Reversible Unfolding of Individual Titin Immunoglobulin Domains by AFM

TLDR

Single‑molecule AFM was used to repeatedly stretch individual titin molecules and record the force as a function of elongation. The force–extension curves displayed a sawtooth pattern with 25–28 nm periodicity, indicating unfolding of individual immunoglobulin domains at 150–300 pN that depended on pulling speed, and refolding was observed upon relaxation.

Abstract

Single-molecule atomic force microscopy (AFM) was used to investigate the mechanical properties of titin, the giant sarcomeric protein of striated muscle. Individual titin molecules were repeatedly stretched, and the applied force was recorded as a function of the elongation. At large extensions, the restoring force exhibited a sawtoothlike pattern, with a periodicity that varied between 25 and 28 nanometers. Measurements of recombinant titin immunoglobulin segments of two different lengths exhibited the same pattern and allowed attribution of the discontinuities to the unfolding of individual immunoglobulin domains. The forces required to unfold individual domains ranged from 150 to 300 piconewtons and depended on the pulling speed. Upon relaxation, refolding of immunoglobulin domains was observed.