Microtubules play a fundamental role in diverse cellular functions including mitosis and migration. EB1 is the prototypic member of a family of microtubule-associated proteins highly conserved from yeast to man. It was first identified through its interaction with the adenomatous polyposis coli (APC) tumour suppressor protein and has since been shown to belong to an emerging class of microtubule-associated proteins that localise to growing microtubule distal tips. An association with components of the cytoplasmic dynein/dynactin microtubule motor complex has also been identified. The human EB1 protein family contains at least three members, but their precise cellular functions remain unclear.

Defining the function of EB1 family proteins could have important implications for our understanding of the cellular defects underlying the development and progression of human cancer. A role for the APC/EB1 interaction during cell migration has been suggested and this may help us understand how cancer cells behave during invasion and metastasis. The budding yeast EB1 homologue participates in a late mitotic checkpoint whose abrogation leads to the uneven segregation of chromosomes between daughter cells. This type of genetic instability is a feature of many human cancers, including those initiated by mutations in the APC gene. Understanding the function of EB1 proteins could therefore give us important insights into how human cells ensure the fidelity of chromosomal transmission during normal cell division and help reveal the ways in which these mechanisms are circumvented in cancer. We are investigating the function of EB1 by generating a series of defined EB1 deletion mutants. Using these tools we have recently identified an interaction between EB1 and the p150^glued subunit of dynactin. We have defined the regions in EB1 that bind to APC and p150^glued and shown that a) the binding sites are located towards the C-terminus of the protein; b) the binding sites overlap; and c) binding of EB1 to one of these proteins precludes binding to the other. We are also making progress in defining the EB1 binding sites in APC and p150^glued. Using EB1 deletion mutants fused to GFP we have shown that EB1 uses a large region towards its N-terminus to interact with microtubules in cells and tubulin in vitro. We are currently devising strategies that will allow us to specifically inhibit the interactions between EB1 and its individual binding partners. Initial results suggest that inhibition of the EB1/p150^glued interaction has serious consequences for microtubule organisation in mammalian cells. We have also cloned the other members of the human EB1 family and are investigating whether functional redundancy exists among family members.