 Therefore, in a protofilament, one end will have the α-subunits exposed while the other end will have the β-subunits exposed.These ends are designated the (−) and (+) ends, respectively.The protofilaments bundle parallel to one another with the same polarity, so, in a microtubule, there is one end, the (+) end, with only β-subunits exposed, while the other end, the (−) end, has only α-subunits exposed.