How to modulate supramolecular protein nanotubes without sacrificing their thermodynamic stability? This challenging issue emerged with an enhanced reality since our successful development of a protein nanotube of chaperonin GroEL(MC) as a novel ATP responsive 1D nanocarrier because the nanotube length may potentially affect the cellular uptake efficiency. Herein, we report a molecularly engineered protein end-capper (SRMC) that firmly binds to the nanotube termini since the end-capper originates from GroEL. According to the single-ring mutation of GroEL, we obtained a single-ring version of GroEL bearing cysteine mutations (GroEL(Cys)) and modified its 14 apical cysteine residues with merocyanine (MC). Whereas SRMC self-dimerizes upon treatment with Mg2+, we confirmed that SRMC serves as the efficient end-capper for the Me(2+)mediated supra molecular polymerization of GroEL(MC) and allows for modulating the average nanotube length over a wide range from 320 to 40 nm by increasing the feed molar ratio SRMC/GroEL(MC) up to 5.4. We also found that the nanotubes shorter than 100 nm are efficiently taken up into HEP3B cells.