Timely and spatially controlled endosomal trafficking and signaling is important for cell proliferation, directed cell migration, and cell invasion, which are frequently misregulated in cancer cells. Cdc42-interacting protein-4 (CIP4) family adaptors promote endocytosis by inducing membrane invaginations via their Fer/CIP4 Homology-Bin/Amphyphysin/Rvs (F-BAR) domains, coupled with activation of the actin assembly machinery to promote vesicle fusion or motility. My thesis focuses on defining the roles of CIP4, and a related protein, Transducer of Cdc42-mediated actin assembly-1 (Toca-1), in regulating Epidermal Growth Factor Receptor (EGFR) endocytosis, EGFR trafficking, cancer cell motility, and invasion. In Chapter 2, I show that CIP4 and Toca-1 localize to early endosomes and promote EGFR trafficking from early endosomes to lysosomes for degradation, thus limiting extracellular signal-regulated kinase signaling from early endosomes and proliferation of A431 carcinoma cells. In Chapter 3, I provide novel evidence that depletion of Toca-1 results in defects in actin-based lamellipodial protrusions that are required for cell motility. The cause of these defects may relate to altered recruitment of the Abelson-interactor-1 and its effector Wiskott-Aldrich syndrome protein family verprolin-homologous protein to the lamellipodia in A431 cells depleted of Toca-1. Results in Chapter 4 identify CIP4 as a negative regulator of breast cancer invasiveness downstream of Src protein-tyrosine kinase. Src is a potent inducer of extracellular matrix (ECM)-degrading structures called invadopodia that function in tissue invasion by cancer cells. I found that CIP4 is a Src substrate that localizes to Src-induced invadopodia in MDA-MB-231 breast cancer cells. Interestingly, depletion of CIP4 results in enhanced ECM degradation, invadopodia formation, and invasiveness compared to control cells. Thus, CIP4 and Toca-1 are multifaceted regulators of EGFR downregulation, EGF-induced cell motility, and Src-induced cell invasion. / Thesis (Ph.D, Biochemistry) -- Queen's University, 2010-08-25 11:44:46.934