If we want to talk about plant proteins here, then we do have to find out how plants differ from other organisms and whether the study of their proteins is helpful for the understanding of these special properties. Cells have to be able to selectively take up and excrete certain molecules. This does often need energy. The performing enzymes are usually elements of the cellular membranes . They have, just like enzymes, a substrate specificity, though they do not change it but release it at the other side of the membrane.
Transport occurs often against a concentration gradient.
Other proteins again recognize special signals, for example physical factors like light of a certain wave length or chemical components like phytohormones or components of other cells' surfaces. After signal recognition do they change their conformation and thus pass on the 'information'. Receptors may be located in the membrane, in the plasma or in the cell wall. Among their tasks is the transformation and the amplification of the signal. Animal cells could be shown to contain receptors connected in series.