Viruses of the family Caliciviridae are non-enveloped, single-stranded, positive sense RNA viruses. Feline calicivirus (FCV), a virus in the genus Vesivirus, is used as a model to study basic mechanisms of replication of caliciviruses because it grows well in cell culture and it has a reverse genetics system. A feature unique to vesiviruses is the presence of a ~14-18 kDa protein of unknown function designated as leader of the capsid (LC) that is expressed as part of a capsid precursor encoded in open reading frame 2 (ORF2). The ORF2 precursor contains the LC fused to the major capsid protein, VP1, which is proteolytically cleaved by the viral protease to release the LC and the mature VP1. The LC is not found in purified virions, and therefore does not appear to have a structural role, but it has previously been associated with promoting the human norovirus RNA replicon when provided in trans. In order to study the role of the LC in virus replication I employed the reverse genetics system to generate recombinant full-length FCV genomes, and performed transient expression experiments with the LC alone. By applying deletional mutagenesis and scanning alanine mutagenesis to the LC coding sequence I identified regions and conserved residues critical for viral replication and virus spread. Transient expression of the LC caused cells to round and die, and the same residues important for virus spread were important for the cell-rounding phenotype. Immunoprecipitation of recombinant LC in FCV infected cells identified the cellular protein annexin A2 as a binding partner, providing a potential mechanism for the cell-rounding phenotype observed in transient expression experiments. Understanding the role of the LC in FCV replication is important because there are currently no antiviral drugs available for FCV and there are numerous reports of vaccine failure. Additionally, elucidating the mechanism responsible for the enhancement of the Norwalk virus replicon may provide new insight into the establishment of a cell culture system for the human noroviruses, which is the leading cause of viral gastroenteritis outbreaks.