Metal Assimilation Pathways - Biological Inorganic Chemistry - page 150

a ferric chelate reductase gene from yeast, which had been selected for enhanced activity at alkaline pH into rice,
increases the yield by 8-fold when the transgenic plants are grown in calcareous soils ( Guerinot, 2007 ) . When we
consider that one-third of the world's soils are alkaline, these results could have a major impact on rice production.

2. Copper and Zinc

Genetic studies in the yeast S. cerevisiae identified two proteins involved in high-affinity copper uptake at the
plasma membrane, CTR1 and CTR3. After reduction of Cu 2 þ by the same reductases, FRE1 and FRE3 described
above for iron, these proteins transport Cu þ . In yeast a third member of the CTR family, CTR2 localises to the
vacuole and is proposed to mobilise copper stored in the vacuolar compartment to the cytoplasm when extra-
cellular copper is limited. There is also a low affinity copper uptake system through FET4 and SMF1 permeases.
Zinc uptake in fungi is carried out to a large extent by the ZIP (Zrt-, Irt-like Protein) family of metal ion
transporters. The family name comes from the yeast Zrt1 protein and the Arabidopsis thaliana Irt1 protein. These
were the first identified members of a family of Zn transporters, which are found at all phylogenic levels, including
bacteria, fungi, plants, and mammals. The mammalian members of the family are given the systematic designation
“SLC39” ( Eide, 2004 ). Without any known exception, the members of the ZIP family transport zinc and/or other
metal ions from the extracellular space, or from the lumen of cellular organelles, into the cytoplasm. This, together
with their transmembrane topography, distinguishes them from the CDF (cation diffusion facilitator)/Znt family:
the mammalian members of the family have been named Znt and given the systematic designation “SLC30”
( Palmiter & Huang, 2004 ) . As we will see when we consider zinc homeostasis in Chapter 8, this family transports
zinc from the cytoplasm into the lumen of extracellular organelles or to the outside of the cell. Thus, CDF proteins
work in the opposite direction to ZIP proteins.
Most ZIP proteins have eight predicted transmembrane domains ( Fig. 7.16 ) and similar predicted topologies
with both the N- and C-termini located on the extracytoplasmic face of the membrane with a His-rich domain

FIGURE 7.16 Predicted membrane topologies for the ZIP/SLC39 and CDF/Znt/SLC30 families of metal ion transporters (a) ZIP/SLC39
(b) CDF/Znt/SLC30. (From Eide, 2006 . Copyright 2006 with permission from Elsevier.)