Proteins are an essential building block of every cell and are ubiquitous throughout the cell. They are involved in almost every process in the cell, from forming structural components to catalyzing reactions to cell signalling and interactions.
Proteins are made of amino acids, which each have their own size, charge and hydrophobicity properties. These wide range of properties allow cells to build proteins with a wide range of functions, binding capabilities and reactivities. Because of this flexibility, proteins can be built that function in many ways.
Below is a general list of all the different functions of proteins in cells.
1) Reaction catalysis. Enzymes are proteins that are instrumental in controlling the rates of chemical reactions in the cell. Without this control, reactions would occur spontaneously and without limit, eating up vital energy and destroying the organism. Enzymes are generally specific to just one or a few reactions in larger pathways and have very tight control. Many have allosteric or feedback regulators that control the enzyme's activity based on cell conditions.
2) Cell signalling. A number of proteins exist to function as cell signalling components. Many of these are embedded into the membrane of the cell to allow for cell-cell signalling. They also may function as ligand binders, like antibodies. These proteins interact with a ligand, then in turn cause some form of signalling cascade that allows the cell to make a response to that exterior signal.
3) Transport within the cell. Connected to cell signalling is the role of proteins in small molecule transport. There are a number of proteins that exist to bind to molecules and move them from area of the cell to another or for export out of the cell. A whole class of proteins exist that move ions in and out of the cell through pores and ion transporters.
4) Structural support and movement. Many proteins are involved in protein structure, including cytoskeleton proteins like actin and tubulin. Some proteins are involved in cell motility, like myosin, which allows the cell to contract and stretch to move and expand. This is critical for cell replication, when the cell has to divide evenly so that daughter cells are fully functional. It is also important for muscle contractions in higher level organisms, where muscle cells must contract and expand to allow for whole organism movement. Other proteins are used by cells to form extracellular connective or structural tissue, such as collagen, elastin and keratin which forms ligaments, cartilage, and other connective and filamentous structures.