The VTC complex
The Vacuole Transporter Chaperon Complex (VTC complex) has first been identified as a complex required for the proper sorting of membrane proteins. Later on, it turned out that the complex actually synthesizes and translocates polyphosphate (polyP) across the vacuolar membrane.
The VTC complex is made of three different subunits: Vtc1, Vtc2/3, and Vtc4. All the subunits have three helices that span the vacuolar membrane. Only Vtc1 has no soluble domains. Vtc2, Vtc3, and Vtc4 share a common N-terminal domain called SPX domain which is supposed to play a role in phosphate signaling. The central domain of Vtc4 is catalytically active while the Vtc2 and Vtc3 subunits are inactive. They are believed to play a regulatory role although no data confirm this hypothesis. Two isoforms of the VTC complex can be described: (1) a complex made of Vtc1/2/4 which is normally localized at the cell periphery and is relocated in the vacuole membrane under phosphate limitation and (2) a complex made of Vtc1/3/4 which is always localized in the vacuole membrane.
The function of the VTC complex is obviously to produce and store polyP but this is not the only activity of the complex in the cell. It is also required for proper vacuole morphology in vitro and vacuole fusion in vitro. Additionally it has been shown to be required for the completion of microautophagy.
I recently identified a novel subunit of the VTC complex that I named Vtc5. It is
encoded by the so far uncharacterized ORF YDR089W. I demonstrated that Vtc5 physically interacts with the other subunits of the VTC complex and is critical to the regulation of the enzymatic activity. Deletion of VTC5 reduces polyP synthesis while the overexpression of
Vtc5 strongly triggers polyP synthesis. Moreover, modulation of Vtc5 expression influences the expression of the PHO genes independently of the environmental phosphate concentrations. These results suggested the existence of an intracellular phosphate sensing machinerywhich I am now trying to identify.