Characterization of Spiroplasma citri adhesion related protein SARP1, which contains a domain of a novel family designated sarpin

Transmission of the plant pathogen Spiroplasma citri by its leafhopper vect or, Circulifer tenellus, involves adherence to and invasion of insect host cells. The S. citri adhesion related protein P89 (SARP1) was purified by im munoprecipitation using anti-SARP1 monoclonal antibodies. The protein's N-t erminal amino acid sequence was determined and used to design a degenerate oligonucleotide. The labeled oligonucleotide hybridized to a 3.5 kb MboI fr agment from S. citri DNA, which was then cloned and sequenced. Additionally , a 1.9 kb RsaI fragment of S. citri DNA, partially overlapping the MboI fr agment, was isolated and characterized. Sequence analysis of the two clones revealed four open reading frames. ORF1 (675 bp) encodes the C-terminal pa rt of a Soj-like protein. ORFs 1 and 2 were separated from ORFs 3 and 4 by a putative transcription termination site, indicated by a hairpin structure . ORF3 encodes an amphiphilic 798 amino acid long protein with a cleavable signal peptide and a predicted transmembrane helix near the C-terminus. The mature protein of 85.96 kDa has a calculated pI value of 5.5 and has an N- terminal amino acid sequence consistent with that determined from the purif ied SARP1 At the N-terminus of this protein is a region consisting of six r epeats, each 39-42 amino acids, a motif belonging to a previously unrecogni zed family of repeats found in a variety of bacterial proteins. The taxonom ically spotty presence of this 'sarpin' domain and the relationship of the repeats to each other suggests a convergent evolution in multiple lineages. (C) 2001 Elsevier Science B.V. All rights reserved.