Protein-protein interactions PPIs are essential for various biological processes in cells, including signal transduction, gene regulation, and metabolic pathways. Different types of protein-protein interactions and complexes contribute to the biological functions of cells in various ways. Some of the main types of PPIs and complexes are:1. Homodimers and heterodimers: Homodimers are complexes formed by two identical protein subunits, while heterodimers consist of two different protein subunits. These interactions are crucial for the stability and function of many proteins, such as transcription factors and enzymes.2. Multiprotein complexes: These are large assemblies of multiple protein subunits, which can be either identical or different. Examples include the ribosome, which is responsible for protein synthesis, and the proteasome, which is involved in protein degradation.3. Transient interactions: These are short-lived protein-protein interactions that occur during specific cellular processes, such as signal transduction or enzyme catalysis. They often involve post-translational modifications, like phosphorylation, which can modulate the interaction between proteins.4. Stable interactions: These are long-lasting protein-protein interactions that form stable complexes, such as those found in the cytoskeleton or extracellular matrix. These interactions provide structural support and help maintain cellular integrity.5. Covalent interactions: Some proteins can form covalent bonds with other proteins, leading to stable complexes. Examples include disulfide bonds between cysteine residues in proteins, which can contribute to protein folding and stability.6. Non-covalent interactions: Most protein-protein interactions are non-covalent, involving hydrogen bonds, ionic interactions, van der Waals forces, and hydrophobic interactions. These interactions are essential for protein folding, stability, and function.7. Domain-domain interactions: Many proteins contain specific domains that mediate interactions with other proteins. Examples include SH2 domains, which bind to phosphorylated tyrosine residues, and PDZ domains, which recognize specific peptide sequences.8. Protein-protein interaction networks: In cells, proteins often interact with multiple partners, forming complex interaction networks that regulate various cellular processes. These networks can be modulated by post-translational modifications, protein expression levels, and cellular localization.Overall, protein-protein interactions and complexes play a crucial role in the biological functions of cells by mediating various processes, including signal transduction, gene regulation, and metabolic pathways. Understanding these interactions is essential for deciphering the molecular mechanisms underlying cellular functions and can provide insights into the development of targeted therapies for various diseases.