Characterization of homologues of the apple proliferation immunodominant membrane protein gene from three related phytoplasmas.

SUMMARY Homologues of the immunodominant membrane protein gene from apple proliferation (AP) phytoplasma have been cloned and sequenced for three further members of the AP subclade, namely European stone fruit yellows, peach yellow leaf roll and a European isolate of pear decline (PD). The putative translation products of all three were similar in size to that of AP and all had a transmembrane region towards the N-terminus and a large C-terminal hydrophilic domain, probably held on the outside of the cell membrane in vivo. Sequence similarities for the putative proteins were compared with interrelationships of the phytoplasmas as measured by rRNA gene sequence similarity. The proteins from AP and PD were more similar (57% identical in the major hydrophilic domain) than those for any other pair (31-34%), but these two phytoplasmas were not more closely related by rRNA gene sequences than other pairs. The possibility that the relative similarities of these proteins is related to the host is discussed. It is suggested that the similarity of the AP and PD proteins may reflect the fact that these two proteins have narrow plant host ranges in two closely related genera in the tribe Maloideae (family Rosaceae), whilst the other two have broader host ranges, mainly in the tribe Prunoideae.

Immunodominant membrane proteins from two phytoplasmas in the aster yellows clade (chlorante aster yellows and clover phyllody) are highly divergent in the major hydrophilic region.

The mechanisms by which phytoplasmas interact with their hosts are not understood. Mollicute membrane proteins may play a role in such interactions and therefore the amp genes encoding immunodominant proteins from two phytoplasmas, aster yellows and clover phyllody, which fall within the largest taxonomic subclade of the phytoplasmas, have been cloned and characterized. The putative translation products, antigenic membrane proteins (Amps), of these genes have properties which are typical for bacterial membrane proteins, and which suggest that each has a single large extracellular hydrophilic domain held by a transmembrane region near the C-terminus, with only a short C-terminal intracellular sequence. Both of the Amps characterized here have bacterial leader sequences which are cleaved during maturation. Whilst the signal peptide and transmembrane regions of the two proteins are very similar, the major hydrophilic domains are highly divergent in both size and sequence. The Amps from the two phytoplasmas are also different in structure and sequence from the immunodominant membrane proteins of three other phytoplasmas whose genes have been cloned previously.