Role of molecular messenger, IP7, in regulating cellular traffic
Ms Manasa Chanduri and Dr Rashna Bhandari (Senior Fellow 2010)
Laboratory of Cell Signalling, Centre for DNA Fingerprinting and Diagnostics (CDFD), Hyderabad
The role of small molecules in maintaining cellular homeostasis is well-established. Inositol Pyrophosphatase 7 (IP7) is one such molecule that consists of an inositol ring decorated with seven phosphate groups (see Figure). The six alpha phosphates in IP7 are attached directly to the carbon atoms on the ring, and a seventh (beta) phosphate is attached to one of the alpha phosphates. Mammals have three IP6 kinases (IP6K1/2/3) that catalyse the synthesis of IP7 from IP6 and ATP (see Figure). IP7 participates in many vital cellular functions such as apoptosis, DNA repair, ribosome biogenesis, and energy homeostasis. In some of these pathways, IP7 has been shown to modulate protein function by directly transferring its β-phosphate to a phosphorylated serine residue leading to protein ‘pyrophosphorylation’.
The transport of fluid-filled sacs or vesicles within cells is critical for intracellular and intercellular communication. Earlier studies on the cellular roles of IP7 revealed its importance in some intracellular vesicle trafficking processes in mammalian cells including insulin exocytosis, and kinesin motor driven trafficking of vesicles towards the cell periphery. We decided to explore whether IP7 affects dynein-driven vesicle transport from the cell periphery to the interior, i.e. towards the minus-end of microtubules. Upon examining mammalian cells lacking IP6K1, which have a 70% reduction in IP7 levels compared with normal cells, we identified a delay in dynein-dependent sorting of vesicles containing the iron-transport protein transferrin. We also noted slower motility in vesicles moving from the cell membrane towards the perinuclear region, and disruption of Golgi morphology, a classic readout of dynein dysfunction.
We collaborated with India Alliance Senior Fellow, Dr Roop Mallik at TIFR, Mumbai, to study the effect of IP7 on dynein-driven motility of isolated vesicles. Dr Mallik’s lab noticed that in comparison with wild-type amoebae, endosomes derived from amoebae lacking IP7 show a decrease in dynein-dependent movement towards the minus-end of microtubules, and a corresponding increase in kinesin-directed movement towards the plus-end. It is known that a continuous tug-of-war between these two opposing motors on an endosome that decides its direction of movement. These experiments also revealed no change in vesicle velocity, indicating that motor function was not compromised, and suggested that compared to their wild-type counterparts, amoebae lacking IP7 recruit fewer dynein molecules to endosomes.
We found that dynein intermediate chain (IC), a subunit of the dynein complex, is pyrophosphorylated by IP7 (see Figure, nextpage). IC interacts with the p150Glued subunit of the protein complex dynactin, which in turn helps in the recruitment of dynein to vesicle membranes. A disruption in dynein-dynactin interaction has been shown to result in phenotypes similar to those we observed in cells with low IP7 levels. In fact, we found a decrease in dynein-dynactin association and reduced recruitment of dynein to membranes in IP6K1 knockout cells.
Our study highlights an important role for IP7 in dynein-driven trafficking. We propose that IP7-mediated pyrophosphorylation acts as a switch favouring dynein attachment to membranes, thereby providing the push needed to move vesicles to the minus-end of microtubules. Our work has also provided a boost to research on serine pyrophosphorylation by IP7. This aspect of the work was emphasized in a commentary on the paper by Dr Adolfo Saiardi from University College London, titled "Protein pyrophosphorylation: moving forward". The commentary highlighted how "this work increases our awareness of this modification, underappreciated by the scientific literature but probably not by the eukaryotic cell".
Inositol hexakisphosphate kinase 1 (IP6K1) activity is required for cytoplasmic dynein-driven transport. Chanduri M, Rai A, Malla AB, Wu M, Fiedler D, Mallik R and Bhandari R.. Biochemical Journal (2016)
Image credit Anna Tanczos, Wellcome Images