Transiently expressed early light-inducible thylakoid proteins share transmembrane domains with light-harvesting chlorophyll binding proteins.

The early light-inducible proteins (ELIPs) of barley chloroplasts are encoded in two multigene families yielding end products of different molecular mass. Sequencing of complete cDNA clones showed that the low and high molecular mass proteins differ by the presence or absence of a 65 amino acid peptide in the amino-terminal part of the mature proteins. Two domains of the ELIPs reveal striking similarity in amino acid sequence with two transmembrane domains of all known light-harvesting chlorophyll a/b-binding proteins from photosystem I and II and may be of importance in anchoring the polypeptides in the membrane. The cDNA sequences of two low molecular mass ELIPs differ by an insert of 5 codons in the putative transit peptide. By in vitro transcription and translation of the cloned DNA and subsequent transport of the products into chloroplasts it could be established that the precursors are processed into products of identical apparent molecular mass. In vitro translated ELIPs were incorporated into thylakoid membranes both as precursors and mature polypeptides. It is suggested that ELIPs are pigment-free substitutes for light-harvesting polypeptides in the assembly of photosynthetic units during early development of thylakoids.