ARIADNE'S THREAD: GUIDING A PROTEIN ACROSS FIVE MEMBRANES IN CRYPTOPHYTES(1)

Cryptophytes are the most archetypal chromalveolates, with their complex plastid having retained many features of the red algal secondary endosymbiont. Most important of these is the remnant nucleus, the nucleomorph, that is kept between the inner and outer membrane pair of the endosymbiont in the highly reduced cytosol, the periplastidial compartment (PPC). Because the nucleomorph's coding capacity is very limited, proteins need to be imported from the host cytosol across the outer two membranes into the PPC and across all four membranes into the stroma. How this is accomplished has puzzled researchers for >20 years. Recent findings show that in both cases, a bipartite topogenic signal, a signal and subsequent transit peptide (TP), is responsible for targeting proteins correctly into these two compartments. An aromatic amino acid-based motif at the +1 position of the TP holds the information determining into which compartment the precursor protein is finally transported. Together with the identification of a novel endoplasmic reticulum associated degradation (ERAD)-derived translocon in the second-outermost membrane, these findings help us to understand the sophisticated targeting mechanisms across four membranes and clarify a key innovation during chromalveolate evolution.