6.4: Contact-dependent Signaling
Contact-dependent signaling, as the name suggests, requires that communicating cells be in direct contact with each other. This is achieved either through receptor-ligand interactions or by specialized cytoplasmic channels that allow the flow of small molecules between cells. In animal cells, channels called gap junctions facilitate contact-dependent signaling in certain tissues, whereas, plasmodesmata perform a similar function in plants.
In animal cells, gap junctions are formed when a set of six proteins called connexins arrange themselves to form a cylindrical structure within the plasma membrane. This cylindrical structure is called a connexon. When connexons of adjacent animal cells align, a channel forms between the two cells. The opening and closing of these channels is regulated by various enzymes. The selectivity of gap junctions allows a single cell to regulate communication with its neighbors and coordinate a complex multicellular response. For example, the gap junctions in cardiac muscles control the flow of ions in a rhythmic manner. This regulates the electrical signal that passes through the cells, allowing the heart muscles to contract and relax in coordination.
In plants, contact-dependent signaling occurs through cytoplasmic channels called plasmodesmata. These channels pass through both the cell wall and the cell membrane.
Unlike gap junctions, plasmodesmata are more flexible in allowing molecules to pass through. At the center of most plasmodesmata channels is a continuous endoplasmic reticulum tubule (ER) that runs between the two cells. This ER extension called the desmotubule is surrounded by the cytoplasmic sleeve. This sleeve permits the passive and active transport of molecules. However, for transportation of larger molecules, like transcription factors, small RNA, or other nucleic acids and proteins, the plasmodesmata can dilate. Because of this ability, plasmodesmata play a key role in spatial organization of cells and their fate. Unfortunately, this ability also permits viral infection to quickly spread between cells.