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<div><em>Citrus tristeza virus</em> (CTV), the most economically important viral pathogen of citrus, encodes a unique protein, p33. Research conducted over the recent years showed that this protein plays multiple important roles in viral pathogenesis. It was found that while p33 is dispensable for infection of a number of citrus varieties, it is required for systemic infection of a few others. Furthermore, p33 is one of the key viral factors mediating virus ability to exclude superinfection with the same or closely related virus. The characterization of p33 demonstrated that it is a self-interacting integral membrane protein, and its membrane localization is important for virus ability to infect an extended host range. In the infected cells, p33 shows plasmodesmata localization and forms extracellular tubules, which are the characteristic features found for a number of movement proteins of other plants viruses. Although p33 is not conserved among other closteroviruses, a few members of the genus <i>Closterovirus</i> encode proteins of ~30 kDa whose corresponding genes are situated at the same genomic position as that of the CTV p33 gene. While primary sequences of those proteins do not share significant homology, some of their characteristics along with their secondary structures show some similarities. Further research is required for a better understanding of the functions that these proteins play in virus infection and the overall genome complexity of closteroviruses.</div>