Protein transport into "complex" diatom plastids utilizes two different targeting signals

Area of Science:

• Cell Biology
• Algal Biology
• Evolutionary Biology

Background:

• Diatom plastids are enclosed by four membranes, unlike higher plant chloroplasts (two membranes).
• Nuclear-encoded proteins in diatoms possess bipartite targeting sequences, including an ER signal sequence and a transit peptide-like domain.
• Understanding diatom protein import is crucial for comprehending plastid evolution and function.

Purpose of the Study:

• To investigate the protein import mechanism into diatom plastids.
• To determine if diatom protein targeting shares similarities with higher plants and green algae.
• To elucidate the functional equivalence of targeting signals in different algal lineages.

Main Methods:

• Heterologous import experiments using the gamma subunit of chloroplast ATPase from Odontella sinensis.
• Analysis of protein targeting sequences, including endoplasmic reticulum signal sequences and transit peptide-like domains.
• Comparative sequence analysis of protein targeting signals across different plant and algal groups.

Main Results:

• Protein import into diatom plastids is a multi-step process, at least two steps are involved.
• The initial step involves co-translational transport through endoplasmic reticulum membranes.
• A subsequent targeting step, similar to higher plants, indicates functional equivalence of diatom and plant transit peptide-like domains.

Conclusions:

• Diatom plastid protein import requires passage through the endoplasmic reticulum, followed by a distinct targeting step.
• The transit peptide-dependent targeting mechanism in plastids is evolutionarily conserved.
• While the core targeting mechanism is conserved, the peptidase cleavage site has undergone significant modification during evolution.