Integration of early light-inducible proteins into isolated thylakoid membranes

An in-vitro system has been established to study the integration of early light-inducible proteins (ELIP) into isolated thylakoid membranes. The in-vitro-expressed ELIP precursor proteins exist in two forms, a high-molecular-mass aggregate which is accessible to trypsin but no longer to the stromal processing protease and a soluble form which is readily cleaved to the mature form by the stromal protease. The mature form of ELIP is integrated into thylakoid membranes; its correct integration can be deduced from the observation that the posttranslationally transported products and the in-vitro integrated ELIP species are cleaved by trypsin to products of the same apparent molecular mass. Trypsin-resistant fragments of high-molecular-mass and low-molecular-mass ELIP appear to have the same size. The processed ELIP species, as well as an engineered mature form of ELIP, are integrated into isolated thylakoid membranes. Integration of the mature protein occurs in the absence of stroma, into sodium-chloride-washed, and trypsin-treated thylakoid membranes. The process of integration is almost temperature independent over 0-30 degrees C. Analysis of the time course of integration leads to the conclusion that, under in-vitro conditions, processing but not integration into membranes is the rate-limiting step. In the absence of stroma, the ELIP precursor is bound to the thylakoid membranes, however, it is no longer accessible to the stromal maturating protease when added after binding has occurred. In conclusion, integration of ELIP differs in many essential details from that of its relatives, the light-harvesting chlorophyll a/b protein family.