules, but its demonstrated role is in shaping the tubule membranes that position CAH3 for CO2 release near Rubisco.