11.4 Cell Signaling leads to Regulation of Cytoplasmic Activities or Transcription

Cytoplasmic and Nuclear Responses  A signal transduction pathway leads to the regulation of one or more cellular activities.  This response may occur in the cytoplasm or may involve action in the nucleus.  In the cytoplasm, a signal may cause the opening or closing of an ion channel in the plasma membrane or a change in cell metabolism.  Many signaling pathways regulate not the activity of enzymes, but the synthesis of enzymes or other proteins, usually by turning specific genes on or off in the nucleus.

Fine-Tuning of the Response Signaling pathways with a multiplicity of steps have two important benefits:  They amplify the signal(which amplifies the response)  Elaborate enzyme cascades amplify the cell’s response to a signal.  The amplification effect stems from the fact that proteins persist in the active form long enough to process numerous molecules before they become inactive again  They contribute to the specificity of response

Signal Amplification

The Specificity of Cell Signaling  The explanation for the specificity exhibited in cellular responses to signals is the same as the basic explanation for virtually all differences between cells:  Different kinds of cells have different collections of proteins.  The response of a particular cell to a signal depends on its particular collection of signal receptor proteins, relay proteins, and proteins needed to carry out the response.  Two cells that respond differently to the same signal differ in one or more of the proteins that handle and respond to the signal.

Signaling Efficiency  The efficiency of signal transduction may in many cases be increased by the presence of scaffolding proteins.  Scaffolding Proteins: large relay proteins to which several other relay proteins are simultaneously attached  Fun Fact: Researchers are finding scaffolding proteins in brain cells that permanently hold together networks of signaling-pathway protein synapses. This hard writing enhances the speed and accuracy between cells.

Signal Efficiency, cont.  Some proteins may participate in more than one pathway, either in different cell types or in the same cell at different times or under different conditions.  The importance of the relay proteins that serve as points of branching or intersection in signaling pathways is highlighted by the problems arising when these proteins are defective or missing.

Termination of the Signal  A key to a cell’s continuing receptiveness to regulation is the reversibility of the changes that signals produce.  The binding of signal molecules to receptors is reversible, with the result that the lower the concentration of signal molecules, the fewer will be bound at any given moment.  When signal molecules leave the receptor, the receptor reverts to its inactive form.  This leads to the relay molecules returning to their inactive form.  Reset and reuse

In Easier Terms…  Basically the section says this:  In the cytoplasm, signaling pathways regulate enzyme activity and cytoskeleton rearrangement.  Other pathways regulate genes by activating transcription factors, proteins that turn specific genes on or off.  Each catalytic protein in a signaling pathway amplifies the signal by activating multiple copies of the next component of the pathway.  This particular combination of proteins in a cell gives the cell great specificity in both the signals it detects and the responses it carries out.  Scaffolding proteins can increase signal transduction efficiently.  Pathway branching and cross-talk further help the cell coordinate incoming signals  Signal response is terminated quickly by the reversal of ligand bonds