To model transport through the grain boundary of a manganite bicrystal we study the conductance of a spin chain doped with itinerant electrons which are strongly Hund coupled to the spin at each site. We induce a domain wall in the chain and propose a single site pinning mechanism. At large pinning we find that the magnetoresistance corresponds to that of two uniformly magnetized domains. Assuming uniaxial anisotropy within each grain we use the above results to calculate the magnetoresistance through the grain boundary and obtain remarkable agreement with experiment. Further, our study solves an apparent contradiction between experiments by different authors.