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 c pSRP43, which is absent from cytosolic SRP, and cySRP54 binds to SRP-RNA, w hich appears to be absent from cpSRP. In the presence of cpSRP43, cpSRP54 p osttranslationally forms a soluble targeting intermediate, transit complex, with the major light harvesting protein of the thylakoid membrane. In cont rast, cySRP54 functions cotranslationally. In this study we investigated wh ether cytosolic and chloroplast forms of SRP54 were interchangeable in thre e 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 subun its able to substitute for each other suggesting that the two proteins are fundamentally different. (C) 1999 Academic Press.