Nanomechanics of Ig-like domains of human contactin (BIG-2).

Contactins are modular extracellular cell matrix proteins that are present in the brain, and they are responsible for the proper development and functioning of neurons. They contain six immunoglobulin-like IgC2 domains and four fibronectin type III repeats. The interactions of contactin with other proteins are poorly understood. The mechanical properties of all IgC2 domains of human contactin 4 were studied using a steered molecular dynamics approach and CHARMM force field with an explicit TIP3P water environment on a 10-ns timescale. Force spectra of all domains were determined computationally and the nanomechanical unfolding process is described. The domains show different mechanical stabilities. The calculated maxima of the unfolding force are in the range of 900-1700 pN at a loading rate of 7 N/s. Our data indicate that critical regions of IgC2 domains 2 and 3, which are responsible for interactions with tyrosine phosphatases and are important in nervous system development, are affected by even weak mechanical stretching. Thus, tensions present in the cell may modulate cellular activities related to contactin function. The present data should facilitate the interpretation of atomic force microscope single-molecule spectra of numerous proteins with similar IgC2 motives.