Signal transduction entails the precise interaction of relevant proteins for transmission of signaling pathways that carry out essential functions. Spatial arrangement and dynamics of these signaling molecules in a complex cellular environment is a critical mechanism by which signaling is precisely and rapidly regulated. Modulation of these spatial arrangements can occur through membrane topology and underlying cytoskeletal structures that confine and regulate protein dynamics to influence signaling. Cellular protrusions are peripheral elaborations that mediate fundamental physiological processes such as chemosensory and motility functions. In cancer, dysregulated protrusion growth underlies metastasis and disease progression. Breast cancer cell protrusions express HER2 and HER2 has been linked to protrusion growth, but details of how HER2 signaling is conducted in protrusions remains unclear. Here, I demonstrate a sensitive and quantitative method for analyzing the expression and spatial localization of HER2 and pAKT using quantum dots (QDs). I also employ novel monovalent HER2-QD probes using various imaging modalities (single particle tracking, EM, and super-resolution) to characterize the spatiotemporal features of HER2 signaling in breast cancer cell protrusions. I found that HER2 and downstream pAKT molecules are primarily localized in protrusions. In protrusions, HER2 motion is diffusive with mobility in filopodia that is faster than in lamellipodia and non-protrusive membrane regions; this is one of the fastest mobilities reported for membrane receptors. When the linear geometry of filopodial actin is perturbed, HER2 motion resembles areas of membrane absent of protrusions and downstream signaling is reduced. I propose that the linear actin structure creates a local environment for efficient movement of HER2 in filopodia that does not require active transport. This implicates the behavior of signaling molecules and their local environment in filopodia in shaping rapid, localized signaling of protrusion growth.