CHANNELS

CHANNELS

Channels mediates the facilitated diffusion of small ions. As diffusion through ion channels does not require complex conformational changes it is significantly faster than  transport mediated by protein carrier. Channels are simply open holes in the membrane. The two important points that distinguish these channels from other openings are- they are capable of discriminating between different ions; thus separate channels exist for transporting important cellular ions such as sodium. Calcium potassium and chloride. In addition channels have gates that can be regulated in response to appropriate stimuli.The ability of channels to regulate the diffusion of specific ions across cellular membrane plays a significant role in many types of cellular communication.

The outward rectifier KCO1 is a member of the “two-pore” K⁺ channel family and is sensitive to the concentrations of cytosolic Ca^2+­­­­. An outward rectifier, KCO1 has been identified at the molecular level from an Arabidopsis EST, based on the highly conserved P-domain motif TxGYGD. This channel is a member of the “two-pore” K⁺ channel family, so-called because there are two P-domains in each subunit. In contrast to Shaker-type channels, KCO1 has only four transmembrane spans in each subunit. Two Ca²⁺-binding motifs called EF hands reside toward the C terminus of the protein. The functional attributes of KCO1 as an outward rectifier have been confirmed by heterologous expression. Analysis of these currents and the underlying channels demonstrates that although these are cation-selective, they do not select as effectively for K⁺ as the inward rectifiers do. Such channels probably constitute a major pathway for Na⁺ uptake into plant cells and could have important implications for salinity tolerance. These channels are partially blocked by external Ca²⁺. An even less selective channel has been observed in the plasma membrane of xylem parenchyma cells. This channel is almost as permeable to anion as to cations; such low selectivity is unusual in plasma membrane ion channels. Perhaps this channel could provide a pathway for release of salts from the symplasm into the xylem.

§   Monovalent cation channels at the vacuolar membrane are Ca²⁺-sensitive and mediate vacuolar K⁺ mobilization. Both types ofchannels activate instantaneously in response to an imposed voltage. The fast vacuolar (FV) channel, which exhibits little selectivity among monovalent cations, in inhibited when cytosolic Ca²⁺ concentrations exceed 1 pM and is activated when cytosolic pH increases. Conversely the vacuolar K⁺ (VK) channels, which is highly selective for K⁺ over other monovalent cations, is activated by cytosolic Ca²⁺ in the nanomolar to low micromolar concentration range and is inhibited by increasing cytosolic pH. A wide range of metabolic and developmental signals in plants trigger an increase in cytosolic free Ca²⁺. Thus, the increase in the concentration of free Ca²⁺ activates downstream targets that further transducer the initial signal into the end response. Voltage-gated Ca²⁺-permeable channels reside in the plasma membranes of a range of cell types and have been characterized by both patch clamp and planar lipid approaches. The channels exhibit various degrees of selectivity for Ca²⁺ over K⁺-from about 2:1 to 20:1. The channels are activated by membrane depolarization.

Channels can interact in two fundamentally different ways, First passage of an ion (usually Ca²⁺) through one channel can result in concentration changes that gate a different channel. Second, channel opening can change V_m, leading to activation of voltage-gated channels. Both aspects of channel interaction are combined in membrane-based signaling pathways that transduce and amplify incoming signals. .

§   Voltage-dependent K⁺ channels at the plasma membrane stabilize and allow controlled K⁺ uptake and loss. Molecular evidence demonstrates definitively that the time-dependent inward and outward currents are carried by separate classes of ion channel. The channels carrying these currents are said to rectify. Like values, rectifying channels carry current in one direction but not the other. For this reason, the channels are known as K⁺ inward rectifiers and K⁺ outward rectifiers. Both are inhibited by millimolar concentrations of tetraethylammonium, a diagnostic blocker of K⁺ channels.

§   Plant cell inward rectifiers are members of the Shaker family of voltage-gated channels. Plant inward rectifier subunits are products of a multigene family, members of which exhibit tissue-specific expression. One member, KAT1 is expressed selectivity in guard cells another, AKT1 is found in roots and hyadothodes. The AKT1 gene of Arabidopsis has been disrupted by using T-DNA insertional mutagenesis. The resulting mutant, referred to as AKT1-1, has dramatically reduced K⁺ uptake less growth in media with low K⁺ concentrations. The analysis of K⁺ uptake in ATK1-1 mutants indicates that AKT1 encodes a high-affinity K⁺ channel.

Several structural features define plant inward rectifying channels as embers of the Shaker family, a super family