Protein functions are highly diverse within a cell and an organism. A distinction is generally made between cellular functions defining the role of a given protein in a cell or organism, and biochemical functions defining the activity of proteins at the molecular level.
Examples of cellular functions:
Structural proteins: they allow a cell to maintain its spatial organisation.
Example: collagen
Regulatory proteins: they modulate the activity of other proteins.
Example: transcription factors
Examples of biochemical functions:
Proteins with enzymatic activity.
Example: DNA polymerase
Ribonucleoprotein complexes, composed of RNA and proteins.
Example: a ribosome
A protein can perform several cellular functions, and express multiple biochemical functions.