The dynamic behavior of the APC-binding protein EB1 on the distal ends of microtubules

Adenomatous polyposis coil protein (APC) is a well-characterized tumor supp ressor protein [1-3]. We previously showed that APC tagged with green fluor escent protein (GFP) in Xenopus A6 epithelial cells moves along a subset of microtubules and accumulates at their growing plus ends in cell extensions [4]. EB1, which was identified as an APC binding protein by yeast two-hybr id analysis [5], was also reported to be associated with microtubules [6-8] , To examine the interaction between APC and EB1 within cells, we compared the dynamic behavior of EB1-GFP with that of APC-GFP in A6 transfectants, T ime lapse microscopy of live cells at interphase revealed that EB1-GFP was concentrated at all of the growing microtubule ends throughout the cytoplas m and abruptly disappeared from the ends when microtubules began to shorten . Therefore, EB1 appeared to be co-localized and interact with APC on the g rowing ends of a subset of microtubules. When APC-GFP was overexpressed, en dogenous EB1 was recruited to APC-GFP, which accumulated in large amounts o n microtubules. On the other hand, when microtubules were disassembled by n ocodazole, EB1 was not co localized with APC-GFP, which was concentrated al ong the basal plasma membrane. During mitosis, APC appeared to be dissociat ed from microtubules, whereas EB1-GFP continued to concentrate at microtubu le growing ends. These findings showed that the APC-EB1 interaction is regu lated within cells and is allowed near the ends of microtubules only under restricted conditions.