3.4. Carriers

1. Carriers exhibit Michaelis-Menten kinetics that indicate conformational changes during transport.

Carriers exhibit saturation kinetics.

Carriers undergo conformational changes during transport (ex, The activity of carrier C)

2. Carriers translocate a wide variety of inorganic and small organic solutes with high specificity.

1) Inorganic nutrients: NH4+, NO3-, Pi, K+, SO42-, Cl-

2) Organic solutes: sugars, amino acids, purine and pyrimidine bases

Functions of carriers

1. At plasma membrane, - nutrient uptake - the mobilization and storage of metabolites.

2. At endomembrane, - sequestration of ions (Na+, Ca2+, Mg2+, No3-, sugars, aa)

3. Most plant carriers are energized by coupling to pmf.

4. Molecular identification of carriers defines them as members of the major facilitator superfamily

Functional analysis

1. By yeast complementation

2. By protein expression in Oocytes

Observation of plant carriers expressed in heterologous systems can provide into carrier function

Structural model illustrating the orientation of a generalized carrier in a membrane

Localization of the sucrose transporter SUC2 to companion cells of the phloem

A. Immunofluorescent localization of SUC2 in Arabidopsis stems.

B. Same section as in A but viewed with transmitted light.

P: phloem X: xylem

Localization of the sucrose transporter SUT1 to sieve elements

A. Immunofluorescent localization of SUT1 in a longitudinal section of a potato stems.

B. Silver-enhanced Immunogold localization of SUT1 in cross-section of a potato petiole

sp: sieve plate n: nucleus cc: companion cells

6. Regulation of carrier activity

1) By transcriptional control

2) By post-translational control

Low K+: 40 uM,

High K+: 2 mM

Different transcription of AtKUP2 and AtKUP3

7. In some cases, ion-coupled solute transport

involves Na+ rather than H+

1. Na+ symport have been found.

2. Na+coupling of K+ transport has been proved by a wheat cDNA

a. Uptake of NO3- and some amino acids is Na+-dependent

b. uptake of K+ at micromolar concentration is also Na+-dependent

Acetabularia, a marine algae