Aquaporins have a highly conserved structure consisting of 6 transmembrane a-helices (Fig. 4), with the amino and carboxy termini localized on the cytosolic side of the membrane. The pore of AQP1 consists of three hydrophilic nodes that bind four water molecules, with the remainder of the pore being hy-drophobic (Sui et al. 2001). The conserved histidine residue in the pore is essential for water specificity. Generally, the 25-30 kD polypeptides form homo-tetramers (Sui et al. 2001, Tornroth-Horsefield et al. 2006) and each monomer functions as a single water pore (Engel et al. 2000). A number of residues in the protein are conserved throughout the aquaporin family, including the aspar-agine-proline-alanine (NPA) motif (Fig. 4). The aromatic/ARG filter, a narrow selectivity filter for water, is conserved in all the Arabidopsis PIPs while the TIPs and NIPs have a number of different regions (Wallace et al. 2006).
The most divergent regions of the protein are the N and C-termini and the membrane connecting loops. The pores of the channels are believed to be narrow so that the water molecules move through in single file. Mercury generally blocks aquaporins by binding to cysteine residues, and can cause conforma-tional changes (Barone et al. 1997). Mercury may inhibit aquaporins indirectly in living cells via general metabolic inhibition (Zhang and Tyerman 1999). Silver and gold compounds have also been shown to strongly inhibit aquaporins (Niemietz and Tyerman 2002).
Some plant MIPs transport other small solutes, such as glycerol and urea, in addition to or alternatively to water. Nodulin 26 (NOD26), is targeted to the symbiosome membrane of nitrogen-fixing nodules, allowing the transport of glycerol and formamide in addition to water (Rivers et al. 1997), and may also be permeable to ammonia (Niemietz and Tyerman 2000). Some members of the TIP subgroup transport ammonia (Jahn et al. 2004). NtAQP1 and NtTIPa transport glycerol, water and urea (Eckert et al. 1999, Gerbeau et al. 1999). Urea transport was also demonstrated for four Arabidopsis TIPs (AtTIP1;1, At-TIP1;2, AtTIP2;1 and AtTIP4;1) (Liu et al. 2003) and a maize root PIP (ZmPIP1;5b; Gaspar et al. 2003). Carbon dioxide permeation in leaf mesophyll may be facilitated via NtAQP1 (Flexas et al. 2006, Uehlein et al. 2008). Other important nutrients transported by MIPs in plants include boron (Takano et al.
2006) and silicon (Ma et al. 2006). Some MIPs also appear to transport toxic metalloids, such as arsenite (Bienert et al. 2008). Signalling molecules may also be transported by MIPs, and hydrogen peroxide has been shown to be transported by AtTIP1;1 and AtTIP1;2 in a heterologous yeast system (Bienert et al.
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