Gluten is a mixture of two naturally-occurring proteins -Gliadin (a prolamin protein),and Glutenin (a glutelin protein) found in wheat, barley and rye, as well as in Triticale and MIR. Both Triticale and MIR are wheat/rye hybrids.
These proteins are important during germination, they help to feed the developing plant.
They are also useful once the grain is milled into flour, when the gluten helps the elasticity of the flour and enables the dough to be kneaded and worked to form different end products.
Gluten is the only known protein found in our food which is completely indigestible.