Dislocations and dislocation networks act as sinks for irradiation-induced point defects such as vacancies and interstitials, or impurities such as helium. Recently, it has been found that the intersection points of the screw dislocation network formed at twist grain boundaries in Au (experimental) and Cu (simulations) act as nucleation sites for He bubbles, which form an array at the interface [J. Hetherly, E. Martinez, Z. F. Di, M. Nastasi, and A. Caro, Scr. Mater. 66, 17 (2012)]. The modeling part of that study was based on Monte Carlo algorithms. Here, using molecular dynamics simulations, we have performed shear deformation simulations of small-angle twist grain boundaries in α-iron. We report on the mechanical properties of this interface in pure as well as He-segregated samples. We find that this particular dislocation array in pure Fe is extremely mobile, with a Peierls stress ten times smaller than the value for a single straight screw dislocation, and that He bubbles induce a colossal increase (>50 times) in Peierls stress with respect to the pristine network. We interpret the results in terms of preexisting kinks and no shear transmission across the He bubbles.