Proteins play a fundamental role in cellular signal transduction, a process that allows cells to communicate with their environment and respond to internal and external stimuli. This process is essential for maintaining homeostasis and regulating physiological activities such as growth, cell division, and environmental responses. 1. Protein Receptors Cell signaling typically begins when signaling molecules such as hormones or neurotransmitters bind to protein receptors located on the cell surface or inside the cell. These receptors act as sensors that recognize the signal and convert it into an intracellular response. Types of receptors include: Membrane receptors, such as G-protein–coupled receptors Intracellular receptors, which interact with lipid-soluble hormones like steroids 2. Signal Transduction Proteins After the signal binds to its receptor, a series of intracellular proteins becomes activated. These proteins act as intermediaries, transmitting the signal from the cell membrane to the nucleus or other cellular organelles. Examples include: G proteins Kinases Phosphorylation-regulating proteins 3. The Role of Phosphorylation in Cell Signaling Phosphorylation—the addition of a phosphate group—is one of the key mechanisms that regulate protein activity. Phosphorylation can alter a protein’s shape or function, activating or inhibiting specific signaling pathways. This mechanism regulates: Cell division Transcription factors Hormonal responses 4. Regulatory Proteins These proteins control the intensity and duration of the signaling process. Examples include: Phosphatases, which remove phosphate groups and deactivate proteins Cell cycle regulatory proteins 5. Biological Importance Protein-mediated signaling helps: Maintain homeostasis Control development and growth Regulate immune responses Respond to environmental stress Coordinate communication among tissues Disruptions in these proteins may lead to severe conditions such as cancer, immune disorders, and hormonal imbalances. Al-Mustaqbal University is the first university in Iraq.