Arc is a repurposed retrotransposon Gag protein that transports mRNA between neurons via assembling into virus-like capsids. An Arc homolog, the HIV Gag, can selectively package the 5’ untranslated region (UTR) of its own genome RNA, leading to more efficient capsid assembly than other abundant host cytoplasmic RNAs. There is currently limited understanding on the mRNA selection mechanism during Arc capsid assembly. Investigating this mechanism can provide insight into Arc’s role in neurological function and impact within neurodegenerative diseases as well as for utilizing Arc as a potential delivery vehicle for therapeutic applications. Here, we investigate any preference that Arc protein potentially has towards binding the Arc 5’ UTR mRNA motif over other mRNA sequences. Dynamic light scattering (DLS) measurements and atomic force microscopy (AFM) confirmed in vitro Arc capsid assembly encapsulates any mRNA that are prominently present within the environment. A custom-made surface plasmon resonance (SPR) biosensor allowed for an in-depth study of biomolecular interactions between mRNA and Arc. The SPR biosensor results showcased Arc’s ability to nonspecifically bind to any mRNA sequence. Furthermore, it confirmed the existence of a selective mRNA binding site on Arc’s CA domain to the Arc 5’ UTR mRNA sequence and how this selective interaction can initiate Arc multimerization for capsomere assembly.