Model for the combined action of CP110 and CPAP at MT plus ends. A model illustrating the molecular mechanism of slow and processive MT growth observed with CEP97^CP110 and CPAP. (A) CEP97^CP110 (green) binds to the luminal side of the MT plus end (gray) through a MT-binding domain present at the C terminus of the CP110 moiety of CEP97^CP110. By doing so, it inhibits incorporation of new tubulin dimers into the MT plus end. (B) CPAP binds on the outside of the MT wall with its MT-binding domain (MBD); with the two parts of its tubulin-binding domain PN2–3, SAC, and LID, it binds to the side and longitudinal interface of tubulin at the PF tip, inhibits catastrophes, and leads to a 4× reduction in the plus-end growth rate. New tubulin dimers incorporated into the MT are shown with asterisks. (C) CEP97^CP110 and CPAP bind to each other through their coiled-coil domains (CPAP-CC1/CP110-CC2), and CPAP overcomes growth inhibition imposed by CP110. The complex of the two proteins leads to slow and processive growth of the MT plus end. Disruption of CP110 homodimerization by CPAP might contribute to alleviation of CP110-driven MT growth inhibition.