Functional divergence of the plastid and cytosolic forms of the 54-kDa subunit of signal recognition particle

Chloroplast and cytoplasmic signal recognition particles (cpSRP and cySRP) each contain a similar subunit, SRP54. The chloroplast homologue binds to cpSRP43, which is absent from cytosolic SRP, and cySRP54 binds to SRP-RNA, which appears to be absent from cpSRP. In the presence of cpSRP43, cpSRP54 posttranslationally forms a soluble targeting intermediate, transit complex, with the major light harvesting protein of the thylakoid membrane. In contrast, cySRP54 functions cotranslationally. In this study we investigated whether cytosolic and chloroplast forms of SRP54 were interchangeable in three types of functional assays: complementation of an Escherichia coli SRP54 mutant, formation of the transit complex, and heterologous binding between the SRP54 subunits, cpSRP43, and SRP-RNA. In no cases were the 54-kDa subunits able to substitute for each other suggesting that the two proteins are fundamentally different. (C) 1999 Academic Press.