What are the functions of membrane proteins?

Membrane proteins are proteins that are attached to the cell membrane and organelle membrane. They serve several vital functions that help cells survive and thrive: 

Form intercellular junctions: Membrane proteins connect and join adjacent cells together through various junctions such as gap junctions and tight junctions. These intercellular junctions help cells communicate with each other.  They also help to transfer materials between cells. 

Facilitate cell-to-cell recognition: An important function of membrane proteins is to act as recognition molecules. Membrane proteins on the membrane surface behave as name tags, which allow other cells to identify the type and identity of the target cell. This is particularly useful in the immune system, as it enables the immune system to differentiate between foreign cells and self-cells and selectively attack the former while leaving the latter untouched.  

Serve as enzymes: All enzymes are types of proteins. In their role as enzymes, membrane proteins facilitate signal transduction and catalyze various chemical reactions including metabolic, hydrolytic, and redox reactions. 

Signal transduction: Binding sites, which are a common feature on many membrane proteins, are characterized by distinct shapes that complement the shape of a chemical messenger or extracellular signaling molecule. On encountering a cell surface, the signaling molecule binds with a matching receptor membrane protein and activates the proteins inside the cell. This results in a signaling cascade, which alters cell structure or behavior.

Enable transportation: Membrane proteins form channels or pores that facilitate the movement of ions, nutrients, and other molecules across the membrane via active or passive transport. 

Provide anchorage: Membrane proteins become points of attachment between the internal cytoskeleton and other cell.