How phosphoinositol lipids can have an effect

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How phosphoinosotol lipids can have an effect

So in the last post I talked about that I am busy with trying to identify phosphoinositol lipids. But did not really say why I would like to know which proteins bind to my phosphoinositol lipid of interest. Phosphoinositol lipids can been seen as markers of the membranes. For example the plasma membrane has the phosphoinositol lipids PI(4)P and PI(4,5)P₂. but membranes of the Golgi has only PI(4)P. In this way the cell is able to distinguish between the Golgi and the plasma membrane. With as result that the proteins whose interaction should happen at the plasma membrane are doing their job at the plasma membrane and not at the Golgi membrane. As such phosphoinositol lipids function as signalling molecules. In cells local differences in the abundance of phospholipids have been seen, also their abundance can be affected by external stimuli, for example salt stress increases PI(4,5)P₂ levels. Thereby specifying the location and timing of the interactions that take place. Of which the exocyst complex is one.

The exocyst complex helps connecting secretory vesicles to the plasma membrane. The secretory pathway can basically divided into five stages, starting with the budding of a vesicle, its transport towards the target membrane, initial attachment of the vesicle to the target membrane, the docking of the vesicle and finally fusion with the membrane. The exocyst complex is involved in the third stage, the initial attachment of the vesicle to the plasma membrane. By doing so it also creates a bigger anchorage point where all the proteins that are needed for successful exocytosis.

The exocyst complex was originally discovered in yeast and is and consist out of eight proteins. Homologues of these eight proteins are present in plants, however as for a lot of plant genes, some have multiple paralogues which can complicate things a bit. Two of the exocyst subunits, Sec3 and EXO70, create contact points with the plasma membrane. Sec3 does this by directly binding to PI(4,5)P₂. Although it is known that EXO70 binds to the plasma membrane the specific phosphoinositol lipid it binds to is unknown. Furthermore there are multiple paralogues of EXO70, suggesting they play a role in specificity of the location of binding to the plasma membrane. The binding of the exocyst to the plasma membrane is further stabilised by the binding of Sec10 to the cytoskeleton.

For the binding of the vesicle to the exocyst complex it is less clear how this might be happening. It is known that Sec15 plays a role in binding the vesicle to the exocyst complex. However Sec15 is not binding directly to the vesicle lipids like Sec3 and EXO70 are doing at the plasma membrane. Instead, vesicles use RabE GTPases to bind to the exocyst. These GTPases are attached in the membrane of the vesicle via an lipid binding domain. For binding to the exocyst RabE first binds the SCD complex which in turn bind Sec15. At the same time the SCD complex activates RabE, enabling it to activate PIP5K which phosphorylates PI(4)P to produce PI(4,5)P₂. As there are multiple paralogous for RabE, SCD as well as for PIP5K, it is possible they have a role in the specificity of the location of vesicle excretion. However not much is known how exocyst subtypes select the vesicles they bind.

Hopefully this made it a little bit clearer how a signalling lipid located in the plasma membrane can actually have an effect. The next question that everybody will ask of course is will those exocyst complex proteins be among those proteins that bind my phophoinositide lipid in response to salt stress. To be able to make a prediction like that we should look about what we know is happening during salt stress to a cell. The initial response will be a loss of water, due to osmosis, shrinking the cell, as well as taking up Na⁺ and Cl^– ions. But at the time of sampling the cell is already trying to make up for this by actively pumping water in and pumping salt out. For this ion channels need to be activated, this can possible go via phosphoinositide lipids. It is also possible that these ion channels are newly deposited at the plasma membrane via exocytosis. Then it is a possibility that EXO70 and Sec3 are among the proteins that are identified as interacting with the phosphoinositide lipids during salt stress. But this is pure speculation based on what we guess is going on at the time of analysis. The only way to find out is actually identifying those proteins that bind my phosphoinositol lipid.