Structural and functional characterisation of the signal recognition particle-specific 54 kDa protein (SRP54) of tomato.

Two representative genes for the 54 kDa protein subunit of the signal recognition particle (SRP54) of tomato were cloned. It was shown that both genes are expressed in the tomato cv. Rentita. SRP54 is encoded by nine exons distributed over 10 kb of genomic sequence. The amino acid sequences deduced for the two SRP54 genes are 92% identical and the calculated protein size is 55 kDa. Like the homologous proteins isolated from other eukaryotes, the tomato SRP54 is evidently divided into two domains. As deduced from sequence motif identity, the N-terminally located G-domain can be assumed to have GTPase activity. The C-terminal part of the protein is methionine rich (14% methionine) and represents the M-domain. In in vitro binding experiments, SRP54 of tomato was able to attach to the 7S RNA of tomato, its natural binding partner in the SRP. This interaction can only take place in a trimeric complex consisting of 7S RNA, SRP54 and SRP19. The latter protein subunit of the SRP complex is assumed to induce a conformational change in the 7S RNA. The human SRP19 was able to mediate the binding of the tomato SRP54 to the 7S RNA, irrespective of whether this latter originated from tomato or man.