Mackinder, L. | bioRxiv Science

bioRxiv2020

Assembly of the algal CO2-fixing organelle, the pyrenoid, is guided by a Rubisco-binding motif

Approximately one-third of the Earths photosynthetic CO2 assimilation occurs in a pyrenoid, an organelle containing the CO2-fixing enzyme Rubisco. How constituent proteins are recruited to the pyrenoid, and how the organelles sub-compartments - membrane tubules, a surrounding phase-separated Rubisco matrix, and a peripheral starch sheath - are held together is unknown. Using the model alga Chlamydomonas reinhardtii, we discovered that pyrenoid proteins share a sequence motif. We show that the motif is sufficient to target proteins to the pyrenoid and that the motif binds to Rubisco, suggesting a mechanism for targeting. The presence of the Rubisco-binding motif on proteins that localize to the tubules and on proteins that localize to the matrix-starch sheath interface suggests that the motif holds the pyrenoids three sub-compartments together. Our findings advance our understanding of pyrenoid biogenesis and illustrate how a single protein motif can underlie the architecture of a complex multi-layered phase-separated organelle. One Sentence SummaryA ubiquitous Rubisco-binding motif targets proteins to the pyrenoid and holds together the pyrenoids three sub-compartments.

cell biology↗

bioRxiv2020

A recombineering pipeline to clone large and complex genes in Chlamydomonas

The ability to clone genes has driven fundamental advances in cell and molecular biology, enabling researchers to introduce precise mutations, generate fluorescent protein fusions for localization and to confirm genetic causation by mutant complementation. Most gene cloning is PCR or DNA synthesis dependent, which can become costly and technically challenging as genes increase in size and particularly if they contain complex regions. This has been a long-standing challenge for the Chlamydomonas reinhardtii research community, with a high percentage of genes containing complex sequence structures, an average genomic GC content of 64% and gene expression requiring regular introns for stable transcription. Here we overcome these challenges via the development of a recombineering pipeline that enables the rapid parallel cloning of genes from a Chlamydomonas BAC collection. We show the method can successfully retrieve large and complex genes that PCR-based methods have previously failed to clone, including genes as large as 23 kilobases, thus making previously technically challenging genes to study now amenable to cloning. We initially applied the pipeline to 12 targets with a 92% cloning success rate. We then developed a high-throughput approach and targeted 191 genes relating to the Chlamydomonas CO2 concentrating mechanism (CCM) with an overall cloning success rate of 77% that is independent of gene size. Localization of a subset of CCM targets has confirmed previous mass spectrometry data and identified new pyrenoid components. To expand the functionality of our system, we developed a series of localization vectors that enable complementation of Chlamydomonas Library Project mutants and enable protein tagging with a range of fluorophores. Vectors and detailed protocols are available to facilitate the easy adoption of this method by the Chlamydomonas research community. We envision that this technology will open up new possibilities in algal and plant research and be complementary to the Chlamydomonas mutant library.

plant biology↗