Transient Receptor Potential (TRP) ion channels are formed by either homomeric or heteromeric assembly of four TRP subunits that have six transmembrane domains (TM) and a P-loop located between TM5 and TM6 defining the channel pore. So far 30 different subunits classified in 6 families sharing as low as 20% homology, have been identified in mammals and about 20 in Xenopus. Although heterotetramers can be formed within a family, the assembly of subunits from members of different families was thought to be unlikely. We here propose that TRPV6 and TRPC1 subunits can co-assemble in the Xenopus oocyte. Western blots performed on naive oocytes lysates revealed that they express both TRPV6 and TRPC1 proteins. Furthermore, co-immunoprecipitation of tagged xTRPV6 and xTRPC1 proteins expressed in oocytes suggested a heteromeric assembly. In mammals, TRPV6 is mainly expressed in intestinal epithelia and in the placenta where it forms a homomeric channel with high calcium permeability whereas TRPC1 is a ubiquitous protein forming cationic channels with an unclear gating mechanism.

In oocytes, over expression of xTRPC1, as previously reported, did not induce any resting ionic current. Conversely, oocytes overexpressing xTRPV6 alone did show an inward rectifying cationic current that could be blocked by La3+ ions and Ruthenium red. As described for mammals, the xTRPV6 channel was largely permeable to calcium ions and displayed an anomalous mole fraction effect. A striking distinction with its mammalian counterpart is the high permeability of the xTRPV6 channel to Mg2+ over other divalents (Mg2+>Sr2+>Ba2+>Ca2+), the Mg2+/Ca2+ ratio being 2.08.

This high permeability to Mg2+ could be abolished by a point mutation in the P-loop (D525N) lowering the ratio to 1.18. Co-expression of xTRPC1 and xTRPV6 lead to a very strong reduction in the current amplitude and in cationic selectivity, the Mg2+/Ca2+ ratio also dropping to 1.18. Further investigations using heterologously expressed xTRPV6 and xTRPC1 in HEK293, allowing whole-cell recordings, will help us clarify the selectivity and the regulation of the channel. The potential interaction between TRPV6 and TRPC1 in mammals will also be examined using tissue extracts and selected cell lines.


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