Vacuole vacuolar sequestration

Protein families involved in vacuolar sequestration may be the NRAMP s, CDF s, and CAX s (reviewed by Hall and Williams [102]) as well as CPx-type ATPases. Until now, already several transporters for vacuolar sequestration of zinc (and possibly cadmium) and nickel have been investigated and could be partially characterized [103-107]. Several CDF transporters for vacuolar sequestration of Zn (and possibly Cd and Co) have been characterized, all are homologous, almost identical in sequence. These are MTPl, ZAT, and ZTP (e.g., [88,89,103,108]). The strongly elevated expression of the CPx-type metal ATPase HMA3 was shown to play a decisive role in Cd accumulation not only in T. caerulescens [109], but also in rice [110,111], reconfirming also the importance of the sequestration into vacuoles for the hyperaccumulation phenotype. The natural over-expression of NRAMPs was identified both in rice and in T. caerulescens to play an important role in Cd tolerance and possibly Cd accumulation [112-114]. When vacuolar... 

As we saw in Chapter 7, there are several plasma membrane zinc uptake transporters in yeast. Within the cell, a number of other proteins are involved in zinc transport within the cell. S. cerevisiae is unusual in that it does not appear to have any plasma membrane zinc efflux transporters. This is to a large extent compensated by the capacity of the vacuole to serve as a major site of zinc sequestration and detoxification, enabling wild-type cells to tolerate exogenous zinc concentrations as high as 5 mM. The zinc stored in the vacuole can attain millimolar levels, and can be mobilised under zinc-deficient conditions for use by the cell. Vacuolar zinc uptake is mediated by two members of the cation diffusion facility CDF family, Zrcl and Cotl (Fig. 8.16). 

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