Noun
protofilament (plural protofilaments)
A filament of polymerized tubulin in a cell, which becomes part of a microtubule
Two of these dimer structures then join side by side, in an antiparallel arrangement, to form a tetramer called a protofilament. Source: Internet
In the A-type lattice, the lateral associations of protofilaments occur between adjacent α and β-tubulin subunits (i.e. an α-tubulin subunit from one protofilament interacts with a β-tubulin subunit from an adjacent protofilament). Source: Internet
Each protofilament is a series of tandem protein chains. Source: Internet
Therefore, in a protofilament, one end will have the α-subunits exposed while the other end will have the β-subunits exposed. Source: Internet