Protein import and the origin of red complex plastids

Area of Science:

• Evolutionary biology
• Cell biology
• Molecular biology

Background:

• The evolutionary history and number of endosymbiotic events involving red algae remain contentious.
• Gene phylogenies, while popular, yield conflicting results regarding the origins of complex plastids.
• Previous studies suggest multiple tertiary or quaternary endosymbiotic events to explain phylogenetic discrepancies.

Purpose of the Study:

• To provide independent evidence for the number of red secondary endosymbiotic events.
• To investigate the role of protein import machinery in understanding plastid origins.
• To propose a unified model for the origin of red complex plastids.

Main Methods:

• Analysis of the protein translocation machinery (SELMA) involved in protein import into complex red plastids.
• Comparison of protein import mechanisms across different plastid types.
• Evaluation of SELMA's evolutionary origin and targeting specificity.

Main Results:

• The protein import machinery, SELMA, evolved once and is conserved across red complex plastids.
• SELMA's conserved function and specific targeting to the second outermost membrane provide strong evidence for a single origin of red complex plastids.
• The unity of protein import across primary plastid membranes supports their single cyanobacterial origin.

Conclusions:

• The conserved SELMA-dependent protein import system strongly supports a single red secondary endosymbiotic event.
• This event is the common origin for all red complex plastids.
• The two outer membranes of red complex plastids likely originated from the host's endoplasmic reticulum during this initial endosymbiotic event.