Absence of plasmids encoding adhesion-related proteins in non-insect-transmissible strains of Spiroplasma citri

In the plant-pathogenic mollicute Spiroplasma citri, spiralin is the major lipoprotein at the cell surface and is thought to be one of the components involved in the interactions of the spiroplasma with its insect vector. With the aim of identifying surface proteins other than spiralin, monoclonal antibodies (mAbs) were produced by immunization of mice with the spiralin-defective S. citri mutant GII3-9a2. mAb 10G3 was found to react with several polypeptides of 43-47 and 80-95 kDa, all of which were detected in the detergent phase after Triton X-114 partitioning of proteins. Mass spectrometry (MALDI-TOF) analyses of the two major polypeptides P47 and P80 of GII3-9a2, reacting with mAb 10G3, revealed that P47 was a processed product and represented the C-terminal moiety of P80. Search for sequence homologies revealed that P80 shared strong similarities with the S. citri adhesion-related protein P89 (Sarp1) of S. citri BR3, and is one (named Scarp4a) of the eight Scarps encoded by the S. citri GII-3 genome. The eight scarp genes are carried by plasmids pSci1-5. Western immunoblotting of proteins with mAb 10G3 revealed that, in contrast to the insect-transmissible S. citri strain GII-3, the non-insect-transmissible strains ASP-1, R8A2 and 44 did not express Scarps. Southern blot hybridization experiments indicated that these strains possessed no scarp genes, and did not carry plasmids pSci1-5. However, S. citri strain GII3-5, lacking pSci5, was still efficiently transmitted, showing that, in the genetic background of S. citri GII-3, the pSci5-encoded genes, and in particular scarp2b, 3b and 5a, are not essential for insect transmission. Whether plasmid-encoded genes are involved in transmission of S. citri by its leafhopper vector remains to be determined.