Minor component of the viral capsid involved in aphid transmission. The RTD domain of the protein is exposed on the surface of the particle and determines the vector specificity and tropism of the virus (By similarity). The RTD domain is not necessary for virus stability in the host hemolymph. This protein is translated as a fusion protein by episodic readthrough of the major coat protein termination codon. It is composed of the major capsid protein fused to a long C-terminal extension called the readthrough domain (RTD). Readthrough of the terminator codon TGA occurs between the codons for 189-Leu and 191-Gly. The N-terminal region like those of many plant virus capsid proteins is highly basic. It has been suggested that these regions may be involved in protein-RNA interaction. Belongs to the luteoviruses readthrough protein family.