Fast exchange model. (A) MT polymerization generates a large number of binding sites (orange ellipses), which disappear with single-order reaction kinetics. Thus, as time progresses, less binding sites are present within the depicted rectangle. (B) Dimeric CLIP-170 exchanges rapidly on binding sites irrespective of the position on the MT end. Several interactions with CLIP-170 molecules can occur during the lifetime of a binding site. The equilibrium between cytoplasmic and MT end–bound CLIP-170 (reaction a) might be determined by posttranslational modifications (reaction c), conformational changes, and/or protein–protein interactions. As we find CLIP-170 exchange on MT ends distal of sites of MT polymerization, copolymerization of CLIP-170 with tubulin (reaction b) does not explain the cometlike distribution of +TIPs. However, it is not excluded (hence the stippled arrow), and modified forms of CLIP-170 (indicated by the purple ellipses) might bind tubulin with higher affinity.