The term 'Proton Constituent' refers to the building blocks of a proton, which are quarks. Quarks are elementary particles that make up protons and neutrons, and they come in six different types, known as flavors. The five-letter answer to the crossword puzzle is 'QUARK, ' which is the most common type of QUARK found in protons. Quarks have fractional electric charges and interact with each other through the strong nuclear force.