Structure, function and regulation of plant Translocator (TSPO) proteins
Arguably as important as inducing responses to a stress is stopping them when they are no longer required. This aspect of stress responses, and signaling in general, has tended to be overlooked. Abiotic stresses including salinity, drought, high light, high temperature, or freezing is perceived by plants, in part, as a transient or permanent water deficit, conducive of abscisic acid (ABA) accumulation. A subset of plant ABA-responsive genes is strictly ABA-dependent in that their expression is almost undetectable in the absence of elevated levels of cellular ABA. Their biological role may be required only transiently and the plant cell under stress therefore needs an efficient regulatory mechanism to transcriptionally and/or post-translationally regulate their expression.
How plants readjust levels of a stress-induced protein, when normal physiological conditions resume has not been addressed. In particular, the questions of how, when, and where the induced proteins are targeted for degradation when their activities become irrelevant await answers. We hypothesized that the recently described Arabidopsis Translocator protein-related (AtTSPO) protein may be a suitable candidate to address these biological questions. AtTSPO (~21 kDa) belongs to the tryptophan-rich sensory protein/peripheral-type benzodiazepine receptor (TspO/MBR) protein family, which are conserved 5 α-helical membrane proteins (Figure 1). The most studied isoform in animal cells, TSPO1, is widely expressed and involved in a wide range of physiological functions and pathologies, including neurodegeneration and cancer.
Using cutting edge molecular, cellular, biochemical and physiological tools, we showed that AtTSPO is a highly regulated protein and is only expressed by plant cells under abiotic stress. Indeed, we demonstrated that heme metabolism regulates AtTSPO stability in the plant and after heterologous expression in yeast. AtTSPO do bind heme in vitro and in vivo and this binding do required a histidine at position 91 (H91). Surprisingly and Interestingly, we found that AtTSPO degradation requires an active autophagic pathway as the induced protein appears to be relatively stable in autophagy deficient atg2, atg5 and atg9 mutant backgrounds. The AtTSPO degradation in the plant cell and in yeast is through a selective autophagic pathway and requires an Atg8-interacting motif on AtTSPO. Indeed, we showed that AtTSPO interacts in vivo with Atg8. We also found a clear connection between AtTSPO role and osmotic stress in that AtTSPO can reduce the level of an aquaporin in the cell plasma membrane (Figure 2).
Some of the fundamental points we are currently addressing are:
- Is AtTSPO a putative autophagic selective cargo receptor?
- What is the molecular mechanism of AtTSPO engulfment by the autophagosome?
- What is the role of Heme binding in this process?
- What is the biological role of the evolutionary conserved, plant-specific N-terminal extension in plant TSPO?