- The Role Of Transport Proteins In Eukaryotic Organisms And Their Potential Exploitation In Genetically Modified Plants
The role of transport proteins in eukaryotic organisms and their potential exploitation in genetically modified plants
There are three major types of membrane transport proteins (Lodish, et. al, 1995).
ATP-powered pumps derive the energy required for energetically unfavorable transport of
ions or molecules via the hydrolysis of ATP. Channel proteins engage in passive
transport, moving particular ions, or water down their respective concentration gradients.
Transporters use the slowest mechanism for transport binding only one or a few substrate
molecules for transport at a time. All three of these types of molecules contribute to
the amazing selectivity of plasma membranes and are, thus, critical to the organization
and function of the entire cell.
ATP-powered pumps, also referred to as ATPases given their apparently enzymatic
properties, catalyze the energetically unfavorable movement of ions against their
concentration gradient. This type of transport protein is further subdivided into three
classes, namely P, V, and F. Common to all three types of ATP-powered pumps are the one
or more binding sites for ATP on the cystolic side of the membrane. Made up of four
polypeptides, two alpha and two beta peptides, P-class pumps are the only class that
become phosphorylated as part of the transport cycle. F and V class pumps are similar in
structure, both having multiple transmembrane proteins and an extrinsic group of least
five kinds of polypeptides called the cytosolic domain. Both do not require
- phosphorylation to be active, but F and V ATPases differ in their functions. Where
V-pumps maintain hydrogen ion gradients across membranes by using ATP, F-pumps primarily
serve to make ATP from ADP and phosphate ions.
Every channel proteins can be grouped into one of two classes by examining its
functionality. Some channels are continuously open. Other transmembrane channels require
a message before they will open to allow for the flow of ions. A fast rate of transport
characterizes either type of this group of proteins.
Transporters use a much slower mechanism to transport ions and molecules. In this case,
either one or a few ions or molecules affix themselves to the transport protein. The
binding of the substrates causes a conformational change such that the ions or molecules
of interest are exchanged from one side of the cell to the other. The many types of
transporters are sub-divided into classes according to the number of substrates that the
protein binds and the relative direction of transport of substrates for transporters that
bind more than one substrate. Uniporters bind and transport one substrate at a time. Two
or more ligands attach to the same face of a symporter; the symporter subsequently
transports both ligands in the same direction. Antiporters are similar to symporters in
that they bind multiple ligands, but the substrates bound to an antiporter are transported
in opposite directions relative to each other. In symporters and antiporters one bound
molecule or ion tends to move along its concentration gradient, while the others move in
an energetically unfavorable direction.
A completely different type transmembrane protein, namely bacteriorhodopsin (Lodish,
1995), which uses light energy to cause the conformational change required for transport,
characterizes lesion-mimic phenomena in plants.
Plants are actually susceptible only to a select and small number of pathogens. The
majority of pathogens are incompatible with plants, but despite incompatible pathogens
imposing no actual threat to plants, when a plant recognizes an incompatible pathogen, it
responds with apparently drastic measures. Plants trigger a cell death pathway, which is
activated by the expression of a bacterial proton pump in tobacco (Mittler, Shulaev & Lam,
1995). This pump, the bO pump, serves to transport hydrogen ions and is activated when
the protein's retinal group absorbs light. Spontaneous lesions, resembling hypersensitive
response lesions, form with bO pump activity. Several protective mechanisms switch on at
this point and systemic acquired resistance manifests itself. Synthesis of
pathogenesis-related proteins increases, as does the volume of salicylic acid stored.
Despite the pathogen being incompatible, the cell reacts as if an actual threatening
pathogen was present.
Earlier research investigated the more general topic of systemic acquired resistance, a
phenomenon that is the result of bO pump activity (Hebers et al., 1996). Systemic
acquired resistance brings about immunity in unaffected parts of the plant that shows