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

Area of Science:

• * Cell Biology
• * Evolutionary Biology
• * Phycology

Background:

• * Cryptophytes, archetypal chromalveolates, feature complex plastids derived from red algal endosymbionts.
• * These plastids contain a nucleomorph, a remnant nucleus, within the periplastidial compartment (PPC).
• * Efficient protein import into the PPC and stroma across multiple membranes has been a long-standing question.

Purpose of the Study:

• * To elucidate the mechanisms of protein import into the cryptophyte periplastidial compartment (PPC) and stroma.
• * To identify the signals responsible for targeting proteins across the four-membraned plastid envelope.
• * To understand a key evolutionary innovation in chromalveolate biology.

Main Methods:

• * Analysis of protein targeting signals in cryptophytes.
• * Identification of novel protein translocation machinery.

Main Results:

• * A bipartite topogenic signal, comprising a signal and transit peptide (TP), directs proteins to the PPC or stroma.
• * An aromatic amino acid-based motif at the +1 position of the TP dictates the final protein destination.
• * A novel ERAD-derived translocon in the second-outermost membrane facilitates protein transport.

Conclusions:

• * The identified bipartite signal and TP motif explain protein import into cryptophyte plastid compartments.
• * The novel translocon is crucial for navigating the complex membrane system.
• * These findings illuminate sophisticated targeting mechanisms and a key chromalveolate evolutionary innovation.