Organelles are subcellular compartments that provide optimal conditions for distinct biochemical reactions. Coordination between these organelles is of vital importance for sustaining cellular function and viability. A particular challenge is imposed on the cell by mitochondria and chloroplasts because these are the only two organelles, other than the nucleus, which contain genetic information. The organellar genome has a small size and a very limited coding capacity compared to the nuclear genome that encodes the vast majority of the organellar proteins. Thus, biogenesis of these organelles requires a continuous crosstalk between two spatially separated compartments. Moreover, mitochondria and chloroplasts fulfill a variety of essential functions. As such, their inheritance and integrity must be faithfully and continuously monitored.
In our lab we maintain a long-standing interest in the mechanisms that determine the morphology and dynamics of mitochondria and the distribution and inheritance of the mitochondrial genome (mtDNA). More than a decade ago we showed that mitochondria in the yeast S. cerevisiae form a single interconnected network that is maintained by balanced fusion and fission events. This observation paved the way for the identification of the machineries involved in these processes. A fascinating turning point was marked by our more recent discovery of the ERMES (ER-mitochondria encounter structure) complex that links together mitochondria and the endoplasmic reticulum at sites of actively replicating mitochondrial genomes. Surprisingly, we found that the ERMES complex is intimately linked to the process of mitochondrial fission, during which the ER wraps around mitochondria to induce a constriction that is subsequently utilized by mitochondrial fission components to complete scission. mtDNA is segregated to both of the resulting mitochondrial tips during this process, pointing to extensive molecular communication between the mitochondrial matrix and the ER. While these findings offer first insights into the mechanisms that facilitate mtDNA segregation and distribution, the molecular players that act directly on mtDNA in this process remain unknown. Another related outstanding question regarding mtDNA is how its copy number is regulated. To explore these questions, we have developed novel genetic and imaging tools that currently provide us with unprecedented insights into these questions.
Very recently, we have introduced the unicellular green algae Chlamydomonas reinhardtii as a model organism to dissect how nuclear gene expression is coordinated with the functional state of the chloroplast. Inspired by the discovery of the unfolded protein response and empowered by the development of a novel tool to specifically manipulate chloroplast protein homeostasis, we have embarked in a genetic screen that aims to identify the molecular players signaling from chloroplasts to nucleus.