This application is based, inter alia, on the discovery of a binding interaction between the Hn-33 hemagglutinin polypeptide of the type A Clostridium botulinum neurotoxin complex and synaptosomal proteins, including synaptotagmin II (Syt II). Methods of screening for compounds that modulate, e.g., increase or decrease, this interaction are provided. Also provided are compositions and methods for targeting compounds to neuronal and cancer cells by coupling the compounds to Hn-33 or biologically active Hn-33 variants.