ABSTRACT Microtubule-associated proteins (MAPs) are an important class of cytoskeletal proteins that are presumed to confer specific functional characteristics on microtubules. The best-characterised is the heat-stable brain MAP, MAP-2. MAP-2 consists of three subunits MAP-2a, MAP-2b (both having an apparent Mrof∼270K; K=103Mr) and MAP-2c (Mr 70K) (Vallee et al. 1981; Garner et al. 1988a). The first insight into the structure of the MAP-2 molecule emerged from the chymotryptic digestion of the protein (Vallee, 1980). Fragments with Mr values in the range of 32 to 39K were found to bind to, and promote the assembly of, microtubules, whilst a larger 240K fragment had no interaction and probably represented an arm domain that projected from the surface of the microtubule and interacted in some way with other cell components, or, indeed, other microtubules (Vallee and Borisy, 1977; Vallee, 1980).